The general arrangement of the nerve-matter in the sub-kingdom Articulata, to which all true insects belong, is that of a double cord, with knot-like protuberances, called ganglia, at more or less regular intervals. The two filaments are in some cases close together: in others, quite distinct; while the larger nerve-masses—the previously mentioned ganglia—also vary in juxtaposition, according to the greater or less importance of the functions they regulate. In the illustration of the larva of Sphinx ligustri (the privet hawk-moth) ([Fig. 22]), the nervous cord is nearly uniform throughout its length, though at its upper portion a separation takes place into three loops. The ganglia also occur at almost equal distances. A very similar disposition of the nerve-structure is seen in the larval condition of the bee; but we may note the absence of loops, the larger development of the cephalic masses, without the separation of their filaments to inclose the gullet together with a more plainly-defined distance between the cords which run parallel through the rest of the body.
Fig. 24.—Nervous System of Perfect Insect.
In the perfect insect we observe some decided modifications to have taken place. The head portions have grown proportionally larger, and show a loop for the passage of the œsophagus, while two large ganglia in the thorax indicate the seat of impressions and impulses connected with the organs of motion—wings and legs—which had no existence in the larval condition. As the functions of the abdominal region, viz., those of digestion and circulation chiefly, remain much the same in the different states through which the individual passes after the hatching of the egg, we find, as we might expect, little change in the nervous system of the posterior segment of the body.
From each nerve-mass will be observed filaments branching on either side to the outer edges of the body. By means of these communication is kept up between all parts of the frame. Sensations are received and conveyed to the sensorial organs, and return-stimuli are sent to the organs whose movements depend for regulation on the different ganglia. This branching of the nerve-fibre is directly proportional to the variety and force of the several functions subserved by the various structures to which they proceed.
The Head.—We will now describe in some detail the structure and functions of the highly-important organs contained in the anterior segment, or head. And first in order let us take the
Eyes.—On either side of the head may be observed an oval lobe, convexly rounded and immovable, brown in colour, covered with a horny tunicle, and exhibiting to the unassisted eye a vast number of distinct points. These points, under a high-power magnifying-glass, are seen to be facets, hexagonal in shape, so as to occupy all available space, without interstices, and each connected with a minute tube and a thread of nerve-matter leading to the cephalic ganglia or brain. These compound eyes, as they are called, are common to most true insects. They may be easily seen in flies, bluebottles, moths, butterflies, &c. The numbers of the facets vary greatly in different families of the Articulata. In the common house-fly there are, it is stated, about 4,000; in the white cabbage-butterfly, 17,000; in the dragon-fly, 24,000. It has been computed that in each compound eye of the bee there are about 3,500 of them.
Fig. 25.—Eyes of a Bee, Greatly Magnified.
Behind the horny covering, or cornea, which consists of two plano-convex lenses, is a layer of dark pigment, which gives the characteristic colour to these eyes. This is pointed like the neck of a vase, and serves the purpose of the iris in the higher animals. This is traversed by a minute aperture or pupil, through which the rays pass by a longer conical lens to the optic nerve. A vertical section shows that each ocellus (or little eye) is the frustum of a pyramid, the large end or base of which is bounded by the cornea, while the other and pointed end terminates against an expansion of the optic nerve. The eminent physiologist, Dr. W. B. Carpenter, says, in describing the minute structure of these organs: "The interior of this pyramid is occupied by a transparent substance, which represents the vitreous humour (of the eyes of vertebrates), and the pyramids are separated from each other by a layer of dark pigment, which completely incloses them, save at the pupillary apertures, and also at a corresponding set of apertures at their smaller ends, where the pigment is perforated by the fibres of the optic nerve, of which one proceeds to each separate eye.