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The presence of two kinds of facetted eyes in pelagic Crustacea, the 

 males of Ephemeridae, and males and females of Blepharocera (certain 

 other Nematocerous Diptera, as Simulmm) show also the two sizes of fa- 

 cets and possess probably the corresponding structural differences in 

 eye-elements), and the general correspondence of the morphologic cha- 

 racters of the differences between the two sorts of eyes in these three 

 widely separated groups of Arthropoda offer an interesting subject of 

 attention. In the case of the Crustacea the Dunkelaugen enable the 

 animal to see its prey (for the Crustacea possessing these divided eyes 

 are all predaceous) in the poorly lighted water levels below the surface. 

 The large facetted eyes of the males of Chloe are also true »Dunkel- 

 augen«, and serve the insects (according to Zimmer's explanation) for 

 perceiving the females during the twilight marriage flights characte- 

 ristic of the genus. The large-facetted eyes of the males of other ge- 

 nera OÎ Ephemeridae are not true »Dunkelaugen« but are a less diffe- 

 rentiated sort adapted especially for the perception of moving objects. 

 They enable the males to recognize the dancing females. In the case 

 of Blepharocera the large-facetted eye-regions probably serve for the 

 perception of moving prey. The females oi Blepharocera are dimorphic 

 and one form only possesses divided eyes, this form having mouth parts 

 (with piercing mandibles) adapted for blood-sucking, while the other 

 form with only the normal small-facetted eyes has no mandibles and 

 is nectar-feeding. The males have also piercing mandibles and are 

 blood-sucking (unusual among Diptera), and have, as already noted, 

 divided eyes. In other words the predaceous males and females of Ble- 

 pharocera have facetted eyes of two kinds while the nectar-feeding fe- 

 males have only the usual small-facetted, strongly-pigmented eyes. 



There is necessary a study of other insects with divided eyes be- 

 fore a satisfactory explanation of this peculiar condition is possible. 

 While in the case of the pelagic Crustacea and of the males of Chloe 

 the necessity of perceiving moving objects by twilight or in dimly 

 lighted water, affords a probable explanation of the extreme differen- 

 tiation of the eye-elements, the lesser differentiation shown in the case 

 of the other Ephemeridae and oî Blepharocera apparently depends on 

 the need of the recognition of moving objects in the light, in one case 

 for the discovery of the females during the mating flight, in the other 

 for the discovery of the flying prey. 



It seems that the Arthropod eye is enabled to overcome to some 

 extent the disadvantages incident to its usual or normal structural 

 condition by a differentiation of the eye-elements. By this differen- 

 tiation there is made possible a certain adaptability of the vision to 

 varying conditions of focus and intensity of light. 



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