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1 From the Medical Division, Army Chemical Center, Maryland and the Biological Laboratories, Harvard University
Wingbeat frequency of Drosophila repleta Wollaston was measured stroboscopically at 25 degrees C. as a function of atmospheric pressure, over the range from 100 mm. Hg to 3860 mm. Hg in air, in nitrogen-oxygen mixtures containing either more or less oxygen than air, and in two helium-oxygen mixtures. Similar measurements were made with D. virilis Sturtevant at 19.3 degrees C. and 25.9 degrees C. in air over the pressure range from 100 mm. Hg to 1520 mm. Hg; and at 760 mm. Hg, 25.9 degrees C., in a mixture of 6.1 per cent oxygen in nitrogen.
The flight response was inhibited when total pressure was less than 80 to 100 mm. Hg, or when the oxygen tension was less than 15-20 mm Hg. Increasing the partial pressure of oxygen above the value for air did not increase the rate of wingbeat.
Within the limits of experimental error, the rate was found equal at equal densities, irrespective of the medium in which it was measured. Wingbeat frequency is therefore independent of total pressure as such, and varies inversely in a logarithmic relationship with the density. The exponents measured for this relationship varied with different individuals between – 0.03 and – 0.15, approximately.
The helium-oxygen mixtures had a detrimental effect on the response of the insects, which was less evident at higher pressures and reversed when the specimens were returned to air.
Amputation of the halteres did not disturb the relationship between wingbeat frequency and density.
Clipping portions from the wingtips increased the frequency of wingbeat. When only one wing was clipped, the increase was less than when both were shortened by equal amounts.
Oxygen consumption of D. virilis was measured during flight in an oxygen atmosphere at 19.3 degrees C., at 760 mm. Hg, 400 mm. Hg and 200 mm. Hg, and was found to be relatively unaffected by variation in density.
Since wingbeat frequency varied less rapidly with changes in density than would be expected if both power output and stroke volume were to remain constant, it is reasoned that partial compensation is effected through adjustments in stroke volume. A decrease in stroke amplitude was observed at higher densities, but it appears unlikely that amplitude can increase enough at lower densities to account for the stroke volume required. Arguments are given to show that the remaining compensation needed may be furnished by alteration within reasonable limits of the angle of attack.
It is concluded that insect flight exhibits homeostatic characteristics, in that the strain which results from density change is distributed over several elements in the wing motion.
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