INSECTA. 



945 



Nerves of the head. When the first pair of 

 ganglia, which always constitute the brain, are 

 viewed from above, they each present a> convex 

 uniform appearance, and are distinguished 

 from each other by a depression between them, 

 which is more apparent on their anterior than 

 thejr posterior surface, and is occasioned by the 

 lateral part of each lobe or ganglion being car- 

 ried a little forwards, so that the two lie across 

 the oesophagus in a curved or lunated direction. 

 On their under surface they are concave, to 

 adapt them to the form of the oesophagus, 

 above which they are situated. From the an- 

 terior and lower part of each lobe originate 

 four remarkable nerves, which belong to the 

 organs of sense and the viscera. The first and 

 largest of these, the optic, passes a little for- 

 wards and outwards to the siemmata, a little 

 behind the mandibles; the second, the anten- 

 nal, passes a little more anteriorly, to the palpi- 

 form antenna; the third, and most inferior, 

 descends at the side of the pharynx, and uniting 

 with its fellow of the opposite side forms a 

 loop or collar around the oesophagus, to the 

 under-surface of which it distributes a few fila- 

 ments. We consider it as analogous to the 

 glosso-pharyngeal nerve of vertebrata. The 

 fourth is situated between the second and third. 

 It passes a little forwards from its origin, and 

 then ascending above the pharynx, meets its 

 fellow of the opposite side, with which it forms 

 a minute ganglion, from the hinder part of 

 which a single nerve passes backwards (6) 

 beneath the brain, in the median line above the 

 oesophagus, to the stomach and viscera. This 

 nerve was discovered by Swammerdam,* who 

 called it the recurrent, from the manner in 

 which it originates and is distributed, and it 

 was afterwards minutely figured and described 

 by Lyonet. Miiller has since described it 

 minutely, and figured it in many species, in his 

 paper on the sympathetic nerves of insects, as 

 the proper visceral nerve, analogous to the sym- 

 pathetic. In a paper in the Philosophical 

 Transactions in 1832, we described it as the 

 vagus,]- of which we believe it is the proper 

 analogue. At that time we were led to sup- 

 pose that it had previously been so described 

 by Straus Durckheim, but such, as we have 

 since found, was not the case. W r e shall pre- 

 sently return to our description of this nerve, 

 in the perfect insect, as belonging to those of 

 the organic functions. Besides these four pairs 

 of nerves from the anterior part of the 

 brain, there is also one minute pair from the 

 posterior, which is directed backwards, and de- 

 veloped on each side of the head into two pairs 

 of little ganglia, (a,) which constitute part of 

 the sympathetic system. The first of these 

 ganglia was discovered and rudely figured by 

 Swammerdam,^ and afterwards more correctly 

 by Lyonet, and the second by Straus Durck- 

 heim. We have designated them from their 

 situation the anterior lateral ganglia.^ 



* Op. cit.tab. xxviii. fig 2. 

 t Part 2, p. 386. 

 J Op. cit. tab. xxviii. tig. 3 i. 

 Philosophical Trans, p. 2, 1832, page 38?. 

 VOL. n. 



These are the proper cerebral nerves of the 

 larva, and belong to the senses and organic func- 

 tions. The medulla oblongata, or first subceso- 

 phageal ganglion, also gives origin to four pairs 

 of nerves. The most anterior pair of these is 

 given to the labium ; the next to the palpiform 

 maxillae; the third, the analogue of the fifth of 

 vertebrata, conjointly to the muscles of the 

 mandibles and maxillae; and the fourth, the 

 most posterior pair, to the silk vessels, the pro- 

 per salivary organs of the larva. 



The nerves of the thorax belong to the se- 

 cond, third, fourth, and fifth suboesophageal 

 ganglia, and their intervening cords. The first 

 pair of nerves from the second ganglion (2) 

 are exceedingly small, and are given to the 

 retractor muscles of the head. The second 

 pair (c) are large, and are divided into many 

 branches that are given to the whole of the 

 muscles of the lateral and superior part of that 

 segment, and the third (d) are directed back- 

 wards, and supply the anterior or prothoracic 

 legs. The third ganglion (3) produces also 

 three pairs of nerves. About midway between 

 the second and third ganglion the cord pro- 

 duces on each side a single nervous trunk (f), 

 which is directed a little backwards, and unites 

 at an angle with the first nerve from the third 

 ganglion. These together from a single trunk, 

 which in the early stage of the larva is exceed- 

 ingly small, but increases much in size as the 

 period of changing into the pupa state ap- 

 proaches. It is the first alary nerve, and is 

 given to the future anterior pair of wings, and 

 is now distributed among the muscles of the 

 anterior part of the segment. It is also con- 

 nected with one set of the transverse nerves (e\ 

 which exist in each segment loosely attached 

 to the cords, and which we shall describe more 

 particularly hereafter. The second pair of 

 nerves from this ganglion produce each at their 

 base a small branch, which has the appearance 

 of a distinct nerve, and which is distributed 

 laterally to the deep-seated muscles, while its 

 main trunk (g) is given to the second, or meso- 

 thoracic pair of legs. Half-way between the 

 third and fourth ganglion the cord again pro- 

 duces on each side a single nervous trunk ('), 

 which, like the corresponding one in the pre- 

 ceding segment, is directed backwards, and 

 unites with the first nerve from the third gan- 

 glion. It is the second alary nerve given to 

 the muscles of the future second pair of wings. 

 Like the corresponding nerve in the preced- 

 ing segment, it is very small during the early 

 period of the larva state, but is greatly en- 

 larged as the period of transformation ap- 

 proaches. It also unites, like the former, with 

 a set of the transverse nerves (h\ and then 

 passes outwards about midway across the recti 

 muscles, between which it penetrates, and 

 pursues its course upwards to the lateral and 

 dorsal muscles of the segments, and which are 

 to act upon the future wings. The second 

 nerve from the ganglion divides, like the cor- 

 responding one from the ganglion of the pre- 

 ceding segment, into two branches, one of 

 which (A;) crosses the smaller rectus muscle, 

 and passes beneath the larger to the dorsal 



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