INSECTS IN GENERAL. 71 



growth, and of the fulfilment of their functions. But what 

 is the mode of this absorption ? This is a question not yet 

 completely answered. M. Cuvier has detailed the reasons 

 which lead us to believe that this absorption, in insects, is 

 operated by a kind of imbibition, because they are destitute 

 of circulatory organs, or of lymphatic, arterial, and venous 

 vessels. It is true that he arrives at this conclusion from 

 negative indications ; but his reasonings are so strengthened 

 by analogy, that they acquire the force of proofs. 



It is certain that no blood-vessels are to be observed in 

 insects. One vessel, indeed, is known, which is a sort of 

 canal, predominating along the middle part of the back in all 

 its regions (see text). This is very visible in the caterpillars, 

 especially in those which have a smooth body, as the cossus. 

 In this there may be discerned a kind of alternate motion of 

 systole, and diastole, or of dilatation and contraction, which 

 appears to pass from the head to the tail ; but neither 

 branches nor roots have been observed, which carry thither 

 any fluid, or proceed from it. 



On the other hand, when we know in what manner respira- 

 tion is performed in insects, we do not find so great a 

 necessity, as in other animals, for the presence of a central 

 agent of circulation, or of canals to convey the fluids towards 

 the place where the air should come especially in contact 

 with them. Animals endowed with circulation, with arterial 

 and venous vessels, which come from the heart and repair 

 thither, have, in consequence of this very circumstance, con- 

 glomerated glands, destined for the performance of secretions, 

 such as the liver, the pancreas, the parotid and salivary 

 glands, &c. But in species deprived of a circulating system, 

 the case is very different. There are no glands designed for 

 any such functions. The secretory organs are composed of 

 numerous and distinct filaments, which are immersed in the 

 midst of the nutritive fluid, from which they are to borrow 



