948 



INSECTA. 



and to the moto-sensitive nerve (n), and, like 

 the other, the obliqui and recti muscles and 

 dorsal vessel. This division of the transverse 

 nerve into two brandies before it arrives at the 

 trachea is not figured by Lyonet in the Cossus. 

 In his delineation the transverse nerve crosses 

 the trachea singly, posteriorly to the spiracle, 

 where it communicates, as in the Sphinx, with 

 the first moto-sensitive nerve from the gan- 

 gliated cord, but does not give off a large 

 branch anteriorly to the spiracle. We are thus 

 particular in our description of these nerves, 

 because it has sometimes been supposed that 

 their distribution is precisely the same in all 

 Lepidoptera, but which it is thus seen is not 

 the case. 



Such, then, are the origins and the distri- 

 bution of the nerves in the larva; of Lepidop- 

 tera, into the description of which we have 

 entered thus minutely in order to show that 

 some nerves are distributed, more especially 

 than others, to parts concerned both in the 

 organic functions and the voluntary motions of 

 the animal, and that others are given almost 

 exclusively to parts that minister entirely to 

 sensation and volition. Of the first kind are 

 those which we have designated respiratory 

 nerves ; of the second are those we have described 

 as the moto-sensitive, the proper nerves of the 

 cord. The distribution of the first so especially 

 to the respiratory organs is a circumstance which 

 justifies us, we think, in still regarding them by 

 that designation, whether they be considered 

 as constituting a distinct system, as formerly 

 supposed, or as being of a mixed character, 

 connecting the organic with the voluntary func- 

 tions of the body, as suggested by Professor 

 Muller, and as we now regard them. In our 

 earliest inquiries into the structure and uses of 

 parts of the nervous system in insects, we first 

 described these nerves with reference to func- 

 tion, as respiratory nerves, but it was after- 

 wards suggested by Professor Grant that these 

 ' might be motor nerves,' an opinion founded 

 analogically upon the existence of a loose and 

 easily detachable structure situated upon the 

 nervous cord in the Scorpion and the Centi- 

 pede, and which was imagined to be the motor 

 tract, but which has since been shown to belong 

 to the vascular instead of the nervous system. 

 The structure which we regard as the true 

 motor column, we have always found in close 

 apposition with the sensitive, and in no instance 

 lying freely upon or loosely attached to it. 

 That the transverse nerves are indeed of a 

 mixed character may readily be inferred from 

 the description we have above given of their 

 peculiar structure, which was in part noticed by 

 Lyonet,* who called these nerves brides 

 e.pinieres. It is distinctly seen that three sets 

 of fibres enter into the composition of them. 

 The commissural set, which runs transversely 

 across the cord to each side of the body, is per- 

 fectly distinct from the longitudinal that form 

 the single loosely attached longitudinal portion 

 of each plexus above the cord. The pecu- 



* Op. cit. p. 201. 



liarity of their distribution is also as remarkable 

 as their structure. We have seen that they are 

 given to the muscles of the wings, not sepa- 

 rately, but approximated to other nervous 

 trunks, which are derived both from the com- 

 pound cord and from the ganglia ; that they are 

 given to the muscle that connects the alimentary 

 canal to the general muscular structures of the 

 body ; that they are connected with the nerve 

 from ganglia of the cord in each segment, 

 are also given separately to the organic struc- 

 tures, the tracheae and dorsal vessel ; and that 

 these nerves alone follow the course of the 

 tracheae inwards to their distribution on the 

 alimentary canal ; from all which it may be in- 

 ferred that their function is certainly in part 

 organic; while the fact of their being also in 

 part continuous with some of the nerves from 

 the cord which are distributed to voluntary 

 muscles renders it equally apparent that they 

 are in part also connected with the function of 

 volition. 



The nervous system of the perfect insect 

 differs considerably in the size and relative 

 position of its parts from that of the larva. 

 Instead of its being almost equally distributed 

 to every segment of the body, the greater pro- 

 portion of it is removed forwards and concen- 

 trated in the head and thorax. This concentra- 

 tion takes place in every insect that undergoes 

 a complete metamorphosis. The great principle 

 upon which the development of the nervous 

 system depends, is the approximation and 

 concentration of the ganglion of the different 

 segments, the shortening of the cords, and the 

 formation of new trunks, by the enlargement, 

 the changing of place, and the aggregation of 

 several nerves into one bundle, occasioned 

 and rendered necessary by other changes that 

 take place in the body at a certain period. A 

 concentration, therefore, of the nervous matter 

 is regarded, both in the perfect and larva con- 

 dition, as a proof of a higher stage of develop- 

 ment in an insect than when the nervous matter 

 is more equally distributed. On this account 

 partly it is that the Coleoptera are considered 

 the higher forms of insects, because, in addition 

 to a more perfectly developed form of the tegu- 

 mentary skeleton, there is also in them a concen- 

 tration of the nervous masses, which, in the 

 more perfect species of the order, are confined en- 

 tirely to the region of the head and thorax. This 

 is the case even in the larva condition of some 

 of the Lamellicornes, Scarabaidtt, Geotrtipidic, 

 and Melotonthida, in which the nervous masses 

 are confined to the first five segments, and the 

 nerves radiate from them into the abdomen, as 

 formerly shown by Swammerdam in Oryctcs 

 nasicornis. But although an aggregation of the 

 nervous masses into one region of the body is 

 usually, it is not invariably, a proof or an 

 accompaniment of high development; since a 

 condition similar to that of the larvae of the 

 Melulonthidie exists even in some of the lowest 

 forms of larvae of other orders, as in the larvae 

 or common maggots of Diptera, and in some 

 of the perfect insects, as in the Gad-fly, (Estrux 

 equi; while, on the contrary, a lengthened 



