22 The Structure and Special Physiology of Insects 



FIG. 42. FIG. 43. FIG. 44. 



FIG. 42. Diagram of ventral nerve-cord of locust, Dissosteira Carolina. (After Snod- 



grass; twice natural size.) 

 FIG. 43. Diagram of the nervous system of the house-fly. (After Brandt; much 



enlarged.) 

 FlG. 44. Nervous system of a midge, Chironomus sp. (After Brandt, much enlarged.) 



commissures are in most insects more or less fused to form single ganglia 

 and a single commissure, but in others the commissures, 

 at least, are quite distinct. In the simpler or more 

 generalized condition of the nervous system as seen 

 in the simpler insects and the larvae of the higher 

 ones there are from three or four to seven or eight 

 abdominal ganglion pairs, one pair to a segment, a 

 pair in each of the three thoracic segments, and one 

 in the head just under the oesophagus. From this 

 ganglion (or fused pair) circumoesophageal commis- 

 sures run up around the oesophagus to an important 

 ganglion (also composed of the fused members of a 

 pair) lying just above the oesophagus and called the 

 brain, or supracesophageal ganglion (Figs. 45, 46, and 

 47). From this proceed the nerves to those impor- 

 tant organs of special sense situated on the head, the 

 antennae and eyes. From the subcesophageal gan- 

 glion nerves run to the mouth-parts, from the thoracic ganglia to the 



FIG. 45. Brain, com- 

 pound eyes, and part 

 of sympathetic nerv- 

 ous system of locust, 

 Dissosteira Carolina. 

 (After Snodgrass; 

 greatly magnified.) 



