STRUCTURAL COLORS I N IKSECTS. I11 BY CLYDE Fv. MASON‘
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Iridescent Integuments Because of their smooth, relatively uninterrupted surfaces and brilliant reflections iridescent integuments are likely to appear distinctly metallic, and the main question has been whether their lustre and colorings were due to selective reflection (such BS is shown by colored metals and solid dyestuffs) or to thin films2 The highly metallic lustre which is so common among certain of Coleoptera is cited as evidence for the first view, while the very marked change of hue with increasing incidence supports the second explanation. The examination of integuments of beetles and other insects is essentially similar to that of iridescent scales, except for the difficulties introduced by the underlying tissue and the heavier pigmentation which is usually present. The surfaces are larger, reflection and polarization may be studied more easily, and ‘there is almost complete absence of striae or other structural features which might interfere with such observations. On the other hand, the colorproducing layer is not easily accessible from the under side or by means of sections, and it is practically impermeable to liquids, besides being so hard as to be relatively resistant to mechanical deformation. However, the criteria enumerated in the study of iridescent scales may be applied with slight modifications to the specimens described below.3 Two main types of iridescent integuments may be recognized : “inetallzc,” as shown by “blue-bottle”flies, and other less familiar insects; and “enameled,” best exhibited by certain of the C e t ~ n i d s . ~ “Metallic” Integuments I n most respects iridescence of the “metallic” type shows the same features on whatever insect it may occur; the characteristic lustre and highly specular reflection are very distinctive. This is dependent on the smoothness of the surface, which is commonly almost perfectly even in small areas under the microscope. To the naked eye grosser structures such as ridges, “dimples,” ~
’The investigation upon which this article is based was supported bv a grant to Messrs. Bancroft, Chamot and hlerritt from the Heckscher Foundation for (he Advancement of Research established by August Heckscher a t Cornell Vniversity. zBiedermann’s paper [Handbuch der verg. Physiol., 3, I, Part z , 1657 (19rq)] gives a very complete discussion of the earlier work along this line, with various arguments for and against the different opinions. %early all of the specimens studied in the present paper were furnished through the kindness of Dr. IT. T. M. Forbes and Xlr. F. C. Fletcher, of the Department of Entomology a t Cornell. The writer’s ignorance of entomology would have handicapped him seriously if the advice and criticism of these gentlemen had not been available. ‘Biedermann deals chiefly w-iththelatter type. lIichelson(Phi1. Mag. (6) 21, 564 (191I ) ) apparently studied only metallic integuments. Onslow (Phil.Trans. 211 B, I (1921))has considered both “metallic” and “enameled” iridescence.
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fissures, and processes, even though complete1.y covered with the color-producing layer, may prevent the high degree of reflection obtainable from a flat surface, with a resultant .dulling of the color. Under these circumstances, because there is some small portion of the surface in position to reflect speculady at any angle, the reflection may be observed through a wide range, and the lustre, instead of being highly metallic, is more matte.‘ Under the microscope a t low magnifications the “high-lights” of such an embossed or convoluted surface may be clearly seen, and the true specular character of the reflection recognized. Lytta cesicatorin (Meloiclae) is typical of a large variety of insects, as regards its structural and optical properties. The deeply convoluted surfaces of its elytra show a polygonal pattern which marks off the portions of t,he cuticle secreted by individual underlying cells. The color is localized in this cuticle, as shown by scraping; fragments of the color-producing layer may be removed by this means, and studied separate from the underlying deeply pigmented layer. Sections give some evidence of lamination in the cuticle, but this is not trustworthy on account of diffraction patterns and the possibility of spurious images. Although the coloring appears substantially uniform to the eye, this is not the case under the microscope, for the green reflection, which is the predominating color, shades into bronze green or blue green in a very small space. This is partly due t o the variation in shape of the surface, which results in different portions being observed at slightly different angles of incidence. The reflection color may be observed from the under side by scraping away the underlying tissues of the elytron. It presents the same appearance as from above. The color of different insects varies in a manner similar to the local variations on the elytron; specimens in the collection range from bronze to greenish blue. This same \,ariation may be seen with the naked eye on different parts of a single insect. By transmitted light no color other than the brown of the pigment is observable. Scrapings from the outer s u r f e e are somewhat pigmented, but show no transmission color, other than this pale neutral brown. Prolonged bleaching in strong hydrogen peroxide (10%) destroys the pigment without damaging the iridescent coloring; the elytra may be a very pale straw color by transmitted light with undiminished reflection color. The change of hue with variation in the angle of incidence is most striking when a small flat portion of the surface is observed under a microscope; the change is evident enough but is somewhat less spectacular if the whole insect is observed with the naked eye. From bronze green, or even yellowish green, a t normal incidence the color changes through green and blue to violet and may go as far as purple. If the insect is copper-red or bronze to the naked eye, it will ordinarily pass through the above series only as far as greenish >This is exactly analogous t o the behavior of metals, which darken as the surface is roughened, and which show a reflection scattered over a considerable angle when thus broken up into a great number of minute reflecting surfaces.
