INSECTA. 



955 



The most posterior one is given to the muscles 

 within the head, and the other passes along the 

 outward part of the interior of the mandible to 

 its apex. The medulla oblongata with the con- 

 tinuation of the nervous cord in all Coleoptera 

 passes under the bony arch or tentorium at the 

 base of the skull, protected on both sides, as 

 in Melolonthu,* Dyticus, and Hydrous, by the 

 lamina posteriores, which inclose it, as in a 

 canal, distinct from the oesophagus, that passes 

 along above it, and from which it is separated 

 by a fine fibrous membrane. The crura and 

 the base of the cerebrum rest upon and are 

 partly protected on each side by the lamina 

 squamosa, which thus, as it were, form a kind 

 of internal skeleton for the protection of the 

 soft part. The optic nerves at their base rest 

 upon the laminae in their course to the eye, 

 and extend as far outwards as the lamina 

 orbitales through the foramen in which they 

 pass, and are immediately expanded into an 

 immense number of filaments which form part 

 of the organ of vision, as we shall presently 

 describe. The whole of the cerebrum is loosely 

 covered by a fine transparent membrane that is 

 continuous with the fibrous membrane that 

 covers the cord. In some instances, as in the 

 Bombus terrestris, it is very distinct, and in 

 others, as shown by Burmeister, is studded 

 with minute opaque rounded elevations, arranged 

 in the form of squares. It appears to be 

 reflected along the course of the optic nerves, 

 and to be continuous in part with the margins 

 of the lamina squamosa, and separates the 

 brain from the muscles, by which it is on almost 

 every side inclosed. In Lucanus cervus, instead 

 of the medulla passing under a simple arch 

 or tentorium, the laminae laterales are approxi- 

 mated and form a double ring (Jig. 388,^, 

 through the inferior of which, as through the 

 ring of a vertebra, the nervous cord passes in 

 its course to the prothorax. In the Orthoptera, 

 as in Blatta Americana and Gryllotalpa, we 

 have seen the same structure, but in these the 

 ring is lengthened and forms a more distinct 

 canal. In the Hymenoptera, as in the hornet 

 and humble-bee, the form of the part is exactly 

 the same, and the cord passes through a short 

 bony ring in its passage to the thorax. There 

 is a somewhat similar structure in Lepidoptera 

 as in Sphinx ligustri, only that it is much less 

 complete, the arch being simply a bar extended 

 across the occipital foramen and dividing it 

 into two, through the lower one of which the 

 cord passes, and also on each side of it the 

 flexor muscles of the head. A like form exists 

 in the Homoptera, but much less perfect. 



The cord and nerves of the thorax, which 

 are usually much larger than those of the 

 abdomen, we regard as the proper cerebro- 

 spinal system, and the abdominal portion as 

 the caudal. This is the view taken of these 

 parts by Burmeister, with whose opinion we 

 perfectly coincide. The prothoracic ganglion 

 is situated immediately before the ante-furca, 

 between which the cord passes to the meta- 

 thorax, when it forms a great ganglion anterior 



* Straus, Considerat. &c. 



to and beneath the medifurca, and then passes 

 onwards over the post-furca to the abdomen. 

 In the Lamellicornes and others in which the 

 cord terminates in the thorax, and the nerves 

 radiate from thence into the abdomen as a 

 caudu equina, they pass over the post-furca in a 

 bundle, and do not separate until they enter 

 the latter region. In the Gryllidae, in which 

 most of the segments are equally developed, and 

 there are three large thoracic ganglia, the meta- 

 thoracic one is situated in the middle of the 

 segment, and the succeeding or fourth sub- 

 CEsophageal ganglion on the rudimentary post- 

 furca. In the Hymenoptera, in which there 

 are but two ganglia in the thorax, the anterior 

 and smaller one is situated at the margin of the 

 metathorax, and the great ganglion at the 

 posterior, and the cord continued from it passes 

 through a strong bony canal or ring in the 

 medifurca, somewhat resembling that which 

 exists in the head, and then forms a smaller 

 ganglion before it enters the abdomen. In 

 the Hemiptera, in which it has been supposed 

 that there is only one ganglion ip the thoracic 

 region, the cord between the medulla and pro- 

 thoracic ganglion is exceedingly short, but is 

 protected in its passage through the elongated 

 neck, and then is developed into a large pro- 

 thoracic ganglion, the second ganglion being 

 situated in the middle of the raeso-thorax before 

 the medifurca. These parts are very distinct 

 in Nepa grandis. In the Lepidoptera, in which 

 the form of the thorax is more compact even 

 than in the Hymenoptera, the cord passes on 

 each side of the medifurca or part to which 

 the triangular muscles are attached, and is so 

 much enlarged as to appear almost like a por- 

 tion or continuation of each of the two great 

 ganglia situated before and behind it. The 

 cords and ganglia of the thorax are covered in 

 by a strong white membrane like those of the 

 head. In the Lepidoptera this is particularly 

 firm, so that the nervous system is not included 

 within the cavity of the thorax. 



In the distribution of the nerves there are 

 some peculiarities. We have seen the auxil- 

 iary connecting nerves of Burmeister, as shown 

 by us formerly in the larva of the Sphinx* 

 (fig. 406), in many species. They exist be- 

 tween the cord and all the ganglia of the 

 thorax in the Gryllida (Jig. 410, e, A,) and 

 between the cord and the nerves to the wings 

 in Athalia centifblia and Panorpa communis. 

 We have seen them also in Oiceoptoma, Pro- 

 scarabaus, Creophilus, Lampyris, Forficula, 

 Blatta , and even in an imperfect form in 

 (Estrus, as well as in some of the Lepidoptera. 

 They are invariably connected with the nerves 

 to the wings, of which they form one portion, 

 and are far more frequently met with than 

 Burmeister appears to have supposed. We 

 formerlyf remarked on a peculiarity in the dis- 

 tribution of the thoracic nerves in the Sphinx,, 

 and the opinion then ventured with regard to 

 its nature we have since had reason to believe 

 was well founded. We have seen the auxiliary 



* Phil. Trans. 1832, part ii. p. 387-8. 

 t Phil. Trans. 1834, part ii. p. 394. 



