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Abstract:
The fruitfly Drosophila and the housefly Musca develop, under conditions of stationary flight in still air, greater thrust in complete darkness than at constant illumination. This effect of “dark activation” has been demonstrated in earlier experiments with Musca by L. MAF-FEI. The present investigations establish similar reactions in Drosophila. The thrust reduction as a function of luminance resembles the transducer characteristics of light receptors. Dark activation can be elicited throughout the visual field. The effect is mainly mediated by the directly stimulated visual element of the compound eye, provided that the source luminance is low enough to prevent stray light effects in neighbouring elements. Covering the ocelli has little, if any, effect on the dark activation. An influence of position and distribution of the light stimulus on the dark activation is expected, respectively, from the discontinuities of the receptive fields and from possible lateral interactions between neighbouring visual elements. However, an estimate of the expected variations shows that a considerable experimental effort would be necessary to distinguish these variations from statistical fluctuations of the dark activation.
