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



exclusively to the male sex ; it seems to me to have arisen in 

 much the same way as the double eye of Bibio. 



The differences in the size of the facets of the male eyes have 

 been noticed and made use of as specific characters. Schiner 

 says that the lower facets of the male of Tabanus sudeticus, Zli\, 

 are very much smaller than the upper — " Untere Facetten des 

 Mannchen auffallend kleiner als die oberen " * — and mentions a 

 similar character in seven other males of his genus Tabanus ; but 

 he does not give it in Haematopota, where, however, it is a 

 microscopic character. I have already noticed the remarkable 

 nature of the differences in H.plmnalis, and I have made a number 

 of observations which show that the character is not uncommon. 



Bibio and Dilophus have already been described. Simulium 

 omatum, F., has the facets in front of the head very much larger 

 than those lower down (Figs. 6, 7). 



The following males in the Stratiomyidae all have a similar 

 character: Chloromyia formosa, Scof., Michrochrysa polita, L., 

 and Chorisops tibialis, Mg. 



In Leptis conspicua, Mg., the front lenses are slightly larger 

 than the lower. 



In the Syrphidae I can point out a number of cases : Platychirus 

 peltatus, Mg., Cnedemon vitripennis, Mg., Syrphus ribesii, L., 

 Rhingia campestris, Mg., Hemophilus pendulus, L., Xylota segnis, L., 

 and Syritta pipiens, L. These vary in proportion from being only 

 slightly larger in Helophilus pendulus to being more than double 

 the size in Syritta pipiens, the difference being thirteen units in 

 the largest and six in the smallest. 



4. Eyes Situated on Lateral Projections. 



This is a character of the Diopsidae, a Muscid family, and 

 therefore not only specialised as to eye-structure, but also in this 

 curious adaptation. In the males of some species the eyes are 

 placed so far apart that a measurement from eye to eye shows 

 a greater distance than a measurement from the head to the 



* Die Fhetjen, vol. i. p. 34, 1SG2. 



