^08 THE INSECTA. -§« 328. 



CHAPTER III 



NERVOUS SYSTEM. 



§ 328. 



The central parts of the nervous system consist, with the Insecta, as with 

 the other Arthropoda, of a Brain and a Ventral Cord.^^' 



The brain is situated in the cephalic segment, and is composed of a 

 Ganglion su'pratESo'phagextm, connecting with a Ganglion infracesophageum 

 which is smaller, by two lateral commissures which embrace the oesophagus. 

 "The first of these ganglia corresponds to the cerebrum of the Vertebrata ; 

 and the second is comparable perhaps to the cerebellum or spinal cord. 



The ventral cord succeeds upon the sub-oesophageal ganglion, and con- 

 sists, sometimes of a single ganglionic mass, sometimes of a chain of ganglia 

 more or less approximated and connected by double, longitudinal commis- 

 ;sures.*-^ The number of the ventral ganglia, which is never greater than 

 that of the segments of the body, as well as the presence and length of the 

 longitudinal commissures, depend often upon the number, the size, and the 

 mobility of the segments of the body to which they belong. With those 

 Insecta whose segments are very short and rigid, the ganglia are closely 

 approximated or even entirely blended together ; while, in most larvae, 

 where the segments of the body are equally developed and flexible, the 

 ganglia are separate, nearly equal in size, and connected by pretty long 

 commissures. These last are rarely united into a single cord, although the 

 ganglia, not only those of the ventral cord, but also those of the brain, 

 appear, nearly always, to be composed each of two united ganglia. 



Aside from the differences presented according as the insect may be a 

 larvae, a pupa, or an imago, the nervous system varies so much even in 

 the same group, that it may be quite dissimilar in species which, in other 

 respects, are very closely allied. These modifications refer to the number 

 •of the ganglia, the length of their commissures, and the more or less com- 

 plete fusion between certain ganglia. 



The superior cerebral ganglion, which is often composed of two hemi- 

 spheres more or less fused together, gives off the two antennal and the two 

 optic nerves. The simple eyes or stemmata, either when alone, or when 

 coexistent with compound or faceted eyes, always receive their nerves from 

 the same ganglion ; although these last are sometimes given off from a trunk 

 in common with the optic nerve. The sub-oesophageal ganglion furnishes 

 nerves chiefly to the mandibles, to the maxillae and their palpi. With the 

 ]ierfect Insecta, the three thoracic are much more voluminous than the ab- 

 dominal ganglia. They send nerves not only to the legs, but also to the 

 muscles of the wings. 



1 For the nervous system of the Insecta in gen- 2 Entomotomists are not apreed as to the numher 



•eral, see Hurmei.tter, llamll). &c. I. p. 290 ; of the abdominal ganglia, for tlie sub-oesophageal 



X,acordaiTe, Introil. &c. II. p. 183 , Newport, ganglion is sometimes regarded as tlie first of tlio 



<'yclopaed. II. ]>. 942, and Jllanchard, Ann. d. Sc. ventral cord. For the nervous system of the larva 



Nat. V. 1846, p. 27:j. L. Diifour (Mem. ])rt'S. t. and pui)a of Sarcvp/iaga, see L. Du/our, loc. cit. 



I'Acad. d. Sc. I.V. 1840, p. 5(52, I'l. I. fig. 16) has I'l. I. fig. 12-15. 

 j;iven a very exact descrijjtion of this system in 

 Harcopkaga haemorrhoidalis. 



