Chemoreceptivity ofDrosophila melanogaster

The occurrence of mutants of Drosophila melanogaster distinguished by the absence or structural modification of the antennae provides a means of assessing the role of the antennae with respect to the reception of various classes of stimuli. Antennaless $(A_{0})$ phenotypes of antennaless stock fail to respond to those chemical stimuli which lead the fly to its food. Their temperature reactions are normal, and their humidity responses are opposite to those of somatically wild-type flies of the same stock or of wild-type controls. $Aristapedia$ $(ss^{a})$, which have leg-like antennae equipped with surface pegs and cones of supposed sensory function present in the normal antenna but absent in the normal leg, respond to chemical stimuli and humidity differences. As compared with that of normal flies, the olfactory response of $aristapedia$ $(ss^{a})$ is somewhat less intense, the humidity reaction being somewhat stronger. These mutants do not give the characteristic responses evoked by thermal stimuli both in normal flies and antennaless phenotypes. The outstanding histological differences between the structure of the antenna of aristapedia and that of wild-type flies is the absence of the pit organ. It thus seems that the pit organ is not essential to the olfactory response and plays no essential part in the humidity response. Since $antennaless$ $(A_{0})$ responds normally to thermal stimuli, none of the putative sense organs of the antennae are essential to the recognition of temperature differences, and since $aristapedia$ $(ss^{a})$ responds more weakly to chemical stimuli than do normal flies, the pit organs may well be long-distance chemoreceptors. What is more certain is that either the peg-like organs or the cones on the surface of the distal joint of the antennae or both are chemoreceptors. The same remark is equally applicable to the perception of humidity differences. Experiments here recorded do not justify the identification of the function of one or other type of sensilla with one or the other type of receptivity. While it is unjustifiable to exclude the possibility that short-distance chemical stimuli play a part in the attraction of flies of opposite sex, it appears that the main role of chemo-receptivity in relation to the mating behaviour of D. melanogaster is to ensure the aggregation of flies of both sexes in situations where food is available and sexual congress can be evoked by other forms of stimulation.