14 



INTEODUCTIOJf. 



they consist of a number of ramifying tubes united on each side 

 of the oesophagus into a siugle duct. The silk glands are pro- 

 bably modified salivary glands. They consist of very long tubes 

 similar in form and situation to the simple tubes of the salivary 

 glands, and are found chiefly in the larvae of the Lepidoptera, but 

 also occur in certain Cheysomeliu^ (Donatio, and Hcemomd) and 

 in Hypera among the Cueculionidje. 



The Nervous System. 



The nervous system consists primarily of a series of ganglia or 

 nerve-centres united by one or two cords of nervous matter. The 

 whole system is very complex and comparatively little is known 

 with regard to many of the minor details. It may conveniently 

 be treated as consisting of the three following divisions : — 



1. The ganglia of the head, sometimes called the cephalic system. 

 Of these ganglia there are two, a large one above the oesophagus, 



B 



Fig. 7. — Nervous system, (A) of Serica brunnea, (Scarabajida.'), showing the 

 concentration of the ganglia, and (B) of Dictyopterus sanguineus, $ 

 (Lycicla3), shewing the decentralisation of the ganglia. (After Brandt.) 



called the supra-cesophageal ganglion, and a small one below the 

 oesophagus, called the infra- or sub-oesophageal ganglion. In the 

 Coleoptera and many other insects these are very closely approxi- 

 mated. They may be regarded as part of a single great ganglionic 

 chain, but are best dealt with separately owing to their complex 



