9 o 



ENTOMOLOGY 



nerve cords. Typically, there is a gan- 

 glion (double in origin) for each primary 



O l segment, and the connecting cords, or 

 commissures, are paired ; these conditions 

 are most nearly realized in embryos and 

 in the most generalized insects Thysa- 

 f- mx 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 Taba n us 



I (Fig. 112, C) the three thoracic gan- 

 glia have united into a single com- 



jr 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- 



2 rax, is single in the abdomen. Various 

 other modifications of the same nature 



3 occur. 



Cephalic Ganglia. In the head the 



4 primitive ganglia always unite to form 

 two compound ganglia, namely the 



5 brain and the subcesophageal ganglion 

 (disregarding a few anomalous cases 



6 in which the latter is said to be 

 absent). 



7 The brain, or supracesophageal gan- 

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



8 of three primitive ganglia, or neuromeres 

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



brum, which gives off the pair of optic 

 nerves; (2) the deutocerebrmn, which 



Central nervous system 



of a thysanuran, Machilis. nerve; b, brain; e, compound eye; I, labial nerve; m, 



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



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



in succession, a, antennal nerve. After OUDEMANS. 



