28 THE SKIN OF INSECTS. 



often of a horny texture, which may be called the 

 horn scale ('). It is not always present, for the spiracle 

 itself is often closed; hut when it is found it is im- 

 portant, as a good guide in the comparison of the 

 other parts. 



It is proper here to remind the student, that, when 

 he examines an insect, he ought to look for all these 

 pieces, just as, in examining a rat, he ought to look for 

 the gall bladder ; but he must not be disappointed if 

 he do not find all the pieces, as some may be wanting, 

 as the gall bladder is in the rat ; and the accounting 

 for such variations opens a fine field of philosophical 

 research. 



The hind breast-plate ; a, its upper flank ; b, its under flank : 

 r, the plate itself; d, the breast prop. 



Having thus gone over the various pieces which 

 compose the breast and the sides of the corselet, we 

 may next turn to the upper part of it, which may be 

 termed the back plate ( 2 ), extending from the neck to 

 the abdomen. In rei'erence also to the division into 

 three rings, and beginning at the head, we may call 



(1) In Latin Peritrema. 

 (2) In Latin Tergum thoracis. 



