THE COMPOUND OR FACETTED EYE 



251 



cerus, etc.). The facets in the lower part of the eye of Dineutes are a trifle 

 larger than in the upper part (about nine to ten). In many insects the reverse 

 is the case, the upper facets being larger than the lower, a notable instance being 

 Anax. The intervening lines between the facets are often beset with hairs, some- 

 times very long and dense, as in the drone bee and Trichophthalmus ; and the 

 modifications of the hairs into scales which takes place on the body occurs on 

 the eyes also, the scales on the eyes of some beetles of the family Colydiidae 

 being very large, arranged in lines over the eyes like tombstones (Trachypholis). 1 



The crystalline lens or cone. Behind or within the facets is a 

 layer composed of the cones, behind which are the layers of retinulae 



FIG. 261. Section through the eye of a fly (Mwsca vomitoria) : o, cornea, or facet ; pc, pseu- 

 docone ; r, retinula ; Jih, rhabdom ; pg l , pg*, pg s , pigment cells ; b.m, basilar membrane ; T, Tt t , 

 7V 2 , trachea ; tv, tracheal vesicle ; t.a, terminal anastomosis ; op, opticon ; c.op, epiopticon ; 

 p.op, periopticon ; n.c, nuclei; n.c.s, nerve-cell sheath; N.f, decussating nerve-fibres. After 

 llickson, from Lubbock. 



and rhabdoms, and which correspond to the layer of rods and cones, 

 but not the retina as a whole, of vertebrate animals. 



The crystalline lens is, when present, usually more or less conical, 

 and consists of four or more hypodermis-cells. 



The cones are of various shapes and sizes in insects of different 

 groups, or are entirely wanting, and Grenacher has divided the eyes 

 of insects into eucone, pseudocone, and acone. As the pseudocone 

 seems, however, to be rather a modification of the eucone eye, the 

 following division may be made : 



1. Eucone eyes, comprising those with a well-developed cone. 

 They occur in Lepisma, Blatta (Fig., 262), and other Orthoptera, 

 in Neuroptera, in Cicadidse, in those Coleoptera with five tarsal 

 1 Waterhouse, Trans. Ent. Soc., London, 1889, p. xxiv. 



