STRUCTURAL COLORS I N INSECTS. I1 B Y CLYDE W . MASON‘
Iridescent Colors Iridescence has for its main characteristic a change in the hue of the object exhibiting it as the angle of vision is varied: frequently iridescence is associated with marked surface lustre, which is more or less pearly or metallic. Whatever may be one’s aesthetic standards, there is something peculiarly fascinating about the glinting, shifting hues shown by iridescent objects, and those which cccur in nature have attracted their share of attention because this property is so markedly different from the ordinary sort of coloring. There are numerous familiar examples : certain insects, mother-of-pearl, oil films on water, tarnish on metals, diamonds and prisms, peacocks and hummingbirds, “lustre” pottery and glasses, opals, laboradorite, “Barton’s buttons” and so on. One might even call “changeable!’ or “two-tone” silks iridescent,,though in this case it is only a question of one lot of colored threads being in the proper position to reflect light while those of the other color are in an unfavorable orientation. Microscopically this is hardly iridescence, but macroscopically it satisfies the criterion. It is commonly assumed that iridescent colors are probably of structural origin but a more definite explanation than this is necessary, and many extensive studies have been made of the different types of iridescence, and their occurence in nature. Most works on physical optics devote a fair amount of space to the discussion of the principles underlying these phenomena, and to the criteria which may serve to identify them. The iridescence observed in insects has been ascribed to almost every possible cause, but in most cases the problem has been attacked from one angle only. For instance, by certain quantitative optical tests a marked resemblance has been shovm between the colors of certain beetles and those of certain solid dyestuffs, which give metallic reflection; however, the same tests have not been applied to film colors, to which they also bear striking resemblances. There has also been a good deal of reasoning of this sort : a grating has many lines, close together and regularly arranged, and it gives iridescent colors ; certain scales of insects have many lines, close together and regularly arranged, and they show iridescent colors; therefore, the colors of the insects must be due to the grating structure. More than this is necessary, of course; it must be shown that in all observable respects the colors are identical with those obtained from gratings, and also that t h e y cannot be due t o any other possible cause. In the case just cited, the lines are present. They must act as a grating and they do: but the grating colors under ordinary conditions are not observable, and the colors seen are undoubtedly due to a different cause. The investigation upon which this article is based was supported by a grant to hlessrs. Bancroft, Chamot and Aierritt from the Heckscher Foundation for the advancement of Research e s t a l h h e d by August Heckscher at Cornel1 University.
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In the work on structural colors of feathers, carried on at Cornell three years ago’ it was necessary to study various types of structural colors; and tests were developed which served very positively to identify those present in feathers, namely, Tyndall blue and thin-film colors, and to exclude the other possihilities. All the “metallic” or iridescent feathers studied were found to owe their colors to thin films, but in the insect world such uniformity is hardly to be expected. The work on feathers involved a study of the different possible causes of their iridescence, and the same methods of distinguishing between these are applicable to the different cases of iridescence in insects. Without repeating the whole discussion of the various types of structural color, for which the reader is referred to these earlier articles, the main types of iridescent insect colors will be considered, classified first with respect to their anatomical occurience.* Iridescent Wing Membranes The changing colors of the wings of many dragonflies, beetles and other membranous winged insects recall the iridescence of soap bubbles, and although this is largely assumption on the part of most people, Gore& established their nature by a very sound and logical study. Obviously, the colors are structural, for they are observable in wings which are wholly unpigmented, as well as in pigmented wings after bleaching. The variety of coloring and its intimate mingling in such small area also make considerable demands upon any theory of their pigmental origin, while the undiminished iridescence in specimens bleached to colorless transparency is radically different from the selective metallic reflection (surface color) of strongly absorbing substances such as dyestuffs. They appear with full brilliancy in highly diffuse light, and are seen from any azimuth, along the line of direct reflection from the wing surface, so they cannot be due to structure of the nature of diffraction, gratings or prisms, all of which require sensibly unilateral illumination, and show their coloring in directions out of the line of the direct beam, and in one plane only (unless an infinite number of crossed gratings or prisms be postulated). S o indication of any such structures is observable bj- microscopic examination. Other special cases of diffraction phenomena are also eliminated by the above simple observations, for there is no “scattering” of light other than that due to the “pebbled” surface of the wing membrane, nor is there any evidence of haloes or coronas from any fine granular structure. A more detailed study of the specimens in questions throws additiona! light upon the nature of these colors. The common house-fly affords an example. Its unpigmented wings exhibit highly variegated colorings, most!y reds and greens, when examined closely with the naked eye or with a lom-
’ Mason: J. Phys. Chem., 27, 2 0 1 , 401 (19231. The insects studied i n this paper were furnished through the kindness of Dr. 11.. T. 11, Forbes, of the Department of Entomology at Cornell. The writer is also indebted to Dr. Forbes for many helpful suggestions during the progress of the investigation. 3Ann. SOC. Ent. fr., ( 2 ) 1, Z O I (18431.
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power magnifier, These are mingled together wholly at random escept near the edges of the wings where they appear in zones. The colors are seen very clearly by reflected light, when observed along the line of direct reflection; and any given point on the surface of the wing changes color as the angle of vision is increased. This change may be from red to green, or oice versa; the zone,s of color near the edge of the wing appear to move outward as the line of vision approaches grazing incidence. “The “pebbled” surface of the wing membrane prevents the observation of any definite polarization effects by reflected light; such a surface has of course no definite ”polarizing angle” and one sees little more by using an analysing nicol prism than without it. The above surface structure also prevents the effective application of pressure, for such severe tangential strains must result when attempt is made to flatten the “embossed“ surface and to compress the membrane that disruption of the structure is likely to result. However, there seems to be a slight shift in the hue of the reflection color. but this is not to be observed with certainty. K h e n examined by transmitted light the membrane shows only very pale tints which are complementary t o those of the same areas seen by reflection. Swelling agents such as S H , O H . water vapor, or phenol vapor alter the hues markedly, causing reds to change t o greens. and rice-versa; these color changes are reversed on the removal of the swelling agent. Immersion in liquid of n = I . j destroys the color, which is restored by washing and drying. The iridescent wing membranes of many common beetles show much the same optical behavior as those of the housefly, but present some significant differences. They are pigmented a neutral brown: their reflections resemble the “temper colors” of steel. both in hue and in metallic lustre, and range from blue through purple t o yellowish brown nearer the edges and tips of the xings. The changes with increasing incidence are marked, and take place in the order just mentioned; on swelling, the change is in the reverse direction. Bleaching does not alter the hue, though the lustre is less in the bleached specimen. Splitting the lamellae of the wing apart does not destroy the iridescence: the original colors are seen only on the outer surfaces. The inner surfaces may show different iridescent colorings. Collodion impressions of either surface show no iridescence, and no structure but the fine wrinkles and stubby hairs of the wing membrane. Immersion of the wing in liquids of index of refraction cn. I . 5 does not result in complete destruction of the iridescence, though the brilliancy of the coloring is very markedly decreased; penetration of the wing of liquids which fill the space between its upper and lower lamellae does not destroy the iridescence.‘ In addition to the above-mentioned examples, a number of other insects were examined with respect to the same properties, T y l o p e laricis, Mesothemis L
Goreau thought that the interlnmellar air s p c e was the color-producing film
