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FIG. £41. 
i 6 
7 
8 
9 
10 
Central nervous system 
of a thysanuran, Machilis. 
The thoracic and abdom- 
inal ganglia are numbered 
in succession. a, antennal 
ENTOMOLOGY 
nerve cords. Typically, there is a gan- 
glion (double in origin) for each primary 
segment, and the connecting cords, or 
commissures, are paired; these conditions 
are most nearly realized in embryos and 
in the most generalized insects—Thysa- 
nura (Fig. 111). In all adult insects, 
however, the originally separate ganglia 
consolidate more cr less (Fig. 112) and 
the commissures frequently unite to 
form single cords. Thus in Tabanus 
(Fig. 112, C) the three thoracic gan- 
glia have united into a_ single com- 
pound ganglion and the abdominal gan- 
glia are concentrated in the anterior 
part of the abdomen; in the grasshop- 
per, the nerve cord, double in the tho- 
rax, is single in the abdomen. Various 
other modifications of the same nature 
occur. 
Cephalic Ganglia.—In the head the 
primitive ganglia always unite to form 
two compound ganglia, namely the 
braim and the subesophageal ganglion 
(disregarding a few anomalous cases 
in which the latter is said to be 
absent ). 
The brain, or suprawsophageal gan- 
glion (Fig. 113), is formed by the union 
of three primitive gangha, or meuromeres 
(Fig. 55), namely, (1) the protocere- 
brum, which gives off the pair of optic 
nerves; (2) the deutocerebrum, which 
nerve; b, brain; e, compound eye; 7, labial nerve; m, 
mandibular nerve; ma, maxillary nerve; 0, cesophagus; 
ol, optic lobe; s, subcesophageal ganglion; sy, sympathetic 
nerve.—After OUDEMANS. 
