CONCLUDING REMARKS. 35 



The ova undergo yolk segmentation, and just before impreg- 

 nation, possess a micropyle, germinal disc, and yolk canal, just 

 like the ova of the highest vertebrates. The skin of the larva is 

 formed from delicate cells, and is complete eight hours after im- 

 pregnation, but neither trachea3 nor viscera can be discerned so 

 early ; these are afterwards rapidly developed, and four and twenty 

 hours after impregnation the larva escapes, its alimentary canal is 

 filled with oil globules and molecular matter, and the trachea3 with 

 air, at the time. The fat bodies are at this period scarcely percep- 

 tible, a number of delicate cells only being attached to thetrachere. 

 The larva commences immediately to feed, and increases rapidly 

 in size, the fat bodies become largely developed, and at the end 

 of three or four weeks it assumes the pupa state. Its further 

 changes have been already described. 



Section XII. Concluding Remarks. 



The nervous system indicates that the Diptera, Hymenoptera 

 Xeuroptera and Orthoptera, are the most highly organized of all 

 insects ; the presence of convoluted nerve centres supported by 

 peduncles, and united by a cominisure, which do not give off 

 nerves, is an approach to the nervous system of the Vertebrata, 

 and such nerve centres are perhaps the most important charac- 

 ters of the above mentioned orders. The elaborate modifications 

 of the mouth in these orders for special purposes, the high 

 development of the sense organs, and in the Diptera the remark- 

 able modification of the posterior wings, and the articulation 

 of the head with the thorax by a pair of condyles in some, point 

 to a high type of organization. But at present it is extremely 

 difficult to decide which of these orders is most highly organized : 

 they will probably eventually form a distinct sub-class of the 

 insecta, characterized by its highly organized nervous system ; 

 and its divisions may perhaps be founded upon the various 



D 2 



