NERVOUS SYSTEM. 



633 



20 



be borne at the end of elongate processes of the body as in 

 Eristalis and in the Culicidae. Water-insects in the imago 

 state have open stigmata and they come to the surface to breathe. 

 They usually have some arrangement which enables them to 

 entangle beneath their elytra or elsewhere some bubbles of air 

 which they carry beneath the surface and use during their 

 lengthy submer- 

 gence. 



The Nervous 

 System. The cen- 

 tral nervous sys- 

 tem of an insect 

 consists of a brain, 

 an infraoesopha- 

 geal ganglion and 

 a chain of paired 

 ventral ganglia 

 extending through 

 the thorax and 

 abdomen. Typi- 

 cally there is one 

 pair of ganglia to 

 a segment, but 

 they tend to un- 

 dergo a consider- 

 able amount of 

 fusion and sup- 

 pression even in 

 the abdomen. 



Embryologi- 

 calJy the brain * 

 consists of three 

 fused pairs of 



ganglia (Fig. 389), (i) the procerebrum f which forms the optic 

 lobes and supplies the eyes ; (ii) the deutocerebrum which is 

 regarded as primitively postoral but which has become preoral 



* See also pp. 320 and 321. 



t The procerebrum of insects is according to Heymons (Zoologica, 

 xiii, Heft. 33, 1901. "Die Entwicklungsgeschichte der Scolopender," 

 p. 143) the homologue of the protocerebrum sensu stricto, together with 

 the syncerebrum of Myriapoda. 



10' 



14 



FIG. 390. Side view of the head of Melanoplus femur-rubrum, 

 the left wall of which has been removed to expose the 

 brain (after E. Burgess). 1 ocelli and nerves leading 

 to them from the two hemispheres ; 2 antenna ; 3 

 median ocellar nerve ; 4 antennal or olfactory lobe ; 5 

 frontal ganglion of sympathetic nerve ; 6 oesophageal 

 commissure to suboesophageal ganglion ; 7 nerve to 

 labrum ; 8 clypeus ; 9 nerve to mandible ; 10 maxillary 

 nerve ; 11 labial nerve ; 12 labrum ; 13 mandible ; 14 la- 

 bium ; 15 suboesophageal ganglion ; 16 unknown nerve ; 

 17 stomatogastric or sympathetic nerve ; 18 posterior 

 sympathetic ganglia ; 19 anterior sympathetic ganglion ; 

 20 optic lobe sending off the optic nerve ; 21 optic nerve ; 

 22 beginning of oesophagus ; 23 base of first maxillae. 



