FIG. 263.—Two separate elements of the eucone eye of a bee; _Lf_, cornea; _n_, nucleus of Semper; _Kk_, crystalline cone; _Pg_, pigment cells; _Rl_, retinula; _Rm_, rhabdom.—After Grenacher, from Lubbock. ] [Illustration: FIG. 264.—Three ommatidia of a pseudocone eye, diagrammatic: _A_, a separate ommatidium of _Musca vomitoria_, semi-diagrammatic: _c_, cornea; _p.c_, pseudocone; _pg′_, pigmented cells surrounding the pseudocone; _p.g_{2}_, additional pigment cells; _p.g_{3}_, basal pigment cells; _n.p.c_, nuclei of pseudocone; _r_, retinulæ; _n.r_, _n.r′_, nucleus of retinulæ; _R_, rhabdom; _b.m_, basal membrane; _t.a_, terminal anastomosis sending nerve-fibrils to the retinulæ. _B_, section through a retinula and rhabdom near the basal membrane, the six retinulæ (_r_) fused into a tube ensheathing the rhabdom (_R_).—After Hickson. ] _a._ Pseudocone eyes; in which, instead of the crystalline lens or cone, there are four cells filled with a transparent fluid medium, and a smaller protoplasmic portion containing a nucleus (Muscidæ, Fig. 264, _pc_). Hickson states that the difference between the eucone and pseudocone eyes lies in the fact that in the pseudocone eye “the refracting body formed by the cone-cell lies behind the nuclei,” and in the eucone eye in front of it. 2. _Acone eyes_, where the cone or refracting body is wanting, but is represented by the four primitive cone-cells. Acone eyes occur in Forficulidæ, Hemiptera (except Cicadidæ), the nematocerous Diptera (Tipula, etc.), and those Coleoptera which have less than five tarsal joints. =The retinula and rod.=—The retinula is morphologically a nerve-end cell, situated at the end of a nerve-fibril arising from the optic nerve. The elements of the retinula of Musca are six in number and surround the _rhabdom_ (Fig. 264), which consists of a bundle of six long, delicate chitinous rods, more or less firmly united together (Fig. 264, _R_). The six elements of the retinula of Musca are in their outer or distal portion free from one another, but towards their base are fused into a sheath (Fig. 264, _r_). They are true nerve-end cells, as shown by Müller and by Max Schultze, their views having been confirmed by Grenacher and by Hickson. The relations of the nerves to the rods after passing through the basal membrane is seen in Fig. 266. =The pigment.=—The cones or pseudocones are mostly buried in pigment, as well as the rods; and the pigment forms two layers. The outer of the two layers is called the iris pigment (Fig. 265, _e_, _iris tapetum_), and the inner (_f_) the retinal pigment. Between the ommatidia internally there occur, according to Hickson, pigment cells (Fig. 264, _p.g_{3}_), each of which stands on the basilar membrane and sends a fine process outwards towards the internal process of the external pigment-cell (_p.g_{2}_). A long, slender tracheal vesicle also passes in between the retinulæ. [Illustration: