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. [Illustration: 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. [Illustration: FIG. 243.—Nervous system of the May beetle, _Lachnosterna fusca_: _w^1_, nerve to 1st,—_w^2_, nerve to 2d, pair of wings; _ig_, infraœsophageal ganglion. ] [Illustration: FIG. 244.—The same of the stag-beetle, _Lucanus dama_, where there are 3 thoracic, and 3 separate abdominal ganglia. ] In certain winged insects the process of fusion or degeneration is carried to such an extreme that there are either no abdominal ganglia (Fig. 242, _D_), or their vestiges are situated in the thorax and partially fused with the thoracic ones, as in the May beetle, in which the prothoracic pair of ganglia is separate, while the two other thoracic ganglia are fused with the abdominal, the latter being situated in the thorax; this fusion is carried to a further extent than in any other Coleoptera yet examined. In many Diptera and Hemiptera the abdominal ganglia are either absent or the vestiges are fused with the thoracic ganglia.