ZooL.— Vol. III.] KELLOGG— NET-WINGED MIDGES. 209 



convex. The other is reddish brown, is composed of much 

 larger facets, faces dorsally, and has a nearly flat surface. 

 This red, large-facetted dorsal eye has the appearance of a 

 flattened mushroom head, or thick plate, resting above the 

 other eye. In the males, the dorsal, large-facetted eye is 

 much smaller and less conspicuous than in the female, but 

 both parts of the eye (or both eyes) are plainly present. 

 This difference in the two parts of the eye is more radical, 

 however, than can be discovered by a mere examination 

 from without. The ommatidia, or eye elements, of each of 

 the regions differ, as shown by sections (see PI. XVII, fig. i . ) 

 in many particulars. Corresponding with the difference in 

 size of the facets (the corneal lens of the ommatidia), there 

 is a marked difference in the diameter of the ommatidia 

 from the two regions. The ommatidia of the dorsal, large- 

 facetted eye are nearly twice as wide, and they are fully 

 twice as long, as the ommatidia of the small facetted eye. 

 Another striking and important difference is this, the larger 

 ommatidia are very much less strongly pigmented than the 

 smaller ommatidia. There are, also, some differences in 

 the character of the inner optic "layers" lying between the 

 hypodermal portion of the eye and the brain; characters 

 too technical for discussion here. In sum, however, it is 

 evident that there is so marked a difference in structure 

 between the two eye regions that there must be a difference 

 in exercise of the function. The seeing by one of the eye 

 regions differs from the seeing by the other eye region. 



In a brief discussion elsewhere of the "divided eyes of 

 arthropods" (Kellogg, 1898) reference has been made 

 to the observations of Chun (1896), who has described the 

 structure of the divided eyes of certain pelagic crustaceans, 

 and to the observations of Zimmer (1898), who has studied 

 the divided eyes of certain male May-flies. In both of 

 these cases the eyes show two sizes of facets, and accom- 

 panying this are both those other structural differences 

 which are apparent in Blepharocera, viz., the large omma- 

 tidia and small amount of pigment of the large-facetted 

 eyes as compared with the small ommatidia and heavy 



