I 



THE METAMORPHOSES OF THE NERVOUS SYSTEM. 39 



The beetles \Coleopterd), the locusts {Orthoptera), and the bees 

 {Hymenopiera), do not have their thoracic ganglions all fused 

 into one mass when they attain adult age, and the nervous 

 centre, which is known as the first of the thoracic segments — 

 the prothorax — never unites with the others. But the next two 

 ganglions do become fused together in those species which attain 

 great perfection of organisation. In the larva of the bee the 

 three thoracic nervous centres are like those of the silkworm 

 caterpillar, but in the perfect insect these ganglions enlarge, and the 

 hindermost are confounded together. This concentration does 

 not exist in all adult insects, and the ganglions remain separate 

 in most of them. 



The nerves which supply the muscles of the first pair of 

 legs in the perfect insect arise from the first ganglion of the 

 thorax or the prothoracic. Those destined for the innervation 

 of the first pair of wings and the second pair of legs come from 

 the mesothoracic nervous centre, and the metathoracic enlarge- 

 ment originates the nerves of the third pair of legs and of the 

 second pair of wings. Nothing appears to interfere with this 

 relation between the ganglions and the legs and wings. 



A great number of larvae have nine pairs of ganglions in the 

 abdomen, and this arrangement often persists during adult age ; 

 as a rule these nervous centres are small, and they even remain 

 detached in pairs in some imperfect insects. In the caterpillar 

 of the silkworm there are eight ganglions in the abdomen. Seven 

 of them are located in the anterior seven segments, and the eighth 

 is situated near the front part of its body ring. It is very large, and 

 evidently is composed of two nervous centres united. But in any 

 other insects whose development is less advanced, the eighth and 

 ninth abdominal ganglions are found to occupy their corresponding 

 segments. The size of the nerves which pass off on either side from 

 the ganglions in the abdomen of the larva, is usually much less 

 than that of the twigs which arise from corresponding situations 

 in the perfect insects. The lateral nerves have much to do with 

 the respiratory efforts, which are feeble in most larvae, but very 

 vigorous in the perfect adult insects. There is no comparison 

 to be entertained between the amount of the breathing energy 

 of a sluggish maggot, grub, and caterpillar, and that of a lively 