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blue, while if it is bluish green at normal incidence the purple at grazing has a very distinct reddish cast. The efective angle of incidence may be considerably increased by placing on the elytron a drop of water or other liquid which does not spread to form a film but stands up with a highly convex surface. This corresponds to the prism arrangement frequently used for viewing Newton’s rings,’ and prevents the critical angle of the specimen from limiting the angle at which light can penetrate its surface. The reflected light may be analyzed by a nicol prism, and shown to have properties similar to those exhibited by the scales of Urania, in that it is elliptically polarized, and, for angles greater than about 60°, the vibrations in the plane of incidence are of color complementary to those perpendicular to the plane of incidence, or to those of unpolarized light. Swelling reagents affect the color very markedly. Steam, ammonia, phenol vapor, water, alcohol and many other liquids all bring about the same sort of color change; of these water (because of its chemically neutral character) and phenol vapor (because no liquid is brought in contact with the specimen) are preferable. In the case of Lytta vesicatoria the action of the swelling agent is more rapid (water, 10-15minutes, phenol vapor, 2-3 hours) than in some of the heavily armored beetles. Blue preen changes through green and brass yellow to copper red, yellowish green changes through brass yellow and copper red to purplish copper. On removal of the swelling agent the color passes through the above series in the reverse direction until the original hue is regained. The colors of insects treated with swelling reagents change with increasing angle of incidence, to colors preceding them in the above series. Pressure is not easily applied in an effective manner, for the integument is highly convoluted and tends to flatten out without itself being much compressed. Any flattening means a change in the inclination of some parts of the surface and a consequent change of color which is not a pressure effect at all. This source of error may be avoided if a small flat fragment of integument is chosen for study and if attention is directed only to portions of this which are not suffering a change of curvature but only a change of thickness. R7ith these precautions, the color change observed may be ascribed to pressure, particularly since the recovery of the original hue is relatively slow when the pressure is slackened, and the points of greatest pressure show a distinct alteration of hue however they may be inclined. All this may be followed under the vertical illuminator. The colors change in the same manner as with increasing angle of incidence, passing through green and blue to violet. As the tissue recovers the colors change in the reverse sequence.* Immersion in liquids of various indices of refraction does not have as pronounced an effect as in the case of the iridescent scales. This is due pri‘Wood: “Physical Optics,” 167 (1911). *Onslowcriticizes Mallock’s statement as t o the effect of pressure on the color of iridescent integuments, on account of being,unable to duplicate his observations. The present writer has had no particular difficulty in demonstrating the color change under the microscope. It is possible t h a t Onslow’s method (pressure under the surface of a convex lens) did not permit microscopic observation, for the changes might be missed by the naked eye.
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manly to the fact that the integument is not readily penetrated and possesses no internal air spaces. However, there is a distinct loss in brilliancy when the liquid is brought into contact with the outer surface, and the color may change from swelling. The foregoing description of the optical properties of Lytta vesicatoria applies equally well to a number of other beetles. Cicindela sexgutta (Cicindelidae) has a very similar convoluted surface, with polygonal cuticular areas; color range and response to various tests are parallel. Other iridescent species of Cicindela behave similarly. Other varieties of Lytta vesicutoria resemble the above very closely in optical properties, but are of particular interest because they may illustrate a n exception to the rule that many writers have tried to establish, regarding the color change with increasing incidence. Instead of changing toward blue or violet (“toward the longer, or most refracted, wave-lengths,” as commonly stated) the deep blue-violet passes through reddish purple and copper red to brass yellow a t grazing. Similarly colored specimens of Calosoma udcoxia (Carabzdae) and Platynus cupripennis (Carabidae) show the same change toward red or orange, as do some color phases of Strongylium bicolor (Tenebrionidae). Beetles having integuments relatively free from convolutions or pitting are more slightly metallic than those just described, and show more striking color changes with changing incidence, but otherwise they exhibit the same optical properties. The Buprestids have been studied by a number of workers. Thzlocteanus rubroaureus is notable for its brilliant metallic lustre, closely resembling copper. This changes to blue-green at grazing incidence; certain other parts of the insect, yellow-green a t normal incidence, change to blue. On swelling, the copper becomes purple, changing to green at grazing. The effect of pressure and the polarizing properties are similar to the preceding specimens, as are the other optical features. The most interesting evidence is furnished by scrapings from the surface face of the integument. These may be obtained by means of a very sharp scalpel, the operation being carried out under the microscope with vertical illumination so that the depth of the removal may be observed. If the blade of the scalpel does not cut, but only slides over the surface a color change may be observed which is due to compression. Actual scraping of the surface removes thin shavings without going deep enough to lay bare the black pigment in the underlying tissue; the scraped area may appear a darker red, due to this black showing through. The shavings are only I - z p thick and may be thinner at their edges. There is doubtful evidence of a laminated structure. They show all the optical properties observable on the integument and are obviously the seat of its iridescence. These properties are obseivable equally well from either surface of the shaving. Pressure causes a striking alteration in hue, from copper-red through yellow-green to blue or violet, this is probably because there is no cushion of softer tissue beneath the color-producing
