THE ORGANS OF DIGESTION. 153 



lops the interior organs and completely fills all portions of the cavity 

 not occupied by them. In larvae the threads and laces of this net are 

 larger and more ragged, particularly in the fat larvae of the crepus- 

 cular and night moths. The nearer it approaches the pupa state the 

 larger are the proportions of this substance ; but as soon as the insect 

 becomes fully developed this material loses its size, and it becomes a 

 broad, delicate, laced web. It is consequently during the pupa state 

 that the greater portion of this substance becomes absorbed, whereby 

 the shreds shrink up, the delicate membrane becomes narrower, and 

 thus the preceding coarse shreds become delicate and fine laces. In this 

 shape the fatty mass not merely represents the rete of the vertebrata, 

 but actually becomes it, for it is the envelope of the intestines, and in 

 conjunction with the air vessels it supports and fixes them. Thence is 

 it that earlier (Malpighi) and more modern (Cuvier) anatomists have 

 called it the net of insects. It is scarcely necessary, after such facts, 

 to adduce other reasons in opposition to the above disputed opinion that 

 this net is the liver of insects ; whoever has but watched the develop- 

 ment of a single butterfly, indeed, whoever shall but have compared 

 an opened caterpillar with an opened moth, to him it will be evident 

 that the fattv mass cannot be the liver. 



tf 



Chemical analysis has as yet contributed nothing towards the 

 removal of the difficulties which still involve the different views upon 

 this subject, although a careful investigation would most certainly settle 

 the dispute. In ants* and r the cochineal insect fat has actually been 

 found, and this consequently may certainly contribute to support the 

 adoption of the opinion of this substance being found in all other insects. 



11G. 



III. THE BLOOD VESSELS. 



We shall find the vascular system just as simple and uniform in 

 insects as we have found their digestive apparatus complex. A vessel 

 which passes along the back from the head to the anus constitutes the 

 only blood vessel to be discovered in insects. That this canal is a true 

 blood vessel, and indeed an artery, is proved by its regular contraction 

 and expansion, which is very easily perceived exteriorly in transparent 

 thin-skinned larvae. Malpighi, its discoverer, considered it as such, 



* Compare Gmelin, Handb. d. Theor. Chemic, vol. ii. Div. i. p. 469, No. 24, ;md 

 p. 508. No. 1 ; 2nd Div. p. 1473, &c. 



