PROBLEM OF SEXUAL SELECTION IN DROSOPHILA 359 



tation. then the effect of smell is negligible. It is not clear from 

 these results whether the effect is produced upon the males, 

 the females, or both. 



In. order to test whether sight is the sense which stimulates 

 the male I carried out the following experiment. Two vials 

 having the same bore were filled with cotton up to about three 

 centimeters from the mouths, and then placed with their mouths 

 together. A thin glass cover-slip was then placed between them, 

 and a male fly was placed in one vial, a female in the other. 

 These flies had been isolated for several days, and were sexually 

 ripe. The male could see the female, but could not touch or 

 smell her. Once or twice they met ' ' head on ' ' with only the 

 cover-slip between them, but the male showed no signs of recog- 

 nition. After thirty minutes the female was let in the vial 

 containing the male; and the vials were left in position. He 

 •courted her w r ithin two minutes, and paired in five minutes. 

 The experiment was repeated with the same result, except that 

 copulation now occurred in three minutes after putting the 

 flies together. 



It is also evident that sight is not necessary for pairing from 

 the facts that Drosophila breeds freely in the dark (see Payne, 

 '11), and that pure stock of white-eyed flies (which may be 

 blind) has been kept for many generations. 



In Fucellia marina and in Musca domestica males seem to 

 see their mates from some distance and fly directly to them, 

 lighting on the back of the mate. Here sight would seem to 

 play considerable part, and in Fucellia still more convincing 

 evidence was obtained from observation of violent courtship 

 of a fly which was separated by thick glass from the courter. 

 Not only did this male court the other fly, but when she walked 

 around rather rapidly, he very accurately followed her on the 

 other side of the glass. This has been observed twice. 



Since the wings play such a conspicuous part in courtship 

 I was led to try the effect of cutting them off. Two males 

 of the same age were used, the wings of one being cut off at the 

 base. These were kept in the same vial for several days, and 

 then placed with a virgin female several days old. The vial 

 was then watched until copulation took place. This has been 

 done 125 times, using different individuals each time. The 

 normal male paired 72 times, the clipped male 53 times. As 



