THE CANADIAN ENTOMOLOGIST 275 



THE NERX'OIJS SYSTEM OF THE LAR\"A OF STIIENOPIS THULE 



STRECKER. 



BY J. M. SWAINE, 

 Entomological Branch, Ottawa. 



The Nervous System of insects is usualh- treated under two divisions, 

 the Central System and the Sympathetic System. The two are so very intimate- 

 ly connected in the caterpillars of the Lepidoptera that there would appear to be 

 little reason for separating them. 



The Central S^ stem. 



There are included h^re the cerebral ganglia and their commissures, with 

 the continuing ventral chain of ganglia and connectives, together with the chief 

 nerves to which they give rise. 



The supraoesophageal ganglion, or brain, lies within the head upon the 

 dorsal wall of the pharynx; it is connected by the crura cerebri, stout connectives 

 which pass around the pharynx, to the suboesophageal ganglion lying immediate- 

 ly below the fore-intestine. The nerves arising from these two cerebral ganglia 

 furnish practically all the nerv^e supply to the head. 



The suboesophagea.1 ga.nglion forms the first of a single chain of ganglia and 

 connectives lying upon the ventral muscles along the median line. There is 

 one ganglion in each thoracic .segment and one in each of the first eight abdominal 

 segments. The ganglia are jointed together by connectives, the whole forming a 

 single median chain. The primitive double connectives have been completely 

 fused into a single stout cord throughout the entire length of the chain; even in 

 the thorax, where all caterpillars outside the JugatT have double connectives, they 

 are as firmly fused as in the abdomen. The only indication of the primitive 

 double nature of the cord is shown in the slight longitudinal split in the con- 

 nective immediately in front of each ganglion, from which the median nerve 

 arises. The fusion of the connectives in the thorax is a most interesting character, 

 the more so since it appears in conjunction with the presence of a distinct eighth 

 abdominal ganglion. 



Most caterpillars have only seven abdominal ganglia, with the last more 

 or less evidently composite. In thule and argenieo7nani!aius the eighth ganglion, 

 composite in itself, is separated from the seventh by a connective nearly as long 

 as that between the seventh and the sixth ganglia. 



Each ganglion of the ventral chain lying behind the suboesophageal gives 

 to its segment typically two pairs of nerves. The anterior pair evidently 

 represent the primitive lateral nerves of the connectives, which in this group 

 have migrated backwards until they appear now as the nerves from the ganglia. 

 The nerves of the last ganglion, three pairs in all, are discussed below. 



The Brain. 



PI. X, Fig. 8. 

 -The Brain, or the Supraoesophageal (ianglion, is situated on the meson 

 a little behind the middle of the head and well below the dorsal wall. It is 

 \"er>' distincth' bilobed, rounded before and behind, and towards the front 

 extended latero-ventrad to the nerves and the crura. The median line is deeply 

 impressed so that the appearance is decidedly that of two ganglia united along the 

 middle line. 



December, 1920 