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layer as there is when on the i n s a t . The transmission color of the scrapings is a very pale, slightly greenish brown. Immersion in liquid appears t o cut down he lustre very little, Tetracha Carolina (Cicindelidae) is similar in optical properties to the beetles already described but the distribution of its coloring is noteworthy. The elytra range from purplish copper on their inner edges, through brass yellow to deep bluish green at the outer edges. With increasing incidence these color zones appear to shift toward the inner edges of the elytra. Platyn u s cupripennis exhibits similar color distribution and shifting with varying incidence. The abdominal segments of Calosoma scrutator and C. wilcoxii show similar properties. An even more striking specimen is Tetraphyllus (Tenebrionidae), which shows the color range in four spots on its back, the centers of these are purplish copper, and they vary through brass-yellow, green, and blue to violet a t their edges, giving patches of color like oil drops on water. Other species of Tetraphyllus show similar coloring. Phanaeus carnijex (Scarabaeidae) also shows the shifting of color in its prothorax. Species of Chrysochroa furnish many excellent examples of shifting of adjacent colors. The optical properties of foregoing insects have been discussed at some length because they illustrate certain typical featues; the following coleoptera are essentially similar in optical behavior to Lytta vesicatoria: Cicindelidae: Cicindela sexguttata, C. limbalis, Tetracha Carolina. Carab-idae: Calosoma scrutator, C. wilcoxii, Platynus cupripennis. Buprestidae: Buprestis striata, Castalia bimaculata, Chrysochroa fulgidissima. Scarabacidae: Dichelonyx elongata, Phanaeus carnifex. Tenebrionidae: Strongylliuni bicolor, S . violeceum, Tetraphyllus. Chrysomelidae: Chrysocleus auratus. A number of unidentified beetles studied also exhibit similar properties. Of Diptera studied, Dolichopus canaliculatus, Lucilia, and ot,her “blue bottle” flies also show typical “metallic” iridescence. I n the above specimens the color-producing layer is relatkely thin, but this is not necessarily always the case. Plusiotis gloriosa illustrates an exceptional structure, and is worthy of detailed description, for it serves as a link between “metallic” and “enamel” iridescence. The integument of this beetle is marked by exceedingly lustrous, pale yellowish-green longitudinal stripes, which appear almost as brightly metallic as burnished silver, particularly when seen against the matte yellow-green back ground of the rest of the surface of the elytra. The reflection from the silvery stripes is specular, and under moderate magnification it is seen that the whole integument exhibits strictly specular reflection also. Its microscopically matte portions consist of a finely “embossed” surface, all parts of which reflect strongly, but since only parts of the tiny bosses are in position to reflect with any given direction of observation and illumination, the rest of the structure appears dark. As a result the microscopic appearance is that of a darker shade and duller lustre than corresponds to the individual elements of the surface as seen under the microscope.
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Bot,h the silvery and matte green portions of the integument are very stiff and brittle, and are distinctly different from the insects described above in that the iridescence is not wholly a t the surface, but comes from a relatively thick layer of tissue. This may be demonstrated by highly oblique sectioning, or by careful scraping or “shaving” of the surface. A considerable thickness of material may be removed without destroying the color and metallic lustre, and the fragments removed by this treatment are themselves equally metallic and colored as seen from either surface. If the underlying tissues are removed, the under side of the color producing layer is quite as colored 8.nd lustrous as its outer surface. The color production appears to take place throughout the layer, rather than being localized in any given strdtum of it. Scraping reveals two other points of interest. The matte rortion remains matte, even when it is scraped to give a plane surface, and its “bosses” still present their original appearance, indicating that their reflecting surfaces are independent of the surface of the layer. When the scraping is carried so far as to remove most of the outer part of the color producing layer, the underlying portions of it present a golden to deep red color wirh undiminished metallic lustre. This indicates that t,here is a certain amount of pigmentation in the layer in addition to the pigmented tissue beneath it. Sections of the elytron confirm the above observations. There is an outer cuticle, 1-2 k thick and beneath it a transparent layer about 8-10 p in thickness. -1moderate amount of yellow to brownish red pigment is distributed diffusely through the under portion of the transparent lnyer. Beneath this lie the heavily pigmented layers and fibrous tissues which compose the elytron. The transparent layer is evidently the seat of the color production. The matte portions of the elytron show its embossed character distinctly, as rather evenly spaced elevations, and corresponding depressions on the under surface. Between crossed nicols these embossed regions of the layer do not extinguish as a unit, as does the smooth region, but on revolution each microscopically curved portion extinguishes in a position tangent to the planes of the nicols. This indicates a curved orientation of the tissue where the bosses rise up (like the folding of geological strata). On account of the hardness and brittleness of the color-producing layer, sectioning is difficult, but sections 1-2 p thick show no trace of any laminated structure. However, fractures oblique to the surface give indication of very fine laminations; these are evidently so intimately fused that they do not result in any tendency toward cleavage parallel to the surface of the layer. There is no indication of any porosity in the color-producing layer, and this is borne out by the total lack of change on prolonged soaking in penetrating liquids. The hue of Plusiostis glorjosa is microscopically uniform except where a “fault” (analogous to “fault bars” in feathers) occurs due to the localized imperfections of developnient. Such faults invariably show the same series of colors: on a normal yellow-green, blue to purple to orange to black at the
