INSECTA. 



953 



to us, as we have elsewhere remarked,* from 

 its seeming to be analogous to similar enlarge- 

 ments on those parts of the spinal cord in man 

 and other vertebrata, from which proceed the 

 nerves to the arms and lower extremities of the 

 body, and corresponds to the apparent greater 

 necessity for accumulations of nervous matter at 

 those parts of the cord. In Lepidoptera the cord 

 is less easily examined in the perfect than in the 

 larva state owing to its increased opacity, but the 

 transverse nerves are not only distinct but have 

 been removed forwards and now pass off on 

 either side midway between the ganglia. 



The brain and its nerves do not acquire their 

 full development until near the termination of 

 the pupa state. The supra-oesophageal ganglia 

 (A) of the larva, which, for convenience of 

 description, we have called after Burmeister 

 the cerebrum, are in reality the analogues of 

 the corpora quadrigemina, and the first sub- 

 oesophageal of the medulla oblongata. Bur- 

 meister has designated this the cerebellum, but 

 of that part of the brain in vertebrata we believe 

 there is no analogue in any of the invertebrata. 

 Instead of the cerebral mass being divided into 

 two ganglia, as in the larva, it has now 

 (Jig. 412, A) acquired a compact form; it is 



Fig. 412. 



A, brain of Timarcha tenebricosa ; B, optic nerves; 



C, origin of the sympathetic and the crura ; 



D, the medulla oblongata ; b, the vagus or 

 visceral nerve passing back from its ganglion ; 

 e, lateral nerves from the ganglion. 



convex on its upper surface with a slight 

 depression in the middle line, and concave on 

 its under, to adapt it to the form of the oesopha- 

 gus, across which it is placed. At its sides it 

 gives off the large optic nerves (B), which are 

 almost equal to it in diameter. They pass 

 directly outwards and are usually swollen into 

 the form of an oblong ganglion, but are again 

 constricted before they arrive at the optic fora- 

 men, through which they pass and are imme- 

 diately expanded into an immense number of 

 fine filaments for the complicated organ of 

 vision. There are enlargments upon these 

 nerves at their base even in the larva state 

 (fig- 405). From the most superior portion of 

 the cerebrum originate the nerves of the ocelli. 

 They vary in number from one to three, and 



* Prize Essay, p. 11. 



are little pyramidal elevations situated on each 

 lobe, as seen in Acrida (Jig. 410), posteriorly 

 to the antennal nerves (D). They are each 

 covered by a dark choroid, and in other respects 

 are distinct nerves of vision. When three exist, 

 as in Hymenoptera, the third is situated in the 

 middle line between the others, and appears to 

 be derived in part from each lobe, so that in 

 this ocellus the vision of both sides of the brain 

 is combined. In the Vespada, however, accord- 

 ing to Burmeister, the three ocelli originate 

 from a single foot-stalk, and not separately, as 

 in the Apid<e. On its anterior surface the 

 cerebrum gives off the antennal nerves. These 

 also, in many instances, have a ganglionic 

 enlargement at their base, as is well seen in 

 some of the Ichnenmonida and other Hymen- 

 optera, and as shown by Straus Durckheim 

 in Melolontha. The antennal nerves vary in 

 position, being sometimes near the middle line 

 and at others close to the base of the optic, 

 but in every instance anterior to them. At its 

 anterior and inferior surface the cerebrum pro- 

 duces the two remaining pairs of nerves. The 

 most external, the glosso-pharyngeal, unites 

 with its fellow of the opposite side to surround 

 the oesophagus, as in the larva. It is seen very 

 distinctly in the pupa state (fig. 415, f) of the 

 sphinx, and also in many perfect insects, as in 

 Acrida (fig. 410). It supplies the under-sur- 

 face of the throat and part of the oesophagus, 

 and a small branch is also given from its base 

 to the sides of the mouth. At its inner side 

 originates the recurrent or vagus nerve, which, 

 after passing a little forwards, ascends and 

 forms its ganglion on the upper surface of the 

 pharynx, and then passes backwards along the 

 oesophagus, as in the larva. In some insects, 

 as in Orthoptera, it originates from a portion 

 of the crura, as in Crustacea, but its course 

 and direction are always the same although 

 appearing to vary, as we shall presently show 

 when describing it as an organic nerve. The 

 sympathetic originates, as in the larva, from 

 the posterior part of the brain. One circum- 

 stance that particularly distinguishes the brain 

 from the other ganglia is its more uniform 

 opacity and greater softness, and disposition to 

 deliquesce when exposed for a short time to 

 the air. It is usually larger than most of the 

 other ganglia, excepting perhaps the meso- 

 thoracic. In Hymenoptera it is larger than in 

 other insects, a curious circumstance this if it 

 may be supposed to have any reference to the 

 comparative instinct of different species. In 

 Diptera and Orthoptera it is also of great size. 

 We were once desirous of knowing whether it 

 contains any cavities or ventricles, but after the 

 most careful search we have been unable to 

 detect any. It is an almost homogeneous mass, 

 penetrated throughout its whole substance by 

 minute air-vessels, which ramify within it and 

 also in the substance of the optic nerve. This 

 is one of the circumstances that lead us to 

 suspect, as formerly suggested by Dr. Kidd in 

 his anatomy of the Mole-cricket,* that the 

 course of the blood in the different structures 



Phil. Trans. 1826. 



