418 Effect of Oil of Citronella on two species of Dacus. 



sent the sexual smell, but owes its attractions to some 

 other cause : the proof is at present incomplete. There 

 remain at least two other solutions of the difficulty. One 

 is that the samples of citronella used contained two or 

 more active ingredients which appealed respectively to 

 zonatus and diversus, and the other is that zonect^cs and 

 diversus are not really distinct species at all, but varieties. 

 I hope to be able to give further attention to these points. 

 If my conclusions are correct regarding the nature of 

 the phenomena, they afford an interesting example of the 

 imitation by artificial means of a sexual attraction probably 

 similar in kind to that which operates in most cases of 

 " assembling." It has occurred to me as possible that the 

 curious predilection of another fruit-fly (Ceo'atitis capitata) 

 for kerosene oil might perhaps be explained in the same 

 way, but I do not remember to have seen any record of 

 the relative numbers of males and females captured by 

 this method. 



Explanation of Plates XXXIX, XL. 



Plate XXXIX. Males of Dacus zonatus attracted to handker- 

 chief moistened with oil of citronella. 



Plate XL. The same three minutes after the flies had been 

 dispersed. 



