INSECTA. 



943 



two distinct columns of fibres, placed one upon 

 the other, and closely united together in every 

 instance. The under or external column, 

 which is nearest to the exterior of the body, is 

 that in which the ganglia or enlargements are 

 situated. The upper one, or that which is 

 internal and nearest to the viscera, is entirely 

 without ganglia, and passes directly over the 

 ganglia of the under column without forming 

 part of them, but in very close approximation 

 to them. In some species, as in the larvae of 

 Timarcha tenebricosa, (Jig- 404, 2, 3, 4,) and 

 Proscalabteus vulgaris, among the Coleoptera, 

 and of the Bee and other Aculeate Hymenop- 

 tera, this column is more apparent than in the 

 larvae of Lepidoptera, in which it is indistinctly 

 seen, excepting when it is beginning to pass 

 over or is just leaving the surface of a ganglion. 



Fig. 404. 



A 



be analogous to the motor column of verte- 

 brata, and the external or under one, in which 

 the ganglia are situated, we regard as the ana- 

 logue of the sensitive. Thus the two cords are 

 each composed of a motor and a sensitive co- 

 lumn, and represent, we believe, the cerebro- 

 spinal system of vertebrata. In the Aculeate 

 Hymenoptera, the ganglia of the cords are in a 

 state of development similar to those of the 

 Talitrus ; they are approximated together late- 

 rally, but still remain distinct from each other, 

 and thus present a transitory condition in the 

 larva state of an insect similar to their perma- 

 nent one in the lower Crustacea. In the fimar- 

 cha, the anterior pair, or supra-cesophageal gang- 

 lia, still continue distinct from each other, (fig. 

 405, A A,) and retain their rudimentary form, 

 as in the Talitrus, and the cords are also sepa- 

 rated ; but the several pairs of suboesophageal 

 ganglia have each coalesced into a single mass. 



Fig. 405. 



Nervous system of the larva of Timarcha tenebricosa. 

 B, natural size. 



The ganglionless upper or internal column 

 of fibres is the part which we believe to 



The supra-cesophageal ganglia, or brain of larva of 

 Timarcha tenebricosa. (Newport, Phil. Trans.) 



A similar coalescence of the ganglia, but car- 

 ried to a less extent, exists in the larvae 

 of Lepidoptera, in which the form of the ner- 

 vous system of insects has been most frequently 

 examined. Malpighi and Swammerdam ex- 

 amined this structure in the Silkworm, and 

 Lyonet in the larva of the Goat-moth, Cossus 

 ligniptrda. We have also examined it in the 

 larva of the Privet Hawk- moth, Sphinx 

 iigustri, in which we shall now describe its 

 general form and distribution. 



Cord and nerves of the larva. In the 

 larva of the Sphinx, (fig. 406,) as in 

 most others of the vermiform type, the nor- 

 mal number of double ganglia is thirteen. 

 The anterior pair (A) situated above the eso- 

 phagus, represent the brain, and the first of 

 those which are situated below it, (ft,) the 

 medulla oblongata. These are the proper 

 ganglia of the head or first segment, and the 

 cords by which they are connected together, 

 and which descend one on each side of the 

 O3sophagus, in like manner represent the crura. 

 Posteriorly to the medulla oblongata. which we 

 shall distinguish as the first suboesophageal 

 ganglion, the cords pass directly backwards into 

 the second segment, where they form the se- 

 cond suboesophageal ganglion (2). They then 



