a. The nervous system as a whole
Fig. 239.—Central nervous system of Machilis maritima: au, eye; lo, optic tract; g, brain; an, antennal nerve; oe, œsophagus passing between the œsophageal commissures; usg, infraœsophageal ganglion; I-III, thoracic ganglia; 1–8, abdominal ganglia, the last (Sabc) consisting of three fused ganglia; s, sympathetic nervous system of the ventral cord.—After Oudemans, from Lang.
The nervous system of insects consists of a double series or chain of ganglia connected by nervous cords or commissures. The first of these is the brain or supraœsophageal ganglion; it is situated in the upper part of the head, above the gullet or œsophagus, while the rest of the system, called the ventral cord, lies on the floor of the body, under the digestive canal.
A ganglion or nerve-centre consists of a mass of ganglion-cells, from each of which a process or fibre passes off, uniting with others to form a nerve; by means of these nerves the ganglia are connected with other ganglia, and with the sensory cells and muscle-fibres. The ganglia may be simple, and arranged in pairs, corresponding to each segment of the body, or they may be compound, the result of the fusion of several pairs of ganglia, which in the early stages of the embryo are separate. Thus the brain of insects is a compound ganglion, or ganglionic mass.
The nerves are of two kinds: 1. Sensory, which transmit sensations from the peripheral sense-cells to the ganglion, or brain; 2. Motor, which send stimuli from the brain or any other ganglion to the muscles.
Of ganglion cells, some are tactile, and others give rise to nerves of special sense, being distributed to the eyes, or to the organs of hearing, smell, taste, or touch.
Fig. 240.—Nervous system of Melanoplus spretus: sp, supraœsophageal ganglion, sending off the large optic nerve (op) to the eyes, and an ocellar nerve to each ocellus (the dotted line oc stops short of the left ocellus); if, infraœsophageal ganglion; 1, 2, 3, thoracic ganglia; 1–5, five abdominal ganglia (the fifth the largest, and sending branches to the ovipositor, etc.) The sympathetic nerve and ganglia are represented by the two main nerves which arise from the medio-cephalic (as) resting on and above the œsophagus, and two ganglia (ps) on the under side of the crop. From each of these ganglia, two nerves are sent under the crop, and a larger nerve on each side to as far as the stomachal cæca, ending the figure at the dotted line 2, near the second thoracic ganglion. u, a round, shining body, connected by a nerve with the medio-cephalic ganglion, its nature unknown.