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simplicicollis, Enallagviae, Dolichopus canalicirlatus, as well as the wings of several beetles; these all show substantially the same behavior as regards their iridescence, color change with swelling, sequence of colors on the wing, and absence of any but a neutral pigment. The disappearance of color on immersion seems to be peculiar to the wings of the housefly, however. Nature of Iridescent Coloring of Wing Membranes Since pigment coloring and diffraction phenomena as causes of the iridescence are eliminated, the most obvious explanation is on the basis of the colors of thin films, and the behavior described above is in good agreement with this very common assumption. Iridescence is the most striking property of thin-film colors, and their definite sequence of hues (Kewton’s series) is almost as well known: As the t,hickness of the film in question increases, a succession of colors occurs which is characteristic to one who is familiar with this series. The series is not a spectrum, and the color changes are not to be described in terms of wave-length, but rather as of higher or lower “order,” the latter corresponding to the thinner films. Increasing the angle of incidence is equivalent to thinning the film’ and the colore change accordingly. Swelling or compression of the color producing films alter their thickness, and consequently their color. The very pale tints observed by transmitted light are due to the large admixture of white t,ransmitted light. Against a dark background the reflection colors are complimentary to t’hose transmitted, and of considerable saturation and brilliancy. With respect to all the above properties, the colors of iridescent wing membranes correspond closely with those of thin films; however, certain discrepancies must be considered before their identity can be called established. If a thin color-producing film is brought in contact with a medium of index of refraction near its own, it should show a very marked diminution of its color, with complete disappearance if the index of refraction is matched exactly. In the case of the common housefly this is readily observed, but with the other wings examined only a decrease in the intensity of the color takes place, even when the wing membrane is immersed in liquid of refractive index near t h a t of chitin. This same behavior is found in the case of iridescent feathers, and is best explained on the assumption that the color is produced by multiple thin films, only the outer one of which would have its action affected by contact with a medium of similar index of refraction, We ordinarily think that a thin film must have a markedly different index of refraction from the surrounding medium if it is to produce color and appear metallic. However, a single thin film of xylene (n = 1.49)on glass (n = 1.52) shows Kewton’s series of colors distinctly as it evaporates. The brilliantly lustrous and iridescent flakes that fall from the surface of ancient Roman or Egyptian glass consist of thin laminae (of Si02?); they offer a convenient “model” for the study of multiple films. When dry, their reflection is highly metallic and their transmission colors are much more saturated than those of a single thin film. If one of these flakes is immersed in liquid, so that the voids between the laminae are filled, 1
C. V. Boys: “Soap Bubbles,” 148, 151 ( 1 g 1 2 )
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its color and lustre are somewhat diminished; hut the index of refraction of the immersion medium must he almost identical with that of the films if all reflection color and lustre are to be destroyed. The iridescent crystals of potassium chlorate studied by Stokes‘ and the fiery patches in the opal are other examples of multiple films in optical contact with a solid medium having nearly the same index of refraction as their own. The effectiveness of such a color-producing system depends on the difference in indices of refraction and on the number of films; both of these factors may be relatively small without serious loss of saturation or lustre. The saturation and brilliancy of the colors of even the brightest iridescent wing membranes are closely rivalled by those of artificial thin films such as can be made by allowing a drop of gold size to soread over water. Such a film, after it hardens: may he picked up on a piece of n e t paper (preferably gray or black, to give a dark background) and the distribution and sequence of its hues, compared with those of the insects, show a most striking similarity. As for the more or less metallic lustre exhibited by the iridescent nlng membranes, this is no more marked than that shown by artificial thin films. If multiple films are present in the wing membrane they might be expected to cause somewhat greater saturation and lustre than a single thin film, but this is opposed by the irregularity and “pebbled” character of the membrane and by its lack of perfect transparency. Biedermann‘s explanation of the iridescent colorings of wing membranes as being due t o an air film between the two lamellae of the wing2is not’ adequate, although he has recognized the nature of the colors by means of the criteria mentioned above. The thickness of the air film in the interior of the wing is many times greater than that corresponding to the colors exhibited; it may be filled with liquids, or it may be closed or opened by pressure or splitting without altering the colors. The highly intricate mingling of colors in a small area of a continuous membrane would involve an extremely complicated pigment distribution, if such were possible, but is readily accounted for by slight variations in the thickness of the color producing film. This is confirmed by the fact that these variations of color do correspond t’o relatively gradual variations in thickness rather than being a mosaic of sharply outlined areas of different colors, and moreover the colors adjacent on the membrane are those adjacent in Xewton’s series. This is particularly striking near the edge or tip of the wing, where a definite sequence of color zones, (first three “orders” of Newton’s series) corresponding to a gradual decrease in thickness, is observed. Such a structural condition is just what one would expect, and conversely, where veins extend out to the edge of the wing, the color zones are altered in a manner that indicates increased thickness near the vein. Swelling with water or other reagents causes the zones of color to shift toward the edge of the wing, without other alteration of their hues or se~~
Wood: “Physical Optics,” 161 ( 1 3 i 1 ) . * Handbuch der vergl. Physiol. 3, I, 2, 1657 (1914)
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quence; on drying, the color zones shift back again t o their original positions. Microscopic demonstration of the actual structure is hardly to be hoped for since the thickness of the films as calculated (ca. 0.211) must be practically a t the limit of resolution, and this limit may be reached only with good “contrasty” objects, suitably prepared. In this case undistorted microtome sections of the same order of thickness as the films themselves, together with highly selective staining of the material of the films would be necessary in order to avoid the possibility of spurious diffraction images. Optical tests are certainly more reliable than histological study, in this instance, and they point to a structure composed of a number (say three to ten) of films of materials of different index of refraction (possibly two varieties of chitin) in optical contact and forming a non-permeable tissue which constitutes the two lamellae of the wing. People were right when they guessed that filmy wings must have thin-film colors. Iridescent Scales The explanation of the iridescence of butterfly and beetle scales not being so obvious as in the case of wing membranes, almost every writer on insect colors has had a try at it. The presence of striations has suggested a diffraction grating, and this is popular11 assumed to be the cause of the color, although most of the later investigators have not fallen into this error. The people who use the terms “diffraction”, “refraction”, “dispersion”, and ‘(interference” indiscriminately and with reference to either gratings or thin films, who speak of “prismatic” colors when they mean iridescent ones, and who believe that “structural color” tells the whole story, are the ones who have perpetuated this untenable opinion. Hagen’, Biedermann2, Onslow,* SUffertj4 M a l l o ~ k , ~ a others, nd have pointed out that most scales, whether iridescent or not, are distinctly striated, and these writers have offered various other objections to a diffraction theory. Walter6 emphasizes the fact that the principal striations are lengthwise yet the colors may be seen with the light falling either lengthwise or crosswise of them. He also points out that a grating requires sensibly unidirectional light for the production of colors, while the scales are fully colored in thoroughly diffuse light. Hodgkinson’ cites the fact that grating colors are always seen out of the path of the directly reflected or transmitted beam, while the scales show specular reflection, and color in the path of the transmitted light. All these properties of gratings have been known to physicists for many years, while those of the insect scales can readily be tested by anyone, yet the serious discrepancies given above have not been accepted as conclusive evidence against the diffraction grating explanation of iridescence on insect
* Proc. Am. h a d . Arts Sci., 17 (1881-2). * Handbuch der. vergl. Physiol. (Winterstein) 3 I, 2,
1892 (1914). Phil. Trans., 211 E, I (1921). 4 Z. Morpho1 Okol. der Tiere, 1, 1 7 1 (1924). 5 Proc. Roy. SOC., 85, A, 598 (1911). fi “Die Oberflachenfarben order Echillerfarben” i1895);cf. J. Phys. Chem., 23,454 (1919). 7 ‘,Manchester Memoirs,” 2, (4) (1889).
