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.
The simplest, most primitive form of the nervous system of insects is seen in that of the Thysanura. That of Campodea has not yet been fully examined, but in that of the more complicated genus, Machilis (Fig. 239), we see that there is a pair of ganglia to nearly each segment, while the brain (Fig. 241) is composed of three lobes, viz. the optic, the cerebral (Fig. 239, g), behind which is the antennal lobe, from which the antennal nerve takes its origin. Behind the opening for the throat (oe) is situated the first ganglion of the ventral cord, the subœsophageal ganglion, which gives rise to the nerves supplying the jaws and other mouth-parts.
Fig. 242, A-D.—The nervous systems of 4 genera of Diptera, to demonstrate their various degrees of fusion of ganglia: A, non-concentrated more primitive nervous system of Chironomus plumosus, with 3 thoracic and 6 abdominal ganglionic masses. B, nervous system of Empis stercorea, with 2 thoracic and 5 abdominal ganglionic masses. C, nervous system of Tabanus bovinus, with 1 thoracic ganglionic mass, and the abdominal ganglia closely approximated. D, highly modified nervous system of Sarcophaga carnaria, in which all the ganglia of the ventral cord behind the subœsophageal ganglion are fused into a single ganglionic mass.—After Brandt, from Lang.
In the Collembola, which are retrograde Thysanura, there are from one (Smynthurus), to three or four ventral ganglia.
In the winged insects, where the ganglia are more or less fused, the fusion taking place in the head and at the end of the abdomen; there are in the more simple and generalized forms, such as Ephemera, the grasshopper, locusts (Fig. 240), etc., thirteen ganglia besides the two pairs of compound ganglia in the head, three pairs of thoracic ganglia, and usually from five to eight pairs of ganglia in the abdomen.
Fig. 243.—Nervous system of the May beetle, Lachnosterna fusca: w1, nerve to 1st,—w2, nerve to 2d, pair of wings; ig, infraœsophageal ganglion.
