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



adjacent to them are small areas of chitin, which are not pierced 

 by the customary round lenses (Fig. 11). 



This is well seen in Ulomyia fidiginosa, Mg., to give one 

 instance out of many ; one preparation of an undetermined 

 Psychoda shows a weaker chitinous structure, with larger facets 

 than those on thirteen other heads belonging to three species 

 which I examined. 



In Corethra plumicomis, F., the chitinous structure, though 

 much less than in Cliironomus, is still very marked ; certainly 

 more so than in Mochlonyx velutinus. Iluthe, or Cidex, where 

 it only defines the lenses, which appear to be showing signs 

 of becoming hexagonal. In many species the compound eyes are 

 of the same shape as the plate in Chironomus. 



Of the Ptychopteridae five species were examined (contaminata, 

 L., paludosa, Mg., lacnstris, Mg., cdbimana, F., scutellaris, Mg.). 

 All have the chitinous structure highly developed, but the lenses 

 incline to the hexagonal form. An even stronger structure is 

 found in Gynoplistia bella, West., and it is very similar in Tipida 

 oleracea, L., the sides of the bands separating the facets appearing 

 to be deeper, a point to which I shall again allude when I come 

 to inquire into the uses of these various modifications. 



In Rhyphus fenestrcdis, Scop., and It. punctatus, F., the 

 chitinous structure is very weak, but can be traced, short hairs 

 being inserted at the apex of each angle. 



In those cases in which the lenses are hexagonal with strong 

 chitinous structure, the hexagonal description only refers to the 

 majority of the facets; in many instances they are modified into 

 very irregular shapes to conform to the curvature of the eye-mass. 



This brings us to the end of the Nemocera, and shows us 

 that, although we constantly find variations, especially in the 

 larger families, this chitinous structure of the eye is a very 

 primitive character, persisting through the whole of the sub-order; 

 it appears to have been lost in the genera Biblo and Cidex, yet 

 it is evident in the more primitive Scatopse and Corethra, The 

 circular facet or lens is also a very archaic character. 