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scales. Dimmock,’ Kellogg,2 and >layers have been conbent with such an explanation, and AIichelson4 has postulated a “saw-tooth grating” t’o reconcile the properties of certain iridescent scales (Enti7nus imperialis) with those of diffraction gratings. Such an unsymmetrical structure is hardly consistent with the highly symmetrical properties of the scales in question, and in addition is open to all the objections given above. I t mill be dealt with further when the insect studied is discussed. It is possible to illuminate any scale having reasonably regular striations so that it can act as a grating.5 but this is entirely independent of whether the scale is iridescent or not as ordinarily seen, and is hardly observable without the aid of the microscope. Lathams pointed this out many years ago. The arrangenient of scales on butterflies wings is ordinarily not suitable for the demonstration of diffraction colors, on account of the lack of perfect parallelism of the striae and of the scales. In the case of the “Pearl Morpho” there appears to be some chance of observing diffraction colors under natural conditions though only when the direct rays of the sun fall through the wing and are observed from a point somewhat to one side of their path.’ The colors seen when the wing is studied with illumination corresponding to this are faint, reminding one of those of mother-of-pearl (another case of diffraction colors in nature), and are very non-uniform.8 They are Iiarely perceptible in the more pigmented 3lorphos even under the most favorable conditions, and the yellow transmission color seen in the field by some observers is mainly due t o pigment and to the complement of the blue reflection. Cinimock. Onslow and others hare isolated the diffraction effect from the other color producing factors present, by making impressions of the striae of the scales in soft collodion and these “replica gratings” may be made “with most T>epicloptera,whether iridescent or not. . . . That all insects, the impressions of whose scales give spectra, do not themselves show iridescence, makes it doubtful whether diffraction is ever a main source of color, especially ~, and others as it was found that the brilliant scales of M o r p h o C y j i ~ l ‘ .\Vestw.. ryith the same structure, gave an almost flat imp~~ession.g”
‘ Psyche, 4, 4j (1883’1. Iiansas Univ. Quarterly, 3. 45 ( 1 8 8 4 1 . ‘Proc. Boston Soc. S a t . Hist., 27, 243 (1896-7:. Phil. Mag., ( 6 ) 21, 564 (1911). A special case of purely microscopic diffraction color is exhibited 11y many unpi.;mentetl. tioldy striated scales. The color liy transmitted lizht is a fairly deep blue if ohserved under the proper conditions, naniel\-: with the aperture of the illuminating heam from the contlensor bearing such a relationship ro the aperture of !he ol)jective that the diffractioii 8 ectra seen at the bark of the ohjective are well defined. the longcr wsi-e lengths (the most &hcteds lieing outside the cone of ritvs grasped IIY the ohjecrire. This phenorneuon is fairly commonly noted in microscopic siutiies of fine’structures ani1 a fuller treatment of i t \vi11 he presented in a later paper ’ h o c . llanchester Lit. and Phil. Soc. 3, 198 (1864). llallock floc. cit) has called uttentioi~to this. The angle at ivhich !hi$ spectrum is diffr:Lcted can lie rcJudilY measured and the “prating space” calculated from i t . with a fair approximation to the ‘interval between the striae as measured microscopically 1 Onslow 2oc. cif. p. 11.
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Selective reflection (surface color) has been thought, to be the cause of iridescence in insects. Walter,’ who saw “Oberflachenfarben” in almost every case of iridescence in nature, failed to reconcile this hypothesis with many of the well-known properties of the objects to which he applied it. Iridescent scales have many such points of difference: sensitiveness to pressure or crushing,* resistance to bleaching or extraction,3 loss of color or permeation with liquids and restoration on drying4transmission color much less saturated than reflection color,j highly complicated mingling of hues.$ very marked change of reflection and transmission colors with angle of in~idence.~ These discrepancies parallel those emphasized by the present writer in eliminating selective reflection from the possible causes of iridescence in feathers. Moreover, it must be borne in mind that the polarization effects and metallic lustreI8and the brilliant colorsgexhibited by selectively reflecting substances are very similar to those of thin films. Suffert,19is an admirably scientific study, has assembled the properties of the various types of color possible in iridescent butterfly scales, and concludes very positively against the “surface color” theory. If it were only supported by Kalter, it would probably have ceased to trouble people; but Michelson’s paper“ gave it a new lease of life. He starts out with the statement that interference colors are ‘‘so rare and so readily distinguished from the true metallic [surface] colors that they may be most conveniently treated as exceptions after the subject of surface color has been considered,” yet the chief characteristics by which metallic reflesion may be distinguished are as follows: “I-The brightness of the reflected light is always a large fraction of the incident light, varying from 50 percent, to nearly I O O percent. “2-The absorption is so intense that metal films are quite opaque even when their thickness is less than a thousandth of a millimeter. “3-If the absorption varies with color, that color which is most copiously transmitted will be part of the incident white light which is least reflectedso that the transmitted light is complementary t o the reflected. “4-The change of color of the reflected light , , , follows the invariable rule that the color nlways approaches the violet end of the spectrurn as the incidence increases.” As Onslow has shown by quantitative measurements, and as,anyone may observe for himself in a qualitative way, thin films, especially multiple films satisfy the first criterion. hlichelson has admitted that the natural iridescent
’ loc. czt.
* Mallock and others J U r e c h : 2. wiss. 2001..57, 306 (1894); Bayliss: Entomologist, 57, j z (1924); Coste: 23, 24 (1890-91); and others. Biedermann, Bayliss, and others. 6 Biederrnann: Eoc. ci:. Rayleigh: Phil. Mag., (6) 37, 98 (1919:. Coste, Rayleigh. Rayleigh. 8 Mason: loc. cit. p. 437. 9 Onslow: loc. cit. p. 27. ’Oloc. cit. I’ loc. cif.