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center of the fault. Slight faults may show only the first colors of this series, They occur in the silvery stripes as well as in the matte green areas. The iridescence exhibited by Plusiotis gloriosa is modified by the embossed portions of the color-producing layer, so that on account of the number of minutely curved surfaces some are always in position to reflect light at any angle at which the general surface may be placed, yet this angle may be far from definitely observable, and strictly normal or grazing incidence may be almost unobtainable. As a consequence the color changes are relatively slight on the matte portions, though the general tendency is from yellow green r i bluish green. The silvery stripes are relatively unsaturated in hue, and their .metallic lustre predominates over their color SO markedly that little change i s observable here. The hue at normal incidence is yellowish silver, becomirig somewhat greener at grazing. The iridescence is much more marked if the bpecimen is observed in benzene, which cuts down surface reflections and permits a larger angle of incidence in the layer. The faults, however, exhibit very pronounced iridescence, their colors changing toward ones following them in the series given above, as the angle of incidence is increased. (This gives the appearance of the fault expanding to cover a larger area.) By transmitted light the color-producing layer is practically colorless in thin sections normal to the surface, except for the pigmentation of its lower portion. Scrapings tangential to the surface, taken above the pigmented stratum, show distinct transmission colors, unsaturated purplish red predominating. The saturation increases with the thickness. Pressure cannot be supplied very effectiveIy to the surface of the elytron, but the slight change observed by reflected light is toward the blue. Fragments show this rhange much more distinctly, and their transmission color also changes through red-orange to yellow. Bleaching has no effect on the metallic lustre or color, except to destroy the dark background furnished by the underlying pigmented tissue. Swelling agents do not act readily, probably because of the thickness and impermeability of the layer: they tend to cause the green to change to brass yellow or even orange. Maceration in acids or alkalies causes some swelling, but the tissue is attacked severely, and the lustre and color are won destroyed. KO evidence of laminations or development of permeability is revealed by this treatment. 1
Nature of Iridescent Coloring of “Metallic” Integuments
It is manifest from the descriptions of the various examples of “metallic” integuments that their optical properties bear a striking resemblance to those of iridescent scales, and a comparison with thin film colors seems justified. Reference to rhe list of properties discussed in connection with iridescent scales’ shows that the first ten of these are shown perfectly by the metallic integuments. The specular reflection, of high intensity, with colors easily recog‘Mason: J. P h p . Chem. 31, 348 (1927).
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nizable as lying on the second and lower third order of Sewton’s series, t’he polarizing properties and the predictability of color change with varying incidence, swelling, pressure, or distribution, constitute strong presumptive evidence in favor of thin films as an esplanation. The eleventh point, the loss of color on penetration by liquid, is applicable only -ivhere air voids are present, and there is no evidence of these in the “metallic” integuments.’ The twelfth point is the basis of the most cogent evidence against the theory of selective reflection (“surface color”). Rayleigh? emphasizes the fact that selective reflection is always accompanied by strong selective absorption, the solid dyestuffs are good examples of this. If any selectively reflecting substance is present in the above specimens it must bc of a character utterly at variance with any now known, with no appreciable transmission color and with great resistance to bleaching agents. The susceptibility to pressure and swelling of course points strongly toward a structural basis of the color. Onslow’s polishing experiments which shoived a more or less sharply defined layer of contrasting color beneath the esternal iridescent material, may be explainedonthebasisof the compression of the structure or by assuming it to have different color-producing properties at different depths. The most obvious explanation, however, is simply that the lower layer of the color-producing structure is pigmented, and that its absorption color is superposed on that due to structure itself, without loss of metallic lustre. The hue of the pigment varies with concentration and thickness from straw yellow t o orange to blood red t o brown, so a variety of colorings are possible. In any case this is not evidence for or against the thin-film t h e ~ r y . ~ The present Lord Rayleigh’s paper4 adds quantitative evidence to the qualitative reasons offered by liis father regarding change of color wibh incidence. This is much more marked in beetles than in any known case of surface color. ‘Xoreover, the broad central band with lateral maxima conforms esactly to the requirements of the theory of multiple reflexion from thin plates. On the other hand, so far as I know there is no absorption spectrum or surface reflexion spectrum having these features.” If thin films are thought to be the cause of the iridescence of “metallic” integumentee, some postulate as to their character is in order. Since both surfaces of the color-producing layer may be brought in contact with liquid without loss of color, multiple thin films are necessary. These need not be ’Biedermann thought that L . vesicntorin had air spaces which were filled slorvly with liquid on soaking, because the color went from green to bronze. Onslow ascribed this t o a “clearing” action, t h e liquid rendering the elytron more translucent, and lessening the dark background. However, the swelling action of the liquid is probably the chief factor, since gaseous swelling agents can produce the same color change. *Phil. l I a g . , (6) 37, 98 ( 1 9 1 9 ) . ?Onslow, after comparing the appearance of some of his “polished” specimens with t h a t of oxidized copper, cites Nallock’s paper (Proc. Roy. Yoc., 94 A, 561 ( 1 9 1 9 ) ) as evidence a ainst the latter showing thin-film colors. Evans (Proc. Roy. Soc., 107 A , 228 (1925))and >fason (J. Phys. Chem., 28, 1233 ( 1 9 2 4 ) )have come to the conclusion t h a t the oxide colors are due to thin films. ‘Proc. Roy. Soc., 103 A, 233 ( 1 9 2 3 ) .
