BEHAVIOR. 9 



CROSS-COPULATION. 



Males of Drosophila melanogaster will copulate with females of D. 

 affinis, D. obscura, or D. simulans. Only the combination with D. 

 obscura has been studied in any detail. As we have seen, the two 

 species differ in the length of time they remain in copula, the average 

 time for D. obscura being about 6 minutes, that for D. 7nelanogaster 

 about 21 minutes. In the case of the cross-mating the time is much 

 more variable than in any intraspecific mating yet studied. The fol- 

 lowing times have been observed: 1/2, 1, 1, 1, 1, 1, 1, 1, 1, 3, 3, 4, 6, 6, 

 7, 8, 10, 10, 11, 12, 13, 14, 15, 16, 16, 18, 18, 23, 24, 25 minutes. This 

 combination has been tried in many ways, but always without the 

 production of offspring. Each sex has been allowed to mate with its 

 own species before cross-copulating, and such preliminary matings 

 have sometimes been interrupted before finished, sometimes allowed 

 to stop normally. Cross-mated females have been mated later to 

 D. obscura males. Numerous different races have been tried, and 

 females of many different ages have been used. Nevertheless, no 

 hybrids have ever been obtained. The combination of D. melanogaster 

 male and D. afinis female has not been tried so extensively, but it 

 has so far failed to give any hybrid offspring. 



The crosses between D. melanogaster and D. simulans are occa- 

 sionally fertile in both directions. The hybrids are discussed elsewhere 

 in this paper (p. 117). The cross-mating has not been studied in 

 detail, but a small series of experiments makes it highly probable that 

 each species is more apt to mate with individuals of its own species 

 than with those of the other form. This selective relation is apparently 

 due chiefly to the females, since males of each species seem to court, 

 but not to mate with, females of either species indiscriminately. 



The hybrid females are mated with by males of both species. In 

 the only case in which the whole process was watched, a male of D. 

 simulans was placed with a female D. melanogaster and a hybrid female, 

 both females being of the same age and size. He mated with the 

 hybrid female, the pair remaining in copula 28 minutes. 



EXPERIMENTS CONCERNING SEXUAL SELECTION. 



Lutz (1911) removed the "sex-combs" on the front tarsi of males 

 of D. melanogaster, and found that such males were as apt to mate as 

 were normal males. The significance of this secondary sexual char- 

 acter — if it has any significance — has not been determined. 



As appears from the description above, the wings of the male play a 

 conspicuous part in the courtship of D. melanogaster. I have per- 

 formed some experiments in an attempt to discover the function of 

 these wing movements (Sturtevant, 1915). These experiments showed 

 that a male with wings excites a female sexually, so that she is ready 

 to mate, more quickly than does a male from which the wings have 



