PHYSIOLOGY OF INSECTS. 119 



mandibles bite severely, and prey on other 

 insects. 



428. Bifid mandibles (mandtbulcz bifdce) are 

 when the extremity is terminated by two distinct 

 points of equal length and similar appearance, 

 otherwise one of them would be called a tooth. 



429. Toothed mandibles (mandibula? dentatce) 

 are when the internal mandibles are beset with 

 teeth : when this is the case, they are hard and 

 bony, have a rapid and powerful horizontal 

 motion, and are capable of gnawing very hard 

 substances. 



430. Setiform mandibles (mandibulce setiformes) 

 are when they are very slender, flexible, thread- 

 like, without horizontal motion, and often com- 

 pletely enclosed in a sheath ; which sheath is the 

 lower lip. 



431. To understand the structure of the head 

 of an insect, it is essential to examine it, to 

 handle it, to dissect it, then the foregoing 

 description will probably prove useful ; but no 

 description, without ocular demonstration, can be 

 made perfectly clear. 



432. The structure of an insect's mouth will 

 be pretty well understood by a comparison with 

 our own, if we suppose it possible for our upper 

 and lower jaw each to be divided down the 

 middle, and the two halves of each to move 

 from side to side, meeting