378 W. WESCHE ON THE STRUCTURE OF THE EYE-SURFACE, 



the females, and I cannot find any unusual sense organs in the 

 genitalia. 



In Bibio the genitalia are of a simple type, and in DilopJms 

 <even more degenerate, and there appear to be no mechanical 

 impediments to prevent the union of many species of the same 

 genus ; nor are there any special sense-organs which might be 

 thought to fulfil such a function, as we find in Tipula, where 

 similar conditions exist. In these two cases the double eyes, 

 and the two sizes of facets (if the means of discrimination, 

 and therefore of the isolation of the species) would, as I have 

 .already pointed out, be of obvious advantage. 



Weighing all these arguments, particularly the fact that the 

 lower parts of the eyes in Bibio and Dilophv.s would look directly 

 down on their partners in coitus, and remembering the innumer- 

 able contrivances in nature to secure isolation, I am inclined to 

 think that the larger facets enable the male to see his mate at 

 a distance, while the normal facets (normal as they are found in 

 both sexes) enable him to recognise her with greater certainty 

 when quite close. 



The presence of the two sizes of facets on the female of Lepto- 

 gaster cylindrica, Deg., does not help to any solution. I have 

 only this solitary example of this genus in my cabinet, and it may 

 be one of those interesting cases of the acquirement of a character 

 belonging to the opposite sex, or, on the other hand, it may be of 

 great advantage to its possessor, this insect being fiercely predaceous. 



It will be noticed that all the species possessing two-sized 

 facets, except in the Bibionidae, where the double eye is found, 

 are insects with the eyes in chitinOus frames, and it is possible 

 that the sight is not so good as in the Muscidae, and that these 

 modifications have arisen in consequence ; but I do not think 

 this idea will hold, as I know no quicker or sharper-eyed insect 

 than a Bombylius, and members of the family to which it belongs 

 exhibit a more highly developed chitinous structure than the 

 Syrphidae, which are by no means slow, especially Helophilm, as 

 most entomologists have found when collecting. 



