9 o 



ENTOMOLOGY 



FlG. III. 



Ol 



! a 



Central nervous system 

 of a thysanuran, Machilis. 



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. in). In all adult insects, 

 however, the originally separate ganglia 

 consolidate more or less (Fig. 112) and 

 the commissures frequently unite to 

 form single cords. Thus in Tabanns 

 (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 

 brain and the subcesophageal ganglion 

 (disregarding a few anomalous cases 

 in which . the latter is said to be 

 absent). 



The brain, or supracesophageal gan- 

 glion (Fig. 113), is formed by the union 

 of three primitive ganglia, or neuromeres 

 (Fig. 55), namely, (i) the protocere- 

 brum, which gives off the pair of optic 

 nerves; (2) the deutocerebrum, which 



nerve; b, brain; e, compound eye; /, labial nerve; m, 



The thoracic and abdom- mandibular nerve; nix, maxillary nerve; o, oesophagus; 

 inal ganglia are numbered ol, optic lobe; s, subcesophageal ganglion; sy, sympathetic 

 in succession, a, antennal nerve. After OUDEMANS. 



