358 A. H. STURTEVANT 



had been removed, and this pairing resulted in the production 

 of offspring. 



The above experiments failed to give any evidence demon- 

 strating that smell alone can cause males to show signs of court- 

 ship. Another series of experiments, now to be described, has 

 indicated, however, that smell may be a secondary factor in 

 causing sexual excitement. Males and virgin females were 

 isolated for three or four days and were then placed, in pairs, 

 in clean vials. The length of time before copulation occurred 

 was recorded; and, after it had taken place, the flies were re- 

 moved and a new pair was placed in the same vial. The only 

 difference between the two sets was that one lot was in clean 

 vials, the other was in vials in which copulation had just occurred. 

 In these experiments an equal number of each sort was done 

 on each day (placing a third pair in some vials when necessary 

 in order to get an equal number), and all flies used on a given 

 day were as nearly the same age and size as practicable, and 

 were from the same culture. Table I gives the result of the 

 experiments. It shows that in the first five or six minutes 



TABLE I 



Minutes Number of times observed 



before — ■ 



Copulation 1st pair 2nd pair 



1-3 13 22 



4-6 12 22 



7-9 5 7 



10-12 7 6 



13-15 11 2 



16-18 7 5 



19-21 8 5 



22-24 4 3 



25-27 5 3 



28-30 2 1 



31-33 7 1 



34-36 * 2 2 



37-39 2 



40. and more, including failures. 24 25 



the flies in the used vials are more likely to copulate than are 

 those in the clean vials; between ten and twenty minutes there 

 are more copulations in the clean vials, and after that the two 

 series are parallel. Apparently there are a certain number of 

 pairs that are nearly ready to mate, and these will mate more 

 quickly if another copulation has just occurred in the same 

 vial. But if much courtship is to be required before copu- 