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many in number, for a single one has lustre and reflection color nearly intense enough, as Onslow admits on the basis of his quantitative comparisons. Moreover, the transmission color, apart from pigment, is too faint for more than a very few films to be present, unless these are separated by a medium of nearly their own refractive index. Probably the situation is like that existing in iridescent wing membranes,’ with several (say three or more, depending on the space available) thin films, substantially uniform in thickness and parallel, enclosed and welded together by material having a different index of refraction than their own.? Such a system is capable of showing all the properties exhibited by the “metallic” integuments, and furnishes the simplest explanation of them. The gradual variations in hue correspond to gradual changes in thickness of the films, while the striking loral variations secn under the microscope are a s easily explained. “Faults,” where the structure is stunted due to hindrance during growth, always are colored to correspond to thinner films. The possibility of several different color phases of a given species is to be expected, since growth conditions or other factors may vary the development of the colorproducing structure enough for this to be noticeableP The effect of dark pigment beneath the color-producing structure would be to enhance its lustre and brilliancy, just as in the case of other thin films. On the other hand, if this structure is itself heavily pigmented the coloring and lustre may be cut down; this might explain n h y some of the beetles, Carabus auratus for instance, are hardly as brilliant as a single thin film. The pale iridescent colorings of freshly hatched beetles and the development of greater brilliancy with age may be explained on the basis of increased pigmentation of a neutral hue. The changes of hue noted in old museum specimens? would correspond to a thinning of the films as they aged. The various cases of color change soon after death, though not investigated by the writer, may well come under a similar explanation, while permanent destruction of the color may be due to irreversible shrinkage or coagulation of some of the more hydrated material of the color-producing system. Ob‘J. Phys. Chem., 31,
322 (1927).
*The thickness required is not inconsistent with t h a t available, since the epidermis of most of t h e specimens is something over I @ in thickness. Plusiotzs gloriosa affords space for 50 or more laminae in the thick color-producing layer of its integument. 38everal specimens of Strongyliuin bicolor ranged from metallic yellowish green to purple. Similar color variation was noted in the case of Lytta vesicat -ia, and the Cicindelzdae,, and indeed it is to be expected where ever a number of specimeni, w e available for comparison. I t might be of interest for the entomologist t o correlate this with t h e age or conditions of development of the insect. ‘Beddard: Animal Coloration ( I 892) savs of a twenty-five year old collection: “Beetles whose natural color was a brilliant red haa not diminished in brilliancy but had changed t o green, pale yellow [pigment] had deepened into brown, blue into black, while the green color of some had been converted into purple.” Each of these changes, with t h e exception of the pigment, is what would he expected if the color-producing films become thinner.
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viously the alteration of the structure need not be very great to bring about the loss of its iridescence.’ The thin-film theory enables one to explain the peculiar behavior of Buprestis, near triszilcatus, which is apparently black a t vertical incidence, changing to metallic red at grazing. Immersed in benzene, the color changes through this red to brass yellow and finally to blue green. Under the microscope it is seen that the areas which appear black to the naked eye are actually a deep purplish blue. The green stripes and spots on the elytra, and the similar coloring of the abdomen behave as typical cases of metallic iridescence. The dark areas are covered with epiderrrial scales, heavily pigmented, but these are almost completely suppressed in the metallic portions of the integuiient. The color changes exhibited, particularly the whole series in bmzenc, locate the color at normal incidence as the blue of the lower third ordrr in Sewton’s series. This is a color of very low brilliancy in metnllic insects and in iridesccnt feathers, and may be a!rnost black when originating in heavily pigmented tissue. At larger angles, where the color changes t o one of greater brilliancy, the pigment cannot suprcss it so completely, and the red beconies clearly apparent. This is further accentuated by the increase of reflecting power of the color-producing structure with increasing incidence. That the pignient does serve to mask the coloring may be slionn by careful scraping to remove the epiderrnal scales. The underlying color-producing structure is revealed, and is much more nietallic and brilliant. nleaching affords another means of eliminating the effect of the pigment, ant1 permitting the full color to be observed. The unique structural character of the color-producing layer of Plusjot is yloriosn is related to its unusual optical properties. l’hc presence of practically resolvable laminations extending through such a thick layer constitutes a system of many more films than in the ordinary rtietallic intcgunient. Such a number is not necessary for brilliance of coloring in an air-chitin system, but where the layer is “solid” and almost optically honiogeneous, the saturation of the colors, particularly by transniitted light, is seriously diminished unless a considerable number of laminae function. This explains why the transmission colors in P . yloriosa arc so much more evident thaii in other metallic integuments. The transmission colors arc distinct enough so that their changes with pressure can be followed just as in the case of butterii3- scales of the Crum’n type. The sequence of colors is seen to be that corresponding to thinning the films. The lack of marked difference betweeen the refractive indices of the iaminated lager renders hopeless any attempt t o resolve the multiple film structure ‘Liesegang (Kolloid. Z., 7, 308) states that turbid B a S 0 4or ;\&I suspensions in gelatine become transparent on drying, due to the increase in refractive index of the surrounding medium. A similar evaporation might he suficient to render the multiple-film sytsem optically homogeneous, and unless the hydration were perfectly reversible the color would not lie restored on moistening. Similarly, reagents which destroy the iridescence (SsOH, HSO,, etc.) may act to change the refractive indices of the films and t o render them more nearly alike.
