368 W. WE8CHE ON THE STRUCTURE OF THE EYE-SURFACE, 



Muscidae. The Caenosia group of the Anthomyidae, and the 

 family Cordyluridae of the Acalyptrates, may be cited as 

 examples in the Muscidae ; for such cases Williston has suggested 

 the convenient term " dichoptic," as the antithesis of holoptic. 



But these are not the only differences. Besides size there are 

 cases in which the eye is divided by an obvious structure into an 

 upper and a lower part, as in some of the Bibionidae (Fig. 2, 

 PI. 28). Again, there may be a difference in the size of the 

 facets, those on the upper part and the front of the head being 

 considerably larger than those on the lower part. This character 

 seems in some measure an approximation to the last mentioned. 

 It is found in a surprising number of families, including the 

 Simulidae in the Nemocera, the Stratiomyidae in the Brachycera T 

 and the Syrphidae in the Cyclorrapha. 



In the curious Muscid family of the Diopsidae, the eyes are 

 placed on long stalks which project from the head. Finally, the 

 eyes may be more or less pubescent, which condition, although 

 often common to both sexes, is in many instances a sexual 

 character, as in Bibio, where the male has remarkably long hairs 

 on the eyes (Fig. 13), while those of the female are bare, or only 

 have a few scattered exceedingly minute hairs. 



A microscopic examination of the eyes, in preparations properly 

 cleared, reveals a great difference in the structure, a difference 

 by means of which we can divide all the species of Diptera into 

 two groups. In the first, a markedly chitinous plate is pierced by 

 the facets, or lenses ; this is found in varying degrees of degenera- 

 tion through an immense series of forms, the partial absence of 

 the opaque structure increasing the light-absorbing capabilities. 

 In the second group the chitinous plate and all opaque structures 

 have disappeared. 



Before proceeding to more minute descriptions, I shall re- 

 capitulate these points. 



General characters. 

 All eyes in Diptera either (a) have a chitinous structure 

 enclosing the lenses, or (6) are without such a structure. 