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colors “are far more vivid than any of the reflexion hues of aniline dyes, or any other case of ‘surface color’ yet discovered.” Hayleigh has taken issue with him on the second count, pointing out that the transmission colors of the insect scales are very much less saturated than those of dyestuffs, while the reverse is true of the respective reflection colors. I n the single instance where Michelson tried to penetrate an insect scale by liquid he notes that t,he iridescence was completely destroyed if the index of refraction \vas between 1.5 and 1.6, and admits that selective reflection is ruled out in this particular case. As many investigators have noted, this loss of color on penetration appears to be without exception in t h e case of the iridescent insect scales. Hayleigh also points out that the third criterion is only roughly correct, and it is well known that more than one type of structural color shows complementary reflection and transmission. He also emphasizes that “with ordinary unpolarized light the surface colors appear to change too little” with changing incidence, and concludes that “hlichelson’s four tests are as well if not better borne by an interference theory.” The present Lord Rayleigh’ and Mason2 have pointed out exceptions to the statement regarding the invariable change toward violet with increasing incidence. Michelson’s quantitative study of the “phase differences” and “amplitude ratios” of the elliptically polarized light reflected from dyest,uffsand from “metallic” insects and feathers shows fair agreement. But the surfaces of the latter specimens are highly complex, as he admits, and the measurements have not been made on any types of struct,ural color for comparison, though thin films are known t o polarize light elliptically, and to behave similarly to surface colors with polarized light,3 and since his “metallic” feathers turned out to have thin-film colors, this phase of the evidence can not be taken too seriously. The present Lord Kayleigh has beer, able, by means of spectrometric examinations, to obtain data showing that some of the liectles which were llichelson’s best examples of surface color exhibit optical behavior corresponding to that of tnultiple thin films. lIerritt4 has made some more recent spectrophotometric studies on feathers and hitterfly scales which show good agreement with the properties of thin films. ICeeley5 came to a similar conclusion as a result of his work with the microspectroscope. As far as qualitative tests go the evidence is certainly against the “selective reflection” theory, and the odds appear to be more than even on the quantitative side as well. The various other theories which have been advanced to explain the iridcscence of insect scales are of minor importance. Resonance has been suggested by Wood.6 Kossonogoff’ assumes that all the colors of Lepidoptera are due Proc. Roy. Soc., 103A, 233 (1923). loc. cit. RIason: loc. til. 4 J. Opt. Soc. America, 11, 93 (1925). 6 Proc. Phil. Aced. S a t . Sci., 63, 1 1 2 (1911). Phil. &lag., (6) 38, 98 (1919). ’Pliysik. Z., 4. 2 0 8 , 258 (1903). 2
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to the resonating properties of fine pigment granules in their scales and Chirvinski’ elaborates this theory. Such an all-inclusive explanation leaves us no better off than before, while resonance of the sort Wood observed with his granular metal films is subject to the same objections as selective reflection, of which it is a special case.
Studies of Typical Iridescent Scales I t will be seen from the studies of typical specimens which follow that, their properties are, as indicated above, markedly at variance with those of diffraction or selective reflection colors: Positive evidence in favor of another explanation will be presented later. The specimens were first examined with the naked eye, both with diffuse and unidirectional light a t varying angles of incidence and falling crosswise and lengthwise of the scales. The inclination, curvature, and arrangement of the scales on the insect were observed with a Greenough-type binocular microscope to which was attached an orienting device so that a variety of relative positions of the scales, wing. illumination and observer were possible. Scales mere removed from the specimen by scraping or by picking up on a cover glass slightly sticky with a film of balsam or glucose. This afforded a n easy method of observing several scales a t one time, and of studying their under surfaces. The reflecting properties of the scales were observed under inclined illumination, and also by means of a vertical illuminator fitted with a movable reflector so that the incidence of the illuminating beam coilld be varied within the angular cone of the objective. An iris diaphragm stop in the objective enabled its aperture to be reduced so that the light was almost wholly vertical rather than convergent. Reflection, transmission, and polarization a t various orientations could be observed systematically by means of a Fuess petrographic microscope with theodolite stage. For detailed study of the fine structures of the scales a number of different objectives, including those of 1.40 N.A. were available, and it was found that a fairly adequate idea of the cross-section and the interior of the scales could be formed by careful focussing of a lens of the appropriate aperture (“optical sectioning”) and by proper manipulation of the condensor t o furnish axial or oblique illumination. The interpretation of appearances by such a procedure was substantially in agreement with that obtained from study of the sections, in cases where these were available for comparison. In this phase of the investigation Onslow’s observations can hardly. be surpassed ; his paper gives excellent figures of sections of a large number of scales, but, as Siiffert says, a clear understanding of the general structure of the scale should precede a study of its cross-section. The latter is useful mainly for relat,ively large details and as a confirmation; the minute structures may often be revealed quite as usefully in torn or crumpled scales. Fracture. behavior under compression, and the process of slow permeation by viscous liquids are of great value in forming a concept of the object which is under examination. Russ. Ent. Rev., 15, 513 (191j).
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Since the structures concerned in the production of color are practically a t the limit of resolution, even under favorable conditions, it is obvious that optical tests are likely to be more significant in establishing the nature of the observed phenomena. The conclusions drawn from such tests should of course be compatible with any structural conditions known to be present. The behavior of iridescent scales in response t o swelling agents is noteworthy. and is best brought out by exposure to phenol vapor,' since this does, not wet the scale, and is readily removed by exposure to the air. Ammonia water, alcohol, and many other liquids hare considerable swelling action also. Aiseries of practically colorless liquids, varying in indes of refraction from 1.33 to 1.8, enables the scale to be surrounded or permeated by inedia of a wide range of refractivity. Liquids of different degrees of volatility (mater, alcohol, benzene. xylene, etc) are useful in the study of the surface and internal structure of the scale as these are esposed by the evaporation of the liquid in which it has been immersed. Bleaching, where necessary t o eliniinate the effect of pigment or to establish its absence, is best carried out with concentrated hydrogen peroxide ( 1 0 - 3 0 5 ).
Urania and Similar Scales C*mnia (Chrysl'ridia) ripheus is perhaps the most striking of all the iridescent scale-bearing insects, both because of its brilliancy and its variegated hues. The fore wings are blue green, changing t o violet a t grazing incidence, and the hind wings are a bronze green shading into reddish purple. The under surface of the hind wings presents a number of hues. At their roots they are brilliant bluish green, and vary in a distance of j mm. through blue, purple: and red to orange. The main area of the under side of the hind wing is greenish brass-yellow, in the center of this space the color varies through orange to red. The above colors are described as seen a t normal incidence. Their distribution most closely resembles the zones of color seen when oil is spilled on wet asphalt pavement. Strztcticrnl Features. The iridescent scales are square-ended, strongly conves about their transverse asis at the outer estremity, so that on the wing the surfaces esposed are highly curved. Detailed study of scales from the different colored areas reveals no observable structural difference. Separate scales are longitudinally striated on their upper surface, the striae being really thin vanes. separated by spaces four or five times their thickness. the under surface is practically smooth. Transmitted light reveals, in addition to the longitudinal striations. a faint irregular tracing which appears to be inside t,he scale, and is probably the residue of the scale contents. Broken or crumpled scales permit the vaned structure to be recognized as part of the upper lamina of the scale, the lower lamina appears thicker (ca. 2 p ) and in close contact with the upper one. Edges of torn fragments indicate a cleavage in the plane of the scale. The individual lainellae are not sharply defined, but the broken Wilson [Psyche. S o . 75 ir880)l noted that an insect packed in a boxwithphenolon cotton, and sent to be painted had its color changed from rich purple to green; fortunately for the picture a short exposure to the air was sufficient, to restor? the original hue.
