The Courtship of Drosophila Melanogaster

Abstract

The courtship of Drosophila melanogasler is described and divided for the purpose of analysis into three elements; orientation, vibration and licking. These do not replace, but are superimposed upon one another. Three possible explanations of the change from one element to another in a courtship sequence are discussed. The most probable one is that the centres controlling the elements receive common excitation but have different thresholds for firing. There is reason to believe that the excitation is continuously fluctuating. Evidence in support of this explanation is as follows. 1. There is a negative correlation between average bouts of vibration plus licking, taken together (= average 'vibration bout-length'), as opposed to average bout-lengths of orientation. This would mean that when, for example, the excitation is for most of the time below the threshold for vibration, it will rise above it only for short periods and vice versa. 2. There is a positive correlation between average vibration bout-length and the percentage of licking in the courtship. This would mean that when the excitation remains above the threshold for vibration for longer periods, then it rises more often above the highest threshold-that for licking. 3. There is a positive correlation between average vibration bout-length and the total time spent courting a female over a long period. This 'total response time' is taken as an independent measure of sexual excitation and the correlation would mean that longer periods above the vibration threshold can be equated with higher sexual excitation. The explanation in terms of fluctuating excitation alone has two main difficulties. 1. Breaks in courtship are irregular and do not fit into the pattern as one would expect. With fertilized females however breaks can be correlated with special repelling movements of the female, suggesting that they are responses to inhibitory stimuli. Virgin females give other, less drastic, inhibitory stimuli and it is probable that their effect accumulates and overcomes the excitation only from time to time. The irregularity of breaks is thus explained because the inhibitory stimuli are indepedent of the excitatory ones. 2. There are consistent differences between the courtship patterns of males with simulans, as opposed to melanogaster females, which cannot be accounted for in terms of different excitation. The regression lines of vibration bout upon orientation bout and upon licking are significantly not parallel for the two types of female. For a given vibration bout-length, males with simulans females have longer bouts of orientation and less licking than those with melanogaster females. We suggest that inhibition while accumulating, interacts with excitation and it is fluctuations in the resultant 'effective excitation' which determine the courtship pattern. It is possible that given variations in excitation will have different effects upon the effective excitation according to the amounts of inhibition and excitation already present, and we assume that the observed difference in pattern is due to the greater number of inhibitory stimuli given by simulans females. The implications of this idea of interaction between inhibitory and excitatory factors in courtship are discussed. It is suggested that a 'conflict' between them may give rise to displacement activities from which the courtship itself is derived. The occurrence of inhibitory stimuli in other animals and the possible evolution of these movements in Drosophila is discussed.