Elizabeth A. Runquist and Olaf Runquist
Passage of Fruit Aies through a Hole
Hamline University
st. pad, Minnesota 55104
A model for a reversible chemical reaction
The passage of fruit flies (drosophila) through a single orifice provides an excellent model for illustrating the principles of equilibrium and chemical dynamics.' To carry out the demonstration, flies are allowed to crawl from one closed chamber, through a small hole, into a second chamber. The number of flies that cross through the hole in unit time is followed visually. "First-order" kinetics are obeyed and rate constants are reproducible. The "reaction" is temperature dependent with maximum rates being obsewed in the 20-21°C range. Adynamic equilibrium is established, and equilibrium constants based upon the concentration of flies in each chamber can be reproduced. The rates a t which flies were ohsewed to move from one chamber to another and the equilibrium constants obtained were found to be dependent upon the concentration of flies per unit of surface area rather than volume (see the table). This seems reasonable in view of the fact that the flies spend most of the time walking or sitting rather than flying (est. 95% or more) and that they find the hole and pass through it by a crawling (rather than flying) process. Discussions with students of the differences between the model experiment with crawling flies in this apparatus and the effusion of gases is instructive. The apparatus employed consisted of a glass tube (50 X 4.6 em), a Teflon barrier, and two movable Teflon pistons. A hole bored a t the center of the barrier was covered by a Teflon "trap door" which rotated freely on a mire pin and which could be opened and closed by rotating the glass tube and barrier. The movable pistons were perforated with small holes to allow the passage of air and were fitted with thermometers for measuring the temperatures in each chamber. A strip of measuring tape pasted on the side of the glass tube facilitated the measurement of surface area of the chambers. The apparatus was mounted on a board in a horizontal position. To conduct an experiment, the "trap door" was closed, the left piston removed and approximately 100 fruit flies were introduced into the left chamber directly from small breeding bottles (flies were of one variety but were a random mixture of males and females of different ages). The piston was quickly replaced, adjusted to the desired position and after a few minutes (to allow the flies to settle down) the "trap door" was opened. The surface concentration of flies in the right chamber was determined a t the end of each minute (alternatively, the number of flies crawling 'An interesting application of t.his phenomenon, used in an attempt to select active drosophila, is reported by ARTHUEW. EU'ING, J. Anim. Behao., 11, 369.
534
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Journal o f Chemical Educafion
time-min Plot of integrated flrrt order equation for fruit fly transport at several different temperatures. X. is the equilibrium concentrotion (Rier/cm2) of fruit Ry and X is the surface concentration at m y time. Diameter of hole, 1.24 cm.
through the hole in the forward and reverse direction in unit time was determined). Typical rate curves obtained at different temperatures are given in the figure and some typical "equilibrium constants" arc given in the table. The equilibrium constant was defined as K*q =
(flies/cm2) right chamber (flies/cmz) left chamber
The dependence of the equilibrium constant upon surface area (rather than volume) was established by inscrting thin glass plates into one chamber, and thus, altering the surface area to volume ratio (see the table). All equilibrium constants given in the table should have a value of 1.0 since the hole in the barrier had Equilibrium Constants for Fruit Fly Transport Through a Hole with Parallel Sides of 1.24 cm Diameter
TT
K., (area)
I
