8 EXTERNAL SKELETON. 



former case the muscles lie upon the outside of the 

 bones upon which they act^ in the latter they are 

 entirely enclosed mthin the firm tubes which furnish 

 them with points of attachment. Hence the term 

 external skeleton has been applied to the integument 

 of the Insects and their allies. 



In most insects this well deser\^es the name of ske- 

 leton, being usually of a horny texture ; but in some 

 it is comparatively soft_, although even in these the 

 division of the body into segments is distinctly per- 

 ceptible, and the tubes of which the limbs are formed 

 exhibit a greater degree of firmness than the rest of 

 the body. As all the more important characters of 

 the different orders of Insects are derived fi^om the 

 peculiarities of this external skeleton, it will be neces- 

 sary to describe its structure at some length. It is 

 usually composed, as already stated, of thirteen seg- 

 ments ; but of these some are often united together in 

 such a way as to form but a single piece, whilst others 

 are sometimes reduced in size and concealed within 

 their neighbours. 



The Head. — The first segment forms the head. It 

 consists of a single piece having an opening in front or 

 beneath for the mouth, and another behind for the 

 passage of the oesophagus into the body. On its sur- 

 face it exliibits the eyes, and gives attachment to a 

 pair of jointed organs called antennse. 



The mouth in Insects is furnished with a variety of 

 organs to enable them to take their nourishment, and 

 as this is exceedingly various, the structm^e of this 

 part appears so different in the different groups of 

 these creatures to suit them for the particular offices 

 assigned to them in the oeconomy of nature, that its 

 peculiarities furnish us with some of the most import- 



