THE INSECT: ITS EXTERNAL STRUCTURE 



but somewhat separated, and such groups are called agglomerate eyes. 

 The chitin of the surface of the body is transparent where it covers 

 the surface of an eye, permitting access of light to the sensory struc- 

 tures within: elsewhere it is usually pigmented and rather opaque. 



The mouth parts of insects vary extremely in their structure. Appar- 

 ently the original mouth parts were for biting and chewing, and this 

 type is very common. In some groups, however, they have been trans- 

 formed into a sucking apparatus. Biting mouth parts, being the more 



FIG. 14. Three types of insect mandibles, greatly enlarged. Somewhat diagrammatic. 



(Original.) 



primitive and simple, are described here, while sucking mouth parts 

 having been differently transformed in different groups will be taken up 

 in connection with those groups. 



In front of (in hypognathous heads), or above the mouth opening 

 (prognathous heads) is the front lip or labrum. It is a thin flap, hinged 

 to the skeleton of the head and 

 moves forward and backward. It 

 is often more or less divided by a 

 central notch at the middle of its 

 free edge. Its inner surface, form- 

 ing the roof of the mouth, is often 

 called the epipharynx. 



At the sides of the mouth open- 

 ing, immediately behind the lab- 

 rum, is a pair of jaws, the mandibles. 

 These differ greatly in form in 

 different insects (Fig. 14). They 

 are often stout, heavy structures 

 with crushing faces bearing blunt 

 projections or teeth ; sometimes they 

 are long, curved and rather slender. 

 In general their form is adapted to the feeding habits of the insect. 



Immediately behind each mandible at the side of the mouth is a 

 second appendage, the maxilla. This differs markedly from the mandible, 

 being much weaker, and composed of a number of pieces (Fig. 15). The 

 tips and outer internal margins of the maxillae usually bear numerous 



FIG. 15. Two types of insect maxilla 

 greatly enlarged. Somewhat diagrammatic. 

 (Original.) 



