Between the ommatidia internally there occur, according to Hickson, pigment cells (Fig. 264, p.g3), each of which stands on the basilar membrane and sends a fine process outwards towards the internal process of the external pigment-cell (p.g2). A long, slender tracheal vesicle also passes in between the retinulæ.
Fig. 265.—Two ommatidia from the eye of Colymbetes fuscus, × 160: a, cornea; b, cone; c, rhabdom; d, basal membrane, with nerve filaments below it: e, iris pigment; f, retina pigment.—After Exner, from Sharp.
The basilar membrane.—This is a thin fenestrate membrane (Fig. 261) separating the cones and rods from the optic tract (Fig. 264, b.m). It is perforated for the passage of tracheal diverticula and of the optic nerve fibrils. It separates the dioptric or instrumental portion of the eye from the percipient portion, i.e. the optic tract.
The optic tract.—This is the optic ganglion of earlier writers, and appears to be the percipient portion of the eye, as opposed to the dioptric portion. If the reader will examine Figs. 249 and 261, he will see that it consists of three distinct ganglionic swellings, i.e. the opticon, epiopticon, and periopticon, whose structure is very complicated. In Musca (Fig. 261) the first ganglionic swelling (opticon) is separated from the brain by a slight constriction, which Berger regards as the homologue of the optic nerve of the other arthropods. It consists of a very fine granular matrix traversed throughout by a fine meshwork of minute fibrillæ, the neurospongium of Hickson. In the young cockroach (Periplaneta) the optic nerve separating the cerebral ganglion from the opticon is much longer in proportion than it is in the adult blow-fly.
Fig. 266.—Periopticon and terminal anastomosis of Agrion, showing the character of the elements of the periopticon (p.op) and the structure of the terminal anastomosis (t.a). 1. The first layer of the terminal anastomosis, consisting of a plexus of fibrils and nerve-cells (n.c). 2. The second layer, in which the fibrils are collected together in bundles. 3. The final optic plexus and nerve-cells. 4. The layer in which the optic fibrils are collected in bundles to be distributed to the retinulæ (r); b.m, basal membrane.—After Hickson.
The second ganglionic swelling (epiopticon, Fig. 261, c.op) is separated from the opticon by a tract of fine nerve-fibrils, which partially decussate; at the decussation two or three larger nerve-cells may be seen. It also contains a few scattered nerve-cells (n.c). The third ganglionic swelling (periopticon, p.op) is separated from the others by a bundle of long optic nerve-fibrils, which cross one another. It is composed of a number of cylindrical masses of neurospongium arranged side by side (Fig. 261, p.op). Between these elements of the periopticon, which do not seem to bear any relation to the number of ommatidia, a single nerve-cell is very frequently seen. The periopticon does not occur in Periplaneta and Nepa (Hickson). The three optic ganglia thus described, together with the cerebral ganglia, are surrounded by a sheath of densely packed nerve-cells.
Bearing in mind the fact that the retinulæ are the nerve-end cells of the fibres passing through the periopticon, it will be well to read the following account, by Hickson, of the terminal anastomosis of the optic fibrils in the periopticon of Agrion bifurcatum, and to examine his sketch (Fig. 266):
“The terminal anastomosis of Agrion may be conveniently divided into four regions. First the region (1) lying nearest to the periopticon in which the nerve-cells are numerous, and the fibrils leaving the periopticon form a complicated plexus; the region (2) next to this, in which the fibrils have collected into bundles separated by spaces occupied by very thin-walled tracheæ in which there are no spiral markings, and lymph-spaces; next, the region (3) in which the fibrils form a final plexus, and in which there are again a considerable number of nerve-cells; and, lastly, the region (4) in which the fibrils are again collected into bundles, separated by spaces containing tracheæ, which perforate the basement membrane to supply the retinulæ.”