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edge resembles a torn group of sheets of paper, and buckling of the scale may cause them to separate somewhat. This lamellar structure is undoubtedly existent, though composed of films too thin to be visible even in Onslow’s excellent sections. There are probably between five and ten lamellae present. Accurate observation of their individual thickness and that of t,he space between them is excluded by the limitations of resolving power of the microscope and the technique of sectioning. Optical Features. As they lie on the wing, the scales show high reflecting power, through a considerable angle, but examination under the Greenough binocular microscope shows that the reflection is strictly specular, and the convex surface of the scale permits some part to be in position to reflect through considerable variation in the inclination of the wing. The reflecting power is high, and the lust,reis metallic, apparently nearly as bright as polished copper in the case of the red scales. The reflection comes from spaces between the vanes. Lustre and specularity of reflection are even more marked on the smooth under surface of the scale. Torn or split scales show that the reflections come from the lower lamina, and the very thin upper lamina with its vanes acts only as a screen thru which these reflections are observed. Under fairly diffuse illumination the reflection color of the scale is not uniform, but varies markedly from center to tip. This is due t o the different inclination of these different portions of its surface, for when the scale is flattened, or when the angle of incidence is actually the same, the color is correspondingly uniform. The uniformity is not perfect, for slight local variations are apparent. These blend into the adjacent colors, and in no case are there marked differences in hue. The reflection colors are highly saturated and are localized in the lower lamina. The transmission color is much less saturated and is tinged with the faint yellowish pigmentation of the scale. The hue varies from center to tip just as does the reflection color, and this variation is eliminated by flattening the scale or by tilting it so that the different portions of its surface are each successively observed with axial light. The transmission colors are complementary to the reflection colors, as shown by combined reflected and transmitted illumination. As the angle of incidence is increased the hues of the reflection colors change very strikingly, this is much more marked in the individual scale than on the wing because of the multiple convex character of the latter, which permits a wide range in the angle of incidence and a consequent lessening of the purity of the colors and of the sharpness of their changes. The sequences from normal to grazing incidence are as follows: green blue -+ purple, blue 4purple -+ red + orange, reddish purple -+ orange -+ yellow grecn. -+
Some scales, violet at normal incidence, go through reddish purple. copper red and orange yellow to greenish yellow, with increasing angle of incidence. The above changes in the reflection colors are accompanied by corresponding
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changes in their complementary transmission colors. On the wing the joint effect of these color changes results in a shifting of the color zones. The bands of blue, purple, red, and orange move to the adjacent portion of the wing, while maintaining the same zonal distribution. This shift of location is toward the portion of the wing blue a t normal incidence, and is due to each of the colors changing to ones following it in the above series. When the change in hue of the reflection color is analyzed by a nicol prism, one notes that if the plane of vibration of the nicol is perpendicular to the plane of incidence of the light the color phenomena are practically the same as without it. If the plane of vibration of the nicol is parallel to the plane of incidence the reflection is practically destroyed a t an angle of about 60” to the normal, and, at greater incidence than this, the color is complementary to that seen with the nicol in the perpendicular position or with it removed completely. If a “% undulation” mica plate is inserted, the plane of the nicol being parallel t o the plane of incidence, the color undergoes a complementary change. Between crossed nicols the scales are seen to be anisotropic, with extinction parallel to their longitudinal striae. This is purely structural anisotropy, for it is dest,royed when the scale is rendered optically homogeneous by permeation by a liquid of the same refractive index as chitin. Any reflecting or refracting surface tends to depolarize light; multiple surfaces, such as t h e grooves of a deeply ruled diffraction grating exhibit this effect and the parallel ridges of the scale undoubtedly act in a similar manner.’ The anisotropy is not sufficient to interfere with the observation of the elliptical polarization of the reflected light described above, for this may be observed with the scale in any azimuth, and depends only on the angle of incidence. The distribution of colors on t’he individual scale, as mentioned above, is not perfectly uniform; a given hue such as red may show variations toward adjacent colors, such as purple or orange. These variations are gradual, though they may take place in a distance of only a few microns. The complementary color shows similar variations, though these are much less marked because of its low saturation. Pressure applied t o a scale causes a very marked change in its hue. This may be carried out under objectives of fairly short, working distance by wedging a speaf-pointed dissecting needle between the mount. of the objective and the cover glass. Transmitted or reflected light may be used for observation. Very slight pressure is sufficient to alter the color to an adjacent one, and increased pressure brings about a succession of colors similar to the sequence seen with increasing angle of incidence. Extreme compression destroys all but the faint yellow pigment color. Unless the scale is actually disrupted the original color is restored on removal of the pressure, particularly if the atmosphere is moist. Under the microscope the effect of pressure may be seen summarized on a single scale which has been bruised locally by a dissecting needle or cornAmbronn [(Kolloid-Z., 6 , z z z ( ~ g r o )has ] termed this phenomenon “rod double-re fraction.”
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pressed only a t one end. The whole series of reflection and the transmission colors is present in a space of a few microns. Yellow-green scales are altered through green and blue to purple by reflected light (more severe pressure disrupts the scale and destroys the color); the transmission color goes through purple and red to orange, and is finally lost in the yellow of the pigment. Redpurple scales go through red and orange to yellow, or yellow green, the complementary transmission color, bluish-green, is lost with moderate pressure and only the yellow of the pigment is seen. Swelling the scales is carried out by exposure t o phenol vapor, ammonia or even water vapor. Here again the color changes very strikingly and in a sequence the reverse of that caused by compression. The original color is rest,ored on exposure to the air. Penetration by liquids affords the most convincing proof of the structural nature of the colors of the scales, for if the liquid is of index of refraction substantially the same as that of the scale (as determined by the Becke test)’ the reflection color is completely destroyed, and only the transmission color of the pale yellow pigment remains. The original color is perfectly restored on removal of the penetrating liquid. If the index of refraction of the chitin is not perfectly matched, the loss of color is not complete, though with liquids of n between 1.5 and 1.6 it is very nearly so. With liquids of index of refraction widely different from that of chitin, the intensity of the reflection and transmission colors is decreased, and in addit,ion t,heir hues are altered. This alteration results in green scales becoming red, and may be due to the swelling action of the liquid, (which would cause a change of this nature) or to the replacement of the air in the voids of the scale by a medium of higher refractive index. Certainly the former plays a considerable part in this color change for one notes under the microscope that the saturation of the color may be restored by replacement of the liquid by air, but the original hue is regained only after a few seconds further drying to remove the swelling liquid in the scale tissues themselves. The progress of the penetrating liquid may be followed under the microscope, if liquids of high viscosity (balsam or glucose) are used, and it is seen that the liquid enters most readily at breaks in the scale, spreading through it independently of the striations or other markings. If only the outside of the scale is wetted no loss in color results. \Then the permeating liquid is alloir.ed to dry out slowly the vaned surface is first exposed, and at this stage the scale is still colorless. On further drying, flashes of color appear over areas in various parts of the scale, and these colors apparently become more saturated with succeeding overlapping flashes until finally the original saturation of color is restored. This spreading of the color throughout, the scale is independent of its striae, and is similar in appearance from either surface. The optical properties of other Uranidae are closely similar to those of ripheus, with minor differences as regards the hues present. Bronze-green,
r.
Johannsen: ‘‘Manual of Petrographic Methods,”
271,
(1918).
