PROBLEM OF SEXUAL SELECTION IN DROSOPHILA 353 



after which copulation may or may not take place, probably 

 depending upon the way the female responds. 



Berlese ('02) and Hewett ('08) find that in the house fly the 

 final step in copulation is taken by the female, which inserts 

 the ovipositor into the genital opening of the male. I have not 

 been able to verify this for Drosophila, but it is probably true 

 here also. In Drosophila, as in some other related flies (I have 

 examined a few Anthomyids and Sepsis violacea), the ovipositor 

 enters the male opening, instead of the penis entering the female 

 duct. But I cannot state positively that the female inserts it 

 instead of the male drawing it in by means of his genital arma- 

 ture. In any case, it is certain that the female is not entirely 

 a passive agent. The time required for copulation to take place 

 depends largely upon whether she stands quietly and allows the 

 male to pair, or moves away when he begins to court. In the 

 latter case very active males have been seen to pair while the 

 female was walking away, but this is exceptional. Occasion- 

 ally a female seems to frighten off a male by spreading her 

 wings and moving quickly toward him. 3 When this happens he 

 moves off, and does not so far as I have seen, then pair with that 

 female, although she has been known to pair with another male 

 within a few minutes afterward. That this is really a threat on 

 the part of the female seems likely from observations of fighting 

 between males. If two males are courting the same female they 

 often grow very excited, especially if she is unwilling to stay 

 quiet. In such cases they may sometimes be seen to spread 

 their wings, run at each other, and apparently butt heads. One 

 of them soon gives up and runs away. If the other then runs 

 at him again within the next few minutes he usually makes off 

 without showing fight. 



The time occupied by the process of courtship varies greatly 

 with the age and condition of the flies and with the temperature. 

 Copulation may occur within a few seconds after the flies are 

 put together, with little preliminary courtship. While experi- 

 menting with flies about 3 or 4 days old, which had never been 

 allowed to pair, I have found that 20 minutes or a little less is 

 about the average length of time before copulation occurs. The 

 flies may remain in copula for only a few seconds, but so far 



3 Compare Howard's ('02, pp. 141 and 145) accounts of Asilid and Empidid 

 flies eating males which were courting them. 



