1900] ENTOMOLOGICAL NEWS. 317 



who has described the structure of the divided eyes of certain 

 pelagic crustaceans, and to the observation of Zimmer,;}: who 

 has studied the divided eyes of certain male Mayflies. In both 

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

 ing this are both those other structural differences which are 

 apparent in Blepharocera, viz.: the large ommatidia and small 

 amount of pigment of the large faceted eyes as compared with 

 the small ommatidia and heavy pigmenting of the small faceted 

 eyes. Here are three groups of arthropods, certain crusta- 

 ceans, May flies and flies, widely separated genetically and of 

 widely varying habits, showing a common structural modifica- 

 tion 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 facted eyes for 

 seeing ia poor light (in the dark) and for readily perceiving 

 moving obj ects (delicate perception of shadows) .* The normal 

 small faceted eyes see more accurately the actual shape of vis- 

 ible objects ; they have better definition, but require much 

 light. Chun explains that the large faceted eyes of the pelagic 

 Crustacea enable them to perceive their prey (for the Crustacea 

 possessing these eyes are all predaceous) in the i)oorly lighted 

 levels of the water. The large faceted eyes of the male May 

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

 ceive the advancing female during the twilight marriage 

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

 large faceted eyes in the case of Blepharocera f 



The females are predaceous; they capture other smaller 

 live insects, and, lacerating them with the saw-edged mandi- 

 bles and blade-like maxilhe, lap their blood. The males, on 

 the other hand, presumably, do not capture insects ; they have 

 no mandibles and are probably nectar- feeding. The females 

 might advantageously be possessed of a number of those large, 

 weakly pigmented eye elements which are specially adapted 



X Zimmer, Carl, "DieFacettenagueuder Ephemeriden/'Zeitsch. 

 f. wiss. Zoologie, 1898, Bd. 63, pp. 236-262, pi. xii-xiii. 



* To present the proof of this statement, which is well based on 

 the researches of Exner and others, would require a technical dis- 

 cussion which space and the character of this paper forbids. 