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changing to blue or purple a t grazing incidence and to copper red on swelling, is their most prevalent color. The changes on compression are very striking, on a single scale one may see absence of color where the pressure has been most severe, and transmission colors of yellow, orange, red, purple and blue in order outward from this point, with a corresponding complementary series of reflection colors. The parallelism between the variation of both reflection and transmission colors with pressure and with increasing incidence is highly perfect.’ Ino statices has iridescent scales of the same general type as those of the Uranidae, but differing from them in certain structural features. Their bronze-green color is localized in the upper lamina of the scales, and shows specular reflections, change with pressure, swelling, or varying incidence. However, the longitudinal corrugations of the upper lamina, instead of being fairly smooth ridges, are studded with rows of raised bosses. The sides as well as the tops of these reflect specularly and evidently this is simply a special case of a very highly “embossed” upper lamina, similar in other respects to that of the scales just described. PapiEio peranthus shows the converse of the above structure, with longitudinal ridges, unevenly spaced cross ridges connecting these, and depressions in the space between these raised structures. Suffert aptly compares the surface to that of a waffle. The reflection is specular from all parts of this embossed surface, just as in Ino statices. The under surface of the scales shows the reverse of these depressions as elevations, though the reflection is hardly as brilliant. Many iridescent butterflies exhibit optical properties very similar to those of the Uranidae. The scales of Papilio philenor contain a dark pigment localized in their upper lamina, which is relatively thick, with shallow longitudinal ribbing. They show the color changes with inclination, pressure, swelling and penetration as do the scales of the Uranidae, but the pigmentation suppresses the clearness of these phenomena somewhat, particularly on the upper surface of the wing. The lower lamina is iridescent, independent of the upper one. Because of its less pigmentation and unstriated surface it is rather more brilliant. A lamellar structure is indicated by the character of torn and crumpled edges, and the lamellae of the upper lamina follow the ridges of its surface. Papilio asterias exhibits much less perfect color producing scales, with hardly perceptible iridescence or metallic lustre. I t is more heavily pigmented, and the striated and cross-marked upper lamina constitutes a screen which practically conceals the iridescence of the lower 1amina.l Papilio neophilus olioencius has a similar mesh structure on the upper surface of the scales, which gives them a matte lustre. The seat of the
’ The foregoing observations are very similar to those made by Siiffert, who considers the scales of I’ranidae as a tvpe example. The colored plates of his paper represent the above variations in hue very vividly. ? Such a structure is similar to that found in many scales not thought of as iridcscent, in % hich the relatively opaque and heavily sculptured upper lamina conceals the lightly pigmented, highly iridescent lower lamina. The under side of the black scales of I.rania ripheus shows brilliant iridescence. Indeed, iridescent lower laminae are very coInmon in the scales of moths and butterflies, whether the scales are iridescent or not as seen on the insect.
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iridescence is beneath this layer, and is very brilliant from the under side of the scale. The scales of P. nireus and P. aalmoxis are essentially similar. Callimorpha dominula is much the same as the preceding specimens, having a heavily pigmented, non-iridescent upper lamina, which masks most of t,he iridescence of the thin, unpigmented lower membrane. Lycaena icarus, a common European butterfly, is a bright lavender; the wing appears light gray at grazing incidence. The inclination of the scales on the wing is partly the cause of this, but another factor is more important, The almost complete loss of iridescent color at large angles of incidence is due chiefly to the character of the upper lamella of the scales. This is longitudinally striated, the striae being connected at short intervals by a fine crossstructure, making a mesh pattern which is relatively thick. The openings of this mesh are large enough so that the iridescence of the lower lamella is visible through them a t small angles of incidence, but as the angle approaches grazing, the iridescence is hidden behind the edges of this boldly sculptured structure. As far as can be observed under these circumstances, the optical properties of the scales parallel those of the specimens already described. Pressure causes a marked alteration of the color of the scales, but since.they are very thin, Xewton’s rings may be seen between the slide and the cover glass in their vicinity, and these colors are likely to be confusing. Very sudden release of the pressure destroys the Newton’s rings before the scale has time to recover its original hue. The lavender color is completely destroyed on penetrating with a liquid of n = I .5 - I .6 and, on drying out, an interesting sequence of changes is noted. When the outer surface of the scale emerges from the liquid, it’s raised striations produce a blue transmission color, as seen wit,h an objective and condensor of the proper aperture.’ Then the liquid in the underlying tissues is gradually replaced by air, and the yellow transmission and lavender reflection colors are restored. The scales of Lycaenn corydon are substantially the same as those of L. icarus, but more closely striated and less lustrous. Basilarchia astyanax has scales similar in structure to those of the Lycaenae, with a “mesh” upper structure and an iridescent lower lamina, the colors of which are ordinarily seen t,hrough the openings of the mesh. Cercyonis alope is similar, but much more pigmented, with iridescence visible only under brilliant illumination. The genus Anaea contains some good examples of iridescence similar to that of the Grania moths. A-morvus is particularly noteworthy for the great change which its colors undergo on exposure to phenol vapor. Light greenish blue at first, it changes through bronze green, orange and copper red, to purple with maximum swelling, and passes through this series in the reverse direction in a few minutes when exposed t o the air. The same reversed sequence is noted as the angle of incidence goes toward grazing, though this is not so easily observable on account of the inclination of the scales on the wing, and because the upper lamina possesses a “mesh” structure similar to that of Lycaenae. See page 327, footnote 5 .
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Helicopis cupido and A r g y n n i s (“Silverspot”) are examples of another sort of scale color, of the same type as that of Uranidae. Their scales are pigmentless, and give a general effect of whit’e on the wing, with bright silvery lustre microscopically. Rather pale iridescent colorings are observable by reflected light, and less saturated, complementary ones by transmitted light. The coloring is not uniform, and the various hues blend into each other giving an appearance much like mother-of-pearl. The behavior with pressure or when penetrated by liquids is closely similar to that shown by the insects described in the preceding studies. The specularity of reflection is another point of resemblance. Cocytia durvilli has iridescent body and leg scales which are of the same general type as the others just described. Their elongated character adds to the satiny lustre as seen on the insect. Of more primitive forms, Eriocrania (purpurella?) shows scales having a very simple struct,ure, consisting apparently of a simple basal membrane bearing longitudinal ridges or vanes. This behaves as if it were a single thin film with no interior air space. The reflection color (purplish to orange) is relatively faint, and is lost if either the upper or lower surface of the scale is in contact with liquid. The foregoing specimens have all been essentially similar as regards the color-producing portion of the scale, and their differences lie chiefly in the various means by which the action of this structure is modified by other portions of the scale. The seat of the iridescence, in the above cases, may be in either lamina of the scale; the upper lamina may vary from a simple vaned film overlying the reflecting structure to a pigmented mesh structure in high relief. The reflecting plane is parallel to the plane of the scale. The scales of Ornithoptera are somewhat different from those just described, but may well be classified with them; 0. poseidon is a typical example.‘ Its wings exhibit only moderate color change with angle varying from somewhat yellowish green to blue-green. At grazing incidence an orange-red sheen is visible in certain azimuths. Structural Features. The scales theniselves are nearly oval, highly convex, and over-lap on the wing very closely. They are uniform in thickness, and much less flexible and fragile than the scales previously studied. N o coarse striation or mesh structure is present, but with high powers very fine and regular striations (about I 1.1between centers) are visible on the upper surface of the scale. By transmitted light the structure appears to be simple, a plate of slightly turbid chitin with a finely striated upper surface; by reflected light it is seen that the body of the scale is not optically homogeneous but shows a fine stippling of reflection color. Thiq is independent of the striations, and is visible from either surface of the scale. It is distinctly beneath the striated surface. Individual bright points of this stippled coloring appear to show something approaching specular reflection. Dr. Forbes points out that it is rather surprising that Papilio philenor, which is really one of the O m i l h o p l e r a , should have scales so different from the other members of this genus.
