2IO CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



pigmenting of the small-facetted eyes. Here are three groups 

 of arthropods, viz., certain crustaceans, May-flies, and flies, 

 widely separated genetically and of widely varying habits, 

 showing a common structural modification of the eyes. 

 We have evidently to do with independent adaptations 

 determined by some common functional need. 



The large size of the ommatidia and the small amount 

 of pigment are characters which adapt the large-facetted 

 eyes for seeing in poor light (in the dark) and for readily 

 perceiving moving objects (delicate perception of shadows). 

 The normal, small-facetted eyes see more accurately the 

 actual shape of visible objects; they have better definition, 

 but require much light. Chun explains that the large- 

 facetted eyes of the pelagic Crustacea enable them to per- 

 ceive their prey (for the Crustacea possessing these eyes 

 are all predaceous) in the poorly lighted levels of the 

 water. The large-facetted eyes of the male May-flies 

 enable them, according to Zimmer's explanation, to per- 

 ceive the advancing female during the twilight marriage 

 flight peculiar to these forms. What is the special use of 

 the large-facetted eyes in the case of Blefharocera ? 



The females are predaceous; they capture other smaller 

 live insects, and, lacerating them with the saw-edge man- 

 dibles and blade-like maxillge, lap their blood. The 

 males, on the other hand, presumably, do not capture 

 insects; they have no mandibles, and are probably nectar- 

 feeding. The female might advantageously be possessed 

 of a number of those large, weakly pigmented eye elements 

 which are specially adapted to the quick j^erception of 

 moving objects. But what makes this explanation less 

 convincing is the fact that the males also possess these 

 large-facetted ommatidia, although, to be sure, in fewer 

 number. Perhaps both males and females are active in 

 twilight. Search as carefully as one might, never but 

 very few of the adult Blepharocera could be found along 

 the stream, from which they were certainly issuing by 

 thousands. Until the habits of our fly are better known, 

 then, it is hardly profitable to speculate on the special use 

 of its large-facetted eyes. 



