30 THE BUTTERFLY VIVARIUM. 



course or treatise, has furnished us with a name 

 for that portion of natural science which concerns 

 insects, and which has heen termed "entomology." 



With but few exceptions, this insection between 

 the two chief portions of the body does not occur in 

 insects in their preparatory or larva stages ; but it 

 is upon the characters of the perfected creature that 

 all classification is founded, and, in that condition 

 of the insect, it forms, as I have stated, a most im- 

 portant feature. The thorax, or front portion, 

 which is, in fact, the trunk or main body, exhibits 

 peculiarities of structure entirely opposite to those 

 of the higher orders of animals. These last have an 

 internal skeleton of bone, clothed with flesh — while 

 in insects it is the skeleton which is external, in 

 the shape of the horny case of the thorax, to which, 

 however, the limbs are attached, as in the higher 

 animals they are to the internal frame-work of bone. 



Of all the tribes which have no cerebro-spinal 

 system, and which have an external skeleton of 

 " horn," instead of an internal one of bone, insects 

 form by far the largest portion. As a whole, 

 creatures of this kind of structure are now divided 

 into five great classes — the Annelida, including 

 leeches, worms, etc. ; the Crustacea, including 

 crabs, lobsters, etc.; the Arachnida, containing 



