6 



DIPTERA 



facetted eve is much smaller and less conspicuous than in the female, but both parts of the eye (or both 

 eyes) are plainl}' present. This difference in the two parts of the eye is more radical, however, than can 

 be discovered b}- a mere examination from without. The ommatidia, or eye elements, of each of the 

 regions differ, as shown by sections, in several particulars. Corresponding with the difference in size of 

 the facets (the corneal lenses 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 that 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 h\podermal 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 difterence in exercise of the function. The seeing b}' one of the eve regions 

 differs from the seeing by the other eye region. 



» In brief discussion elsewhere of the « divided eyes of Arthropods » (KeWogg, Zoo I. Anzeig. i8g8) 

 reference has been made to the observations of Chun {Bibliotheca Zool., 1896). who has described 

 the structure of the divided eyes of certain pelagic crustaceans, and to the observations of Zimmer 

 {Zeitschr. f.wiss. Zool., Bd. 63, p. 236-202, i8g8), who has studied the divided eyes of certain male Mav-flies. 

 In both of these cases the eyes show two sizes of facets, and accompanying this are both those other 

 structural difference which are apparent in Blepharocera viz., the large ommatidia and small amount of 

 pigment of the large-facetted eyes as compared with the small ommatidia and heavy pigmenting of the 

 small-faceted eyes. Here are three groups of Arthropods, viz., certain crustaceans, certain !\Iay-flies, 

 and the Blepharoceridae, widely separated geneticalh' and of widely varying habits, showing a common 

 structural modification of the eyes. We have evidenth' lo 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 e3-es for seeing in poor light (in the dark) and for readiU- perceiving moving objects 

 (delicate perception of shadows). The normal, small-facetted eyes see more accurately the actual shape 

 of visib'e objects ; they have better definition, but require much light. Chun explains that the large- 

 facetted e3^es of the pelagic Crustacea enable them to perceive 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 Ma^'-flies enable them, according to Zimmer's explanation, to perceive the advancing fem.ale during 

 the twilight marriage flight peculiar to these forms. What is the special use of the large-facetted eyes, in 

 the case of Blepharocera ? 



» The females are predaceous; they capture other smaller live insects, and, lacerating them with 

 the saw-edge mandibles and blade-like maxillae, lap their blood. The males, on the other hand, presu- 

 mably, do not capture insects; the}- have no mandibles, and are probably nectar-fc-eding. The female 

 might advantageousl}' be possessed of a number of those large, w-eakly pigmented eye elements w'hich 

 are specially adapted to the quick perception 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 few-er 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 ». 



Distribution. — All the six species of Blepharoceridae so far found in Europe occur in Italy or 

 adjoining islands; but most of them occur also elsewhere in southern or central Europe. Of the eleven 



