INSECTS THEIR MOUTHS 165 



called chitine, the same substance as the hard parts of all 

 Insects and Crustacea are made of. 



From beneath the sides of this there project on each 

 side two broad hooked pieces, which, as you see, I can 

 with a needle force out laterally, so as to show their form 

 better, for they hinge upon the sides of the face, beneath 

 the head-shield. Each forms the half of a crescent, the 

 curved points of which are turned toward each other, and 

 can work upon each other, the points crossing, like shears. 

 These are the proper biting jaws, or mandibles, and in 

 many of the larger beetles they have great power of hold- 

 ing and crushing. Sometimes their inner side is cut into 

 strong teeth, but here this side forms a blunt cutting edge; 

 the upper surface, however, is scored with ridges and fur- 

 rows, like a file, and this structure is best seen in the left 

 jaw, which, when the pair close, crosses over the right. 

 This is an action of the jaws the reverse of ours, but it is 

 characteristic of all the articulate classes of animals, in 

 which the jaws, whenever present, always work horizon- 

 tally, from right to left, and not vertically, up and down. 



I will now, by making the forceps revolve, bring the 

 under side of the head into view; for without separating 

 the parts by dissection (which, however, is by no means 

 difficult) it is impossible to see them all from one point of 

 view. The part nearest our eye now is the chin, a wide 

 horny piece, like the upper lip, jointed to the head by its 

 straight hind edge, but, unlike it, having its front edge 

 hollowed out with two deep notches, the central piece be- 

 tween them itself notched at its tip. Immediately above 

 this notched central tooth (I speak of the relative position 

 of the parts, supposing the insect to be crawling on the 

 ground, without reference to the way in which we turn it 



