10 APPLIED ENTOMOLOGY 



spines or hairs, but this condition varies according to the nature of the 

 food of the insect. Attached on the outer side of each maxilla not far 

 from where the latter articulates with the head, is a sort of tiny antenna- 

 like structure consisting of from one to six (usually five) segments, which 

 is called the maxillary palpus. The function of the maxillae appears to be 

 to hold and retain the food in the mouth while it is being worked upon 

 by the mandibles, and also to aid these in breaking it up. The presence 

 of sense organs on the maxillary palpi suggests that these are possibly 

 concerned with the sense of smell. Both mandibles and maxillae move 

 sideways. 



Fig. 16. — Two types of insect labium much enlarged. Somewhat diagrammatic. 



(Original.) 



Behind the maxillae and closing the mouth opening behind, is the 

 hinder lip or labium (Fig. 16). This was evidently once a pair of jaws 

 somewhat similar to the maxillae, but with no mouth cavity between to 

 separate them, their inner edges have grown together to varying 

 degrees in different insects. In some, only one or two of the pieces 

 nearest the head have fused: in others, fusion all the way to the tip 

 has been accomplished, and all intermediate stages also occur, thus 

 producing a structure which now moves forward and backward like the 

 front lip, but which may be complete, partially, or almost entirely cleft 

 in the middle line. 



Like the maxilla the labium has a palpus on each side arising from 

 near its base, and composed of three (rarely four) segments. The func- 

 tion of these labial palpi appears to be similar to that of the maxillary 

 palpi. 



Near the base of the labium on its inner or mouth side there is fre- 

 quently a fleshy swelling more or less covered by bristles or hairs, which 

 is called the hypopharynx, lingua or tongue. It varies greatly in size 

 and form. 



The thorax has its three segments usually quite clearly marked. 

 Each segment bears a pair of legs, but the prothorax, or first of the 

 three behind the head, bears no wings. On the second or mesothorax, 

 and on the third or metathorax, both wings and legs occur in the majority 