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in sections, for the limit of resolution of the microscope (ca 0.z.p) can only be approached with “contrasty” preparations. With objects which are almost optically homogeneous, structures three times as coarse as this may be unresolvable. The only histological evidence of the laminated structure is given by obliquely fractured surfaces, and even here the whole layer is so coherent that there is hardly any tendency for the fracture to occur along the laminae the way it does in the air-chitin lamellar structure of iridescent scales. The silvery stripes of P. gloriosa do not show as marked color change with angle as is common on smooth metallic integuments. This is due to the fact that its reflection color (2nd order greenish yellow) is not as sensitive to changing angle as are either green or red iridescent colors. It is also probable that the uniformity of the laminae is less perfect in the stripes, thus decreasing the purity of the color and its changes, without diminishing the metallic lustre which is so striking. At the edges of the faults in the silvery stripes the colors (of the lower orders) show typical iridescence in accordance with the thin-film theory. It may be that the thinned laminae are more uniform here also. The thin-film theory of the iridescence of metallic integuments has been widely held by workers in the field of structural color, but hiiehelson’s conclusions in favor of selective reflection (“surface color”) apparently caused some misgivings, and even Onslow appears to incline toward this latter theory. It need only be repeated herel that we know of no selectively reflecting material which does not absorb light very strongly and show a n intense color even in very thin layers. The brilliant color and lustre of specimens bleached to a pale straw color by transmitted light is utterly at variance with the properties of substances which exhibit selective reflection. The effects of swelling and pressure, the amount of color change with angle, and the distribution of hues present are also thoroughly inconsistent with the known properties of these substances. Michelson’s quantitative physical studies are opposed by other quantitative studies by Rayleigh and by Merritt, the latter workers having used for comparison specimens known to consist of thin films. hlichelson did not take this precaution, and there is reason to believe that the curves which he obtained might also be obtained from specimens of single or multiple films, as well as from the various feathers and insects which he considered to owe their iridescence to selective reflection. It i s unfortunate that a n indirect study of such doubtful positive character should have been the basis of a conclusion which ignores the multitude of independent investigations and easily verifiable properties thoroughly inconsistent with it. Biedermann considers thin films to be the cause of metallic iridescent coloring, but puts most emphasis on another type of iridescence, which he characterizes as “hmail.” ‘The objections to the “surface color” theory have been discussed in detail in Part I1 of the present series of articles, and in Part I1 of that on insect colors.
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1867
“Enameled” Iridescent Integuments The type of iridescence which Biedermann compared to enamel is distinctly different in appearance from the metallic iridescence just described, though its optical nature is closely related, particularly to the structure possessed by PZusiotze glorioaa. The characteristic feature of “enameled” iridescence is its satiny matte lustre and its apparent “depth.” Specular reflection is absent and the color appears to be scattered from beneath the surface of the integument. A peculiar shifting luminous shadow i s also noticeable. Cetonids of several genera offer good examples, and this group seems to be the only one in which this type of iridescence is at all common.
Euphorza fulgida is typical of a number of Euphorias as well as of the “enameled” beetles in general. Structurally it is rather similar to Plusiotzs glorzosa. The integument possesses a color-producing layer 20 p or more in thickness] all levels of which give rise to the color. Surface examination and tangential or oblique sections reveal a finely stippled or dotted appearance in this layer. The “dots” are very minute (say 0.3 p in diameter) and are less than I p from center to center, unsystematically arranged but uniformly distributed. Sections normal to the surface show that these dots are the ends of fine rods extending through the color-producing layer, in general perpendicular to its surface, but occasionally slightly tilted. S o other periodic structure is evident; fractures oblique to the surface give little if any indication of laminations. The surface of the cuticle exhibits a thin outer layer (1-2p ) divided into polygonal areas. This may be removed without affecting the iridescence. Sections of the enamel layer are anisotropic and between crossed nicols the rod structure shows up plainly. E. fzdgzda is very lightly pigmented as compared with the other “enameled” beetles, the elytra being a dark straw color by transmitted light. Painting the under surface of the elytra with india ink cuts down the effect of the transparent underlying tissues, and serves as a dark background to enhance the iridescent coloring very markedly. The color of the enamel layer is a slightly yellowish green by reflected light (some specimens range from green to blue), pale straw (pigment) by transmitted light. Although the brightness is greatest in the vicinity of the angle of the reflected ray, light is scattered also, and the color may be observed over a considerable angle with unidirectional illumination. The intensity of reflection is high, and the specimen under favorable light may appear almost self luminous. “Faults” in the color-producing layer are blue green to purple. Apart from these the color is uniform. With large angles of incidence the reflection color is blue green. This is best observed when the specimen is immersed in benzene so as to eliminate surface reflections which decrease the saturation of the color as grazing incidence is approached. Such procedure also insures that the angle of the incident light in the color-producing layer shall not be limited by the critical angle of chitin. The“fau1ts”change from blue-green through purple to dark a t grazing. The angle of vision, rather than the angle of illumination, determines the color changes observed. The
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CLYDE W. MASON
tissue beneath the color-producing layer may be removed by dissection or by The exposed under surface shows the same maceration in dilute "03. color phenomena as the outer surface. Very thin scrapings, if not completely distorted, are capable of producing the color. Sections normal to the surface do not give the iridescent color, but with unilateral dark field illumination may show non-uniform diffraction colors, as would be expected from such a periodic structure thus illuminated. The color is of full intensity in diffuse light, and is independent of the azimuth of the illumination or observation. N o polarization effects were observed, except such as are manifested by any finely divided system (similar to Tyndall blue). Th- luminous shadow, which is characteristic of enamel iridescence, is exhibited only a t small angles of incidence, and is seen along the line of direct reflection (in the position of the high light on a smooth surface). Swelling agents, such as NaOH, cause the green to change through brass yellow to orange red, and this change is reversed on thorough washing or neutralization of the alkali by an acid. Bleaching does not destroy the iridescence, though all the pigment coloration may be destroyed. This enables the true transmission color of the iridescent layer to be observed, and it is seen to be a pale pink. Pressure alters the color from green through blue t o purple, and the transmission color (as seen in bleached specimens) changes through orange to yellow. Penetration by liquids is not possible in the unThis reagent treated layer, but may be effected after maceration in "03. renders the color-producing structure porous, probably by dissolving one variety of chitin, (of which the "rods" consist). The attack spreads from cracks in the layer, and the unaltered material is thus available for comparison of hue and brightness. The HSO? may be replaced by liquids of various refractive indices with the following changes in appparance : Air n = 1.00 Whitish blue scattered; dark brown by transmitted light; pores distinct. Water n = 1.33 similar to above, but less bright and opaque. Xylene n = 1.49 perfect match for unaffected portion of color-producing layer. Rlixture n = 1.58 no color effect whatever; pores wholly invisible even with dark field illumination. a -Monochlornaphthalene n = 1.63 similar to unaltered portion in hue and brightness Methylene iodide n = 1.74 brighter colored and more opaque; pores more distinct. Methylene iodide sulphur n = 1.78 whitish blue replacing normal iridescent color. Refractive index of layer 1.594 by Becke test. The same specimen may be run through the series of liquids in any order, with the corresponding changes in appearance. The penetration or evaporation of liquid appears to take place in patches, as if the color-producing layer were composed of fairly large cells so perfectly fused together as to be invisible ordinarily.
