INSECTS IN GENERAL. 39 



and especially from those which are winged, in having a 

 complete system of circulation, and a different mode of respi- 

 ration. From these motives, in his lessons on comparative 

 anatomy, he formed a particular class of the Crustacea, and 

 subsequently admitted that of the arachnida into the " Regne 

 Animal," comprising however, in this last, those species only 

 which have no antennae. The insects are, accordingly, dis- 

 tinguished from the Crustacea, by M. Latreille, by the 

 absence of peculiar circulating vessels, and by their tracheae ; 

 from the arachnida, by having two antennae, by the tracheae 

 forming two parallel cords through the whole length of the 

 body, with centre of branches, like verticillas at certain inter- 

 vals, and by the stigmata not being limited to a particular 

 space. 



We shall now add to the text a few brief remarks on the 

 structure of insects in general. 



There are three principal regions of the body to be consi- 

 dered in insects : the head, the corslet, or thorax, and the 

 abdomen, or belly. 



The head is constantly articulated with the thorax, but 

 the mode of articulation varies according to the orders, fami- 

 lies, and genera. All the true insects have the head articu- 

 lated, and mobile on the trunk, and furnished with antennae. 



The mouth is a very important portion of the conformation. 

 Its parts differ not only in the different orders, but by slight 

 modifications in all the genera, and more than probaby, in 

 all the species. On these modifications, some entomologists 

 who have studied them deeply, have established both orders 

 and genera. 



It cannot be expected that we should enlarge much on the 

 anatomy of this part. It will be sufficient to say that with 

 regard to it, all insects may be referred to two grand divi- 

 sions : the species with free mandibles and jaws, disposed 

 isolatedly in mobile parts ; these are the masticating or grind- 