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Fractured or distorted scales show little tendency to cleavage along the striations, and these cannot be isolated or flattened sidewise. There is little indication of a cleavage in the plane of the scale, but the edges of the fractures show some evidence of a lamellar structure. Cross sections show that the striae are closely packed, between I and z p high, and might equally well be thought of as a system of fine parallel grooves in the upper surface of the scale. The body of the scale appears rather more turbid than the upper or lower portions, but the differentiation between the cuticle and the interior is not very pronounced under the microscope. Oblique, broken, or distorted sections show distinct evidence of a laminated structure which apparently extends throughout the interior of the scale, parallel to the surface. This portion is not visible if the section is penetrated by a liquid, but on evaporation and entry of air, its opacity increases. Optical Features. The most distinctive characteristic of the scales of Ornithoptera is the absence of true specular reflection. The scales show “high lights,” but these are not sharply defined. and there is obviously a certain amount of diffuse reflection. Because of this, their lustre is hardly metallic, and their coloring less bright than that of the I’rania scales. On the other hand, the color is visible over a wider angle. The reflection from either surface of the scale is essentially the same, for the striae are hardly visible except by transmitted light. On focusing up and down through the scale with an objective of very low “penetrating power” the reflecting surface cannot be definitely located; instead. reflection appears to take place from any level. This is borne out by study of the under side of the scale. The reflection coloring of the scales is matte rather than metallic; yellowish green predominates, and the scales are remarkably uniform in hue. They are strongly colored by a non-granular yellow pigment, and this undoubtedly modifies both the reflection and the transmission colors. By transmitted light the scales are orange pink. The orange-pink transmission color is what is seen at grazing incidence when the scales are on the wing. This may be studied under the binocular microscope and the various reflections and transmissions of the light by the overlapping scales may be clearly seen. The convexity of the scales is a considerahle factor in the “reflection” of their transmission color, for light may be totally reflected a t the lower surface of the scale, and travel a considerable distance in it before emerging. It is also noted that the orange sheen is greatest when light falls across the striations of the scales as the lie parallel on the wing. Here the yellow striae, which project above the portion of the scale where the green color originates, present an ideal finely divided transparent medium to scatter and to color the light which falls across them. Moreover, the curvature of the surface is greater crosswise. S o orange sheen is observed on the under surface of scales stripped from the wing as a group. Their reflection color is green (instead of yellowish green), changing to violet a t grazing incidence. If the scales on the wing are surrounded (but not penetrated) by a liquid their surface reflection is minimized, and the color from the interior shows up clearly at any angle, with none of the orange sheen.
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The color of the scale varies with the angle of incidence, the reflected yellowish green changing toward blue and the transmitted orange-pink toward yellow. Polarization effects cannot be observed distinctly on account of the scattering of the reflected light. Swelling reagents cause less alteration in the color than in the case of the L’ranidae. The green becomes more yellowish, and the transmission color becomes more pink. These changes are reversed on exposure to the air. Pressure brings about a very distinct change in t’he reflect,ion color, from green through blue to reddish purple, before it is finally destroyed. The transmission color loses its pink and finally only the yellow of the pigment remains. Removal of the pressure with exposure to a moist atmosphere is sufficient to restore the original hue. Pressure on swelled scales first restores their original color and then alters it as indicated above. Penetration by liquids is not so rapid as in C’rania scales; the scale may lie completely immersed in liquid for some minutes before it is affected, though broken scales are more rapidly permeated. With a liquid of index of refraction near that of chitin (1.5 - 1.6) the iridescent color is perfectly destroyed and only the yellow pigment color remains. Retnoval of the liquid restores the original coloring. (Most, liquids have a certain amount of swelling action, judging from the “lag” between the restoration of color and lustre, and the recovery of the original hue.) The progress of penetration by liquids is similar t o that in Cranidae, and is independent of the striae. The loss of color is gradual and more or less by stages, a given spot on the scale appearing less and less green until all it’s reflection color is lost,. On evaporation of the liquid, irregular patches of faint color first appear, and, as these extend throughout the scale, successive additional patches of color appear, developing over the areas already colored; finally the original saturation of hue and intensity of lustre is completely restored. Morpho and Similar Scales In a number of respects the iridescent scales of the well known family of Morphos are different from t,hose of the insects just described. X brilliant, highly met’allic blue is the prevailing color, and this is present only on the upper surfaces of the wings. The variation in hue with change in angle of incidence is easily observed, reddish purple being the extreme color noted. The reflection colors are seen best looking from the base toward the tip of the wing (“with the grain” of the scales). This is only partly due to the inclination of the scales on the wing. Structural Features. The scales of the Morphos are practically plane, and lie close to the wing, like shingles on a roof. M . menelaus has a double layer of iridescent scales. The outer ones are very thin, long, and narrow, shaped like the blade of a paddle, with longitudinal striae rather far apart (ca. I . j-zp) and standing up like vanes on the upper lamella of the scale. Broken or crushed scales show these vanes flattened or torn loose from the scale proper, while scales folded crosswise permit their elevation to be seen; they are from z t o 3 p high. The vanes usually break cleanly, but may give evidence of a
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longitudinal “cleavage” slanting very gradually from the upper edge to the base. There is no indication of a hollow interior in the scale. Optical Features. On account of their great transparency the outer scales are best studied after being removed from the wing. On examining them by reflected light with a 4 mm. objective, it is evident that the vanes are the seat of the color, and that the laminae of the scale do not contribute to its iridescence. The reflection consequently is not specular from the surface of the scale as a whole, but is so from the upper edges of the vanes. Torn or split’scales, in which the vanes are loosened or removed from the lamina of the scale afford convincing proof of this; a single, isolated vane shows just as brilliant reflection and coior as when in its normal position on the scale. However, with axial illumination and an objective of narrow aperture, no reflection is noted even though the plane of the scale is perpendicular t o the illuminating beam. It is only when markedly oblique illumination is supplied that light is reflected up along the axis of the microscope. Evidently the reJEecting structure i s not parallel to the plane ojthe scale. This may be confirmed by tilting the scale until it reflects along the axis of the microscope with vertical illumination, or by rotating the stage in the plane of the scale after the illumination has been adjusted to such an obliquity as to be reflected along the axis of the microscope. I n the latter case the vanes appear bright only once in a revolution; this behavior corresponds to the “oriented lustre” of etched met,al specimens, and of laboradorite, cats-eye and similar minerals, all of which possess systematically arranged reflecting surfaces oblique to their own general surface. The plane of reflect,ion by the above tests is found to “dip” toward the root of the scale, making an angle of between 10’ and zoo with its laminae and with the edges of the vanes. Since there is practically no reflection from vanes flattened in the plane of the scale, it appears that the tilting of the reflecting structure is only lengthwise of the scale, and that it passes through ths vanes rather than following up one side, over the upper edge, and down the other side. The ‘‘dip’’ of t,he reflecting plane in the vanes is the equivalent of tilting the scale on the wing, so that although the scales of the Morphos lie nearly flat, the reflection from the wing surface is that corresponding to markedly inclined scales. The intensity of the reflection is very marked, the narrow edge-views of the vanes appearing as highly luminous blue lines. The iridescent color is obviously of structural origin and connected with the peculiar reflecting properties of the individual vanes, since it shows its full intensity on an isolated vane or on vanes which have been t’wisted laterally, and does not depend on their arrangement or proximity to other vanes on the scale. The color changes with angle from greenish blue for illumination normal to the reflecting plane to purplish red at grazing. If the color change is referred to this plane, it is consistent for all azimuths, but if referred to the plane of the scale the behavior is not so simply described.