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STRUCTURAL COLORS I N IXSECTS
1869
h very brilliant Cetonid, Sphellorrhina guttata’ might be described in practically the above words, except that it is niore of an emerald green, and changes a t grazing incidence to blue. The prominent spots consist of white scales overlying depressions in the enamel-layer, which is thinned and stunted beneath them, and colored in the same manner as if a “fault” were present. Scraping the enamel layer has no effect on the color or lustre until the inner portion is reached, when the color goes toward blue. Beneath this is a layer of dark pigment. Heterorhina (Coryphocern) decora Ill. is the most striking of the enameled beetles, with brilliant red thorns and stripes across black elytra. These stripes appear almost as luminous as glowing coals. With increasing incidence their color changes through yellow and green to blue-green. At the edges of the red stripes their coloring is graduated through the above lines, before it meets the surrounding black-pigmented portion of the elytron. Faults, and the inner portions of the enamel layer (as revealed by scraping) show this same series of colors. The red portions of this insect, even when thoroughly bleached, show a pale green by transmitted light, while those areas which are green eshibit a pink transmission color, as does the green of Sphellowhinn g u t t n t n . The effect of pressure on fragments of the red areas is noteworthy, in that the green transmission color changes to pink and then to orange, while the red reflection color passes through yellow-green and blue-green to blue. Nature of Iridescent Coloring of “Enameled” Integuments I n spite of the marked differences in lustre, “enamel” iridescence resembles that of thin films rather consistently. The colors and their changes with angle of incidence, pressure, swelling, or “faults,” the complementary character of the transmission colors, and their low saturation, the loss of color on penetration, after treatment with H903, the persistence after bleaching or in diffuse light-all these criteria are fully satisfied, and they could not be met by any other known type of structural color. The discrepancies which esist may be esplained on the basis of the riiodification of the structure by the fine rods which perforate it perpendicular to the surface. I t was first thought by the writer that these were the chief cause of the enamel iridescence, and that they acted as some sort of a point grating by virtue of their uniformly minute dimensions. This cannot be the case, since diffraction colors demand unidirectional light, while these :we unimpaired even in thoroughly diffuse light. Comparison with a number of other Cetonids, not “enameled” but either metallic or non-iridescent, reveals a similar rod structure in their integuments, so there can hardly be an)- significant connection between it and the color production. It is probable that the color-producing layer is essentially similar to that of Plusiotl‘s gloriosa, consisting of many unresolvable laminae of chitin ( n = ‘This is probably closely sirniliar in appearance and properties t o Hcteroririiin nf)icana studied by Onslow, and Smaragdisthes nfricaria studied by Biedermann.
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CLYDE W. MASON
1.59)perforated and welded together by another variety of chitin (n = I . 5-2) the dimensions being such as to give colors of the second and lower third orders. I n the untreated specimen the specular reflection from a system of multiple thin films is broken up by the rods, and light is scattered over a considerable angle. At vertical incidence the rod structure acts as a “light trap,’’ like velvet’ and since it constitutes a considerable portion of the area of the enamel layer, there is a lessened reflection that is manifest as the luminous shifting shadow (the pseudopupille of Biedermann). At larger angles of incidence the rods serve to diffuse both incident and reflected light, which may suffer many reflections before it emerges from the multiple-film structure, colored. A definite polarizing angle, and the other polarization effects characteristic of uninterrupted films are obviously impossible in such a system.
