The facetted eye is composed of numerous simple eyes called ommatidia, each of which is complicated in structure. The elements which make up an ommatidium are the following: (1) The facet or cornea, which is a specialized portion of the cuticula; and (2), the crystalline lens or cone; (3), the nerve-ending or retinula, which is formed out of the retinula cells and the rhabdom or rod lying in its axis; and (4) of the pigment enclosing the lens and rod; the last three elements are derived from the hypodermis. The single eyes are separated from each other by pigment cells.
The facet or cornea.—This is biconvex, clear, transparent, usually hexagonal in outline, and refracts the light. The corneal lenses are cast in moulting.
The corneal lenses are circular in most cases where they are very convex, as in Lathridius and Batocera. The hexagonal ones are very irregular. When they are very convex the eye has a granular appearance, but when not greater than the convexity of the eye itself, the eye appears perfectly smooth (Bolbocerus, 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, sometimes 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 Colydiidæ being very large, arranged in lines over the eyes like tombstones (Trachypholis).[[45]]
Fig. 261.—Section through the eye of a fly (Musca vomitoria): c, cornea, or facet; pc, pseudocone; r, retinula; Rh, rhabdom; pg1, pg2, pg3, pigment cells; b.m, basilar membrane; T, Tt1, Tt2, 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 Hickson, from Lubbock.
The crystalline lens or cone.—Behind or within the facets is a layer composed of the cones, behind which are the layers of retinulæ 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 Cicadidæ, in those Coleoptera with five tarsal joints, in the dipterous genus Corethra, and in the Lepidoptera and Hymenoptera (Fig. 263).
