CHAP. VI.] NERVOUS SYSTEM OF ARTHROPODS. 



105 



all Insects with a separate sub-oesophageal ganglion, from 

 which nerves are given off to the mandibles, the maxillae, 

 and the labium. But, as in Spiders, the oesophageal ring is 

 often very narrow, owing to the greatly diminished size of 

 the O3sophagus in the imago forms of higher Insects. In 

 Spiders and Myriapods, as before stated, the sub-oeso- 



FIG. 43. 



FIG. 44. 



FIG 45 



FIG. 43. Nervous System of a White Ant (Termes}. (Gegenbauer after Lespes.) 



FIG. 44. Nervous System of a Water Beetle (Diitiscus). (Gegenbauer.) 



FIG. 45. Nervous System of a Fly (Musca). (Gegenbaner after Blanchard.) o, 



Eyes ; gs, supra-oesophageal ganglia (brain) ; gi, sub-oesophageal ganglion ; gr, g*, g 3 



fused ganglia of the thorax. 



phageal ganglion has no separate existence apart from 

 the thoracic ganglia. 



In many Insects the three thoracic ganglia preserve 

 a separate existence (fig. 43), though in others of tho 

 higher types above referred to these ganglia are more 

 frequently fused into a single lobed mass (fig. 45). The 