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As seen by reflected light, the edges of the vanes are not clear lines of blue, but each one is crossed, a t intervals of 1-2 p or more, by fine dark markings, which give them a somewhat “beaded” appearance. By transmitted light the scales are seen to be unpigmented, and only faintly yellow along the color-producing vanes.’ Polarization effects comparable to those shown by rrania and other similar insects are not to be observed accurately on account of the complexity of the surface of the scales, and the strong possibility of interference by polarization from its deeply ribbed structure. Between crossed nicol prisms the scales are seen to be anisotropic with parallel extinction; this is purely a structural phenomenon, since it is destroyed by permeation of the scale x i t h liquid of the proper refractive index. The anisotropy is localized in the vanes, a single isolated one showing it fully. When t,he vanes are flattened by pressure or distortion of the scale they are only faintly anisotropic. The extinction shown by the vanes seen flatwise is not sharp, on account of the weakness of their polarization color, but is apparently parallel or of small obliquity. With one nicol the scales may show an apparent pleochroism, blue for vibrations parallel to their vanes, and greenish for transverse vibrations. This is related to the diffraction color described on page 3 2 7 (footnote 5). The blue is at a maximum for vibrations parallel to the diffracting structure; diffraction is at a minimum for vibrations perpendicular to this direction, and the blue, lessened in intensity and combined with the pale yellow transmission color, appears greenish. The reflection colors of the scales are less variegated than are those of the insects studied previously, though tints of green and purple may be seen mingled with the blue; a single vane may show slight non-uniformity of hue in different parts. Pressure may flatten the vanes sidewise, dest,ro>-ingt,heir reflection, but if this does not occur the hue is seen to be altered through dark blue and violet to reddish purple. It may be restored by exposure to a moist atmosphere. Swelling the scales with phenol vapor causes the hue to change to yellowish green (going to violet at grazing). The original coloring is restored on exposure to the air. Penetration by liquids of index of refraction close to that of chitin (I. j-I. 6 ) results in complete loss of color. The original color is perfectly restored on removal of the penetrating liquid. Liquids of refractive index markedly lower than that of chitin cut down the reflection color somewhat, and alter its hue to yellowish green, which is due partly to swelling, and perhaps also to the replacement of air in t,he scale by a medium of higher refractive index. Wetting of the under lamina of the scale by the liquid does not alter the color, this occurs only when the liquid penetrates between the vanes and wets them thoroughly. On evaporation of the permeating fluid the color only reappears 1 Onslow was disturbed by the fact that the scales may appear blue both by reflected a n d by transmitted light, but the k t t e r is true only with objectives and illumination of the proper relative aperture, and is due to the causes indicated above (page 327, footnote j).
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after the vanes have been exposed, there is frequently an appreciable time interval after their exposure before the color develops in each vane separately. The under scales of the upper surface of the wings of M . menelaus are superficially different from the transparent, outer scales, but are similar in all important features. Structural Features. The scales are flat and broad with blunt, uneven ends, and are very closely striated longitudinally. These striae are really fine vares, as in the outer scales, so closely spaced as to be practically in contact, with no space visible between them. They are less than I p between centers! though not perfectly even or parallel, and project about 2-3p above the upper lamella. They may extend slightly beyond the tip of the scale. Fractured scales show that the vanes and upper lamina constitute the greater portion of the scale. and that the lower lamina is a simple thin membrane. Fractures tend to follow the vanes, giving the scales a well defined longitudinal cleavage. On account of their close packing, bhe fracture of the individual vanes is not readily studied, nor are they easily flattened sidewise. Optical Features. The reflections from the scale originate in the individual vanes, and are seen only when these are in their normal position, perpendicular to the plane of the scale. A single isolated vane exhibits the full color and lustre. The reflecting plane is not parallel to the edge of the vane and in the plane of the scale, but is inclined to these, dipping toward the root of the scale at an angle of between 10' and zoo. With respect t o this plane, the reflection is specular enough so that it is observed through a narrow angle only for any given angle of incidence. In cont,rast to the outer scales, the under ones arc pigmented fairly heavily, with a dull brown pigment, which is dilute in the lower lamina and is mostly in fine granules along the lines of the vanes. Since the scales are dark from the under side, it is evident that t,he pigment is mainly beneath the vane. The under scales show color changes with varying angles of incidence precisely similar to those exhibited by the outer scales. Their polarization effects by reflected light are not well defined. Between crossed nicols they show anisotropy of structural origin, with parallel extinction, flattened vanes seem to show slightly oblique extinction. On account of the close proximity of the vanes on the scale, pressure can he very effectively employed without flattening them sidewise, and the sequence of color changes from the normal light greenish blue to purplish red is easily observable. Their behavior on swelling is the same as that of the outer scales. Penetration by liquids reveals the presence of a spongy interior, to which the thick, pigmented upper lamina with vanes is attached. Pressure on the empty scale forces air out of the interior and bubbles may be seen in the surrounding liquid, or if the scale is filled but not surrounded by liquid, this may he squeezed out as from a sponge. The scale possesses a considerable amount of elasticity. On drying out, the vanes are cleared of liquid first, and their color is restored. A t the same time, or in,inediately after, the spongy interior empties, and the opacity of the scale increases further. The restoration
STRUCTURAL COLORS IN INSECTS
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of the color on drying is along the vanes, as is its destruction 011 penetration. The condition of the interior of the scale or of the lower lamina does not govern the appearance of the color. The lower lamina has a slightly wavy surface, and is iridescent from either above or below. I t is similar to the iridescent lower laminae possessed by nearly all butterfly scales, and plays no part in the color of the scales on the insect. The scales of Morpho sulkowskyi ("Pearl Morpho") have practically the same optical properties and structural features as those of M . menelaus. The pigmented scales are a deep brownish orange by transmitted light but this is not all due to pigment, for penetration by a liquid of n = 1.55 destroys all transmission color but a weak grayish brown. Pressure also destroys much of the color. This dark orange transmission color is related to the blue reflect'ion color, for it develops simultaneously with it as the permeated scale dries out. The scales are distinctly pleochroic, from dark orange for vibrations crosswise of the vanes to lighter orange for vibrations parallel to them; this pleochroism is lost when the scale is penetrated by a liquid. A number of other butterflies have scales which are similar in their optical properties to those of the Morphos. A p a t u r a (chlorippe) seraphinu is a widely studied example, which shows the vaned scales and inclined reflection typical of the insects just described. In the violet scales near the outer edge of the wing, the plane of reflection is distinctly more tilted than in the Morphos, and makes an angle of about 30' with the plane of the scale. The vanes of the scale, flattened sidewise, show oblique extinction, at an angle between 2 0 and 30'. The scales in the brilliant greenish blue band across the center of the wing have less inclined reflection, and their vanes do not show markedly oblique extinction when seen flatwise. A p a t u r a ilia is similar in optical behavior to A . seraphinu, but its colors are not as brilliant, possibly because of greater pigmentation; they range from greenish blue, to reddish purple at grazing incidence. The tilting of the reflecting plane, and the localization of the iridescence in the vanes are unmistakeable. Ancyluris meliboeus is another insect having scales with iridescent and inclined reflection. It exhibits a very brilliant lustre, and colors similar to that of the Morphos, showing rather more variation in hue with increasing incidence, with pressure or with swelling. The reflecting plane makes an angle of I O " - Z O ~ with the plane of the scale. Callicore eluina has scales showing practically identical properties with those of Ancyluris meliboeus as regards their hue and its changes with pressure, swelling or varying angle of incidence. The Morphos and the specimens described after them constitute a second group with essentially similar optical and structural properties throughout. They are characterised by their vaned scales, the iridescence of which is localized in the indi>