When the color-producing layer is treated with HNOI the less refractive variety of chitinous material is removed, and this may then be replaced by various media. With air the scattering action of the pores (rods) is so pronounced that it obscures any effect from the thin films, and only an opalescent white, almost opaque even in very thin layers, is observed. Under these conditions the layer functions as a Tyndall blue medium.2 Highly refractive permeating media have a similar action, and it is only when the pores are filled with a medium of refractive index fairly near that of chitin that the structure becomes transparent enough for the effect of the multiple films to be evident. This is best seen when the refractive index of the penetrating liquid lies between 1.45 and 1.65, 1.55 matching the appearance of the untreated layer perfectly. If the refractive index of the penetrating liquid is such as to render the system optically homogeneous ( n = 1.59)then no color effect whatever is observable. In the case of some of the more brilliant enameled beetles the treated layer, when filled with a liquid of high refractive index, may show a color different from its ordinary hue, This may be due to swelling and change of thickness of the color-producing films, or t o the fact that the highly refractive layer is of greater effective thickness and acts as a thicker film. The original color is restored by pressure, which fits either of these explanations. The descriptions of enameled iridescence given above check very closely with Biedermann’s observations. He recognized its unique properties, and came to the conclusion that “the colors of thin plates are the chief factor, and the rod layer is important only in so far as it may give blues of a turbid medium and may increase the lustre because of its high reflecting po~ver.”~ He compares the pseztdopzcpTlle to that shown in the luminous tapetum in certain animal and insect eyes, and considers that it limy function to give an effect analogous to rod double refraction. ’Bancroft: “Applied Collid Chemistry,” 242 (1926) ; X o o d : “Physical Optics,” 443 (l?II). W a s o n : J. Phys. Chem., 27, 201 (1923). Structural Colors in Feathers, I. 310~.cit. p. 1918.
STRUCTURAL COLORS I N ISSECTS
1871
As Onslow points out, Biedermann considers the rod layer of importance in cases of simple metallic iridescence, where it is entirely beneath t’he colorproducing structure. Onslow classes enamel iridescence with effects due to scattering of light by fine particles, but his observations are rather incomplete, and he makes no postulates as to how the color might be produced by such a system. Diffraction Iridescence Since such positive opposition to diffraction as an explanation of iridescence has been expressed in various places in this series of papers, the writer is glad to be able to demonstrate that this is not purely prejudice, and that he can recognize diffraction colors when they really occur in nature. Sericn sericea is typical of a number of Sericne, a11 of which are dark-brown, highly convex beetles, showing a brilliant iridescence in direct sunlight and much less in ordinary light. In thoroughly diffuse light (such as on top of a building in a fog), no trace of iridescence is detectable. With light from a point source, even of low intensity, the colors are distinct and two orders may be observed. There is no color in the directly reflected light, and this absence of color in the high-lights is another good test for this type of iridescence. K i t h light and line of vision falling lengthwise of the elytra, as the line of vision is moved away from the directly reflected ray a sequence of colors is observed. Through about 90’ they are roughly as follows: blue, green, yellowv,orange, red, purple, blue, green, orange, red. With light and line of vision falling across the elytra, no color effects are noted. The above variation of iridescence with azimuth leads one to look for some oriented structure, which is found in the form of fine striae running crosswise of the elytra. These are shallow ( 0 .j-1.0k ) , and very evenly spaced (:-I.s p! or about 2 0 , 0 0 0 per inch). That they are at the very surface of the cuticle may be shown by means of collodion impressions‘ which retain the same pattern. The iridescence is exhibited by these impressions quite as well as by the insect, and may also be observed by transmitted light. If the surface of the elytron is covered with liquid, the iridescence is decreased markedly and is not observable if its refractive index differs from that of chitin by less than about i 0 . 1 . -111 the above properties are perfectly in accord with those of diffraetion gratings, and there can be no question as to the striae functioning as a remarkably perfect grating, particularly since their orientation on the elytra is crosswise of the direction in which the light is diffracted. The fact that the red is deviated more than the blue is also a significant point in the identification. S o such set of properties could be manifested by thin films, surface color, turbid media, or any other knon-n color-producing structure except a grating. The high degree of convexity of the el>-tra enhances the effect, and renders several colors visible a t one time, as a sort of “rainbon” transverse of the ell-tra. ‘These impressions are made by flowing syrupy collodion (or duPont cement) over the surface, and stripping it off after it hardens.
1872
CLYDE W. hL4SON
Conclusion It appears from the evidence discussed in the foregoing pages that, thin films are the cause of nearly all types of iridescence in insect integuments, as they are also in insect scales and in feathers. Although this seems a coniplicated way for nature to produce the color, yet it must be borne in mind that once the possibilit?7 of a laminated structure is admitted, the hue is only a matter of thickness, while the degree of saturation and lustre are dependent on the number and uniformity of the films. The most significant property, that of hue, can thus be varied through the whole range of color by a much more simple means than bj- the synthesis of a variety of special pigments. Incidentally, the colors so produced are “fast” enough so that they have endured in the inlaid ornaments of Egyptian tombs. The conclusions of this paper are as follows: I. “Metallic” iridescent insect integuments owe their color phenomena to a thin laminated layer at or just beneath the surface, which acts as a multiple thin film. 2. The color-producing structure may be much thicker, with many more laminae, as in Plusiofis yloriosa, where it is embossed rather than smooth. “Enameled” iridescent integuments owe their color phenomena to a 3. thick multiple-film layer, the properties of which are modified by the closely spaced, minute rods n-hich perforate it. 4. Diffration iridescence has been identified in Serica, but does not occur in “metallic” or “enameled” integuments. 5 . The properties and criteria for identification of the above types of iridescence are discussed in detail. 6 . Selective reflection (“surface co1or”j is not exhibited by iridescent insect integument,s. Laboratory of Chemical Jlicroscopy Cornell Cnicersity
