The existence of the secondary optic nerves (6) and common retina (c) is likewise disputed by Muller and Duges, who consider the proper optic nerves to arise immediately from the surface of the brain.

Structure of the eye of a Cockchafer. A, sectional view: a, optic ganglion.

Fig. 166. Structure of the eye of a Cockchafer. A, sectional view: a, optic ganglion; b, secondary nerves; c, general retina, in front of which is a layer of pigment, d; e, proper optic nerves, supplying the individual facets of the compound eye. B, a group of ocelli, much magnified: f, bulb of optic nerve; g, layer of pigment; h, vitreous humour; i, cornea.

(874). With regard to the wonderfully complex structure of these organs, Straus-Durckheim suggests that, the eyes of insects being fixed, nature has made up for their want of mobility by their number, and by turning them in all directions; so that it might be said that these little animals have a distinct eye for every object. But here we are naturally tempted to inquire whether insects see at the same time distinctly with every one of these eyes, or if they distinguish with one eye only. Upon this point Straus-Durckheim observes that, if they saw clearly with all, the great number of images would necessarily produce confusion, and would prevent creatures so organized from paying special attention to any determinate point. It is probable, therefore, that one ocellus only is at any given time placed in circumstances precisely adapted to the complete examination of an object, the animal seeing things imperfectly with the rest, in the same manner as we see objects situated nearer to us or further off than that upon which we fix our attention; so that, according to this supposition, insects would see very distinctly with one eye only, exactly as we see confusedly an extensive landscape, although we only distinguish a small part of it.

(875). In all insects the sexes are quite distinct; and the generative apparatus, both of the male and female, consists of various secreting organs with their excretory ducts: in the male, such glands furnish the impregnating secretions; and in the female give origin to the ova, and provide the covering wherein the eggs are enveloped.

(876). Commencing with a description of the male organs, we find in the Cockchafer various parts represented in the accompanying figure, taken from the admirable work of Straus-Durckheim already so often quoted. The testicles of Melolontha (fig. 167, a a) are six in number on each side of the body; but, in the engraving, those of one side only are delineated. Every testis consists of a vesicular organ, hollow internally, which, being immersed in the juices of the insect, separates therefrom the seminal fluid. Six ducts (b b b) may be called vasa deferentia, and convey the spermatic liquor into a common canal (c c), of very considerable length and much convoluted. Although slender at its commencement, this tube ultimately expands into a wider portion (d), wherein, no doubt, the semen accumulates, and which has been called by authors the vesica seminalis.

Male generative organs of Melolontha vulgaris: a a a, testicular glands.

Fig. 167. Male generative organs of Melolontha vulgaris: a a a, testicular glands; b b b, vasa deferentia; c c, common canal; d, its dilated portion; d', termination of the corresponding canal from the opposite side; eef,f, auxiliary glands; gg, common duct enclosed in a sheath, h; ii, ejaculatory apparatus; 11, nn, section of penis.

(877). The canal (d) terminates by joining the corresponding duct from the opposite side (d') to form a common tube (g); but just at the point of junction they are joined by two long auxiliary vessels (f,f), that have been named sperm-vessels, gluten-vessels, and gum-vessels by different authors, but which appear to be appropriated to the production of some fluid, perhaps analogous to the prostatic fluid of mammalia, whereby the bulk of the seminal liquor is increased in order to facilitate its expulsion. Each of these auxiliary vessels consists of two parts - a long and much-convoluted portion (e e e), forming the secreting organ; and a dilatation (f), that must be looked upon as a reservoir for the fluid elaborated. The common canal (g) receives all these secretions: it is at first enclosed in a kind of sheath (h), but, soon becoming muscular, it dilates into a strong contractile canal (g, i), called the ductus ejaculatorius, which is continued to the extremity of the penis.

(878). The intromittent organ itself is composed of two parts - a pro-trusible corneous tube (l l), and an external horny sheath (n n), in which the former is usually concealed and protected.

(879). Great variety, of course, is found in the number, form, and general arrangement of all the parts alluded to in the above description, when examined in different insects. In the Hive-bee, for example, the testes (fig. 168, a) are only two in number, and are simple oval vesicles; the vasa deferentia (b b) are short, and the seminal receptacles (c) form membranous sacculi. The auxiliary secreting organs (d), although placed in the same position as in Meloloniha, are represented by capacious caeca; while the common excretory duct (e) swells into a strong and muscular bag (f), which constitutes the ejaculatory apparatus. Still, however, it is easy to see that, although diversified in appearance, the parts here found are essentially similar to those met with in the Cockchafer, and represent respectively the same organs.

(880). The female apparatus of reproduction presents a general correspondence, both in form and arrangement, with the sexual parts of the male insect. The ovaria are simple secreting sacculi, or elongated tubes, in which the germs or ova are produced instead of the seminal liquor; and the excretory canals, or eggpassages, with the organs appended to them, although appropriated to different functions, strikingly resemble the organs met with in the other sex.