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. ]
FIG. 241.—Section through the head of Machilis, showing the brain (_br_), and subœsophageal ganglion (_soe. g_); _cl_, clypeus; _lbr_, labrum; _oc_, ocellus. ]
While the supraœsophageal ganglion, or “brain,” of the insect is much more complex than any other ganglion, consisting more exclusively both of sensory as well as motor ganglia and their nerves, it should be borne in mind that the subœsophageal ganglion also receives nerves of special sense, situated on the palpi and on the tongue, as in the bee and other insects; hence this ganglion is probably complex, consisting of sensory and motor cells. The third thoracic ganglion is also, without doubt, a complex one, as in the locusts the auditory nerves pass into it from the ears, which are situated at the base of the abdomen, while in the green grasshoppers, such as the katydids and their allies, whose ears are situated in their fore legs, the first thoracic ganglion is a complex one. In the cockroach and in Leptis (Chrysopila), a common fly, the caudal appendages bear what are probably olfactory organs, and as these parts are undoubtedly supplied from the last abdominal ganglion, this is probably composed of sensory and motor ganglia; so that we have in the ganglionated cord of insects a series of brains, as it were, running from head to tail, and thus in a still stronger sense than in vertebrates the entire nervous system, and not the brain alone, is the organ of the mind of insects.