498 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 supraesophageum, connecting with a Ganglion infrawesophageum 
which is smaller, by two lateral commissures which embrace the cesophagus. 
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-cesophageal 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 alivays, 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 
coéxistent 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-cesophageal ganglion furnishes 
nerves chiefly to the mandibles, to the maxillae and their palpi. With the 
perfect 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- 
eral, see Burmeister, Handb. &c. I. p. 290; 
Lacordaire, Introd. &c. II. p. 183, Newport, 
Cyclopaed. 11. p. 942, and Blanchard, Ann. d. Sc. 
Nat. V. 1846, p. 273. ZL. Dufour (Mém. prés. a. 
PAcad. d. Sc. IX. 1846, p. 562, Pl. I. fig. 16) has 
given a very exact description of this system in 
Sarcophaga haemorrhoidalis. 
2 Entomotomists are not agreed as to the number 
of the abdominal ganglia, for the sub-cesophageal 
ganglion is sometimes regarded as the first of the 
ventral cord. For the nervous system of the larva 
and_pupa of Sarcophaga, see L. Dufour, loc. cit 
PL. I. fig. 12-15. 
