10 THE NORTH AMERICAN SPECIES OF DROSOPHILA. 



been removed. But when the female is ready to mate, she will mate 

 with a wingless male almost, if not quite, as readily as with a normal 

 winged one. 



An extensive series of experiments with various mutant types 

 (Sturtevant, 1915) indicated that neither males nor females exercised 

 any ''choice " of mates with respect to the characters studied. Usually 

 the mutant-type is less active than the wild-type. In such a case, 

 the mutant male mates less often than does the wild-type male, since 

 he courts less vigorously and persistently. But the mutant female is 

 mated with more often than is the wild-t3'^pe female, since she is less 

 active in running away from the male. 



SEX RECOGNITION BY THE MALE. 



All attempts to induce courtship by means of visual or of olfactory 

 stimuli alone have failed in Drosophila. IMating occurs in the dark, 

 indicating that sight is not necessary. Males without antenna? wdll 

 mate, and Barrows's experiments (see above) indicate that the organs 

 of smell are located in the third antennal joint. Yet there is evidence 

 that both sight and smell may play a part in the process. 



I compared the time before copulation occurred in two parallel 

 series of pairs of D. melanogaster. One series was placed in clean 

 vials; the other in vials in which another pair had just copulated. 

 The second series mated significantly sooner, on the average. This 

 can only mean that olfactory stimuli had hastened sexual excitement. 



When a male of D. melanogaster is courting a female she frequently 

 walks or flies aw^ay. He orients toward her and follows her accurately 

 if she is only a few millimeters away, but never orients accurately if 

 she is as much as a few centimeters away. In the latter case he often 

 becomes excited, and shows movements characteristic of courtship; 

 but he finds the female again only by accident. This behavior is not 

 changed if one antenna is removed. Circus movements do not then 

 occur when the male becomes sexually excited, and the female, if she 

 is close enough, is followed as accurately as before. This seems to me 

 to indicate that orientation toward the female is by means of visual 

 stimuli. This view is borne out by the fact that sexually excited 

 males will sometimes orient tow^ard and follow other males, though 

 only rarely does one male cause sexual excitement in another one. 

 The hypothesis is also in agreement with Barrows's observation that 

 orientation toward olfactory stimuli (food) occurs at distances much 

 greater than those at which a sexually excited male can orient toward a 

 female. 



The failure of olfactory stimuli alone to produce courtship, the fact 

 that males without antennae will mate, and some observations of normal 

 courtship, all suggest that tactile stimuli may be involved; but no 

 direct evidence for this conclusion is at hand. 



