CLASS V. INSECTS. 67 



ences in the formation, structure, and situation of their mouth, 

 gills, gill-covering, fins, &c. But an account of them here 

 would be of little ueeor interest. 



SECTION VI. 



Class V. Insects. 



THE animals of this class, although less complicated and 

 perfect in their internal structure, than those of some of the 

 following classes, are yet remarkable for a greater variety of 

 powers and a more wonderful display of instinct and intelli- 

 gence, than any .other of the invertebral animals; and they 

 are, therefore, placed first among them in this description. 



Insects are destitute of a heart, but instead of it they have 

 a vessel or reservoir situated along the back, extending from 

 one end of their bodies to the other, and filled with a trans- 

 parent, viscous fluid. This vessel undergoes an irregular 

 contraction, which is supposed to be analogous to the con- 

 tractions of a heart. No branches have been discovered go- 

 ing off from it, and yet it is highly probable that this reservoir 

 contains the blood or nutritious fluid of the animal, which is 

 slowly conveyed, by absorption, to the various organs. In- 

 sects have no particular organ for respiration, but their bodies 

 are penetrated in every direction by tubes, called tracheae, 

 which convey the air to every part. These tubes com- 

 municate externally by openings called stigmata. The blood, 

 therefore, undergoes the changes wrought upon it by air, 

 throughout its whole circulation. Instead of a brain and 

 nervous system, they are furnished with two knotted cords, 

 running the length of their bodies, which perform the same 

 functions. They possess the senses of seeing, tasting, smell- 

 ing, and feeling ; but organs of hearing, if they exist, have 

 not yet been discovered. 



Being destitute of any internal skeleton, insects are pro- 

 vided with a hard external covering, which serves to support 

 their motions and protect their organs. The nature of this 

 covering differs in different species ; in some it forms a com- 

 plete shell or case of a horny or shell-like substance ; and in 

 others it consists merely in a tough, muscular coat, divided 

 into rings, which surround the body. 



The greater part of insects are winged, but some are not 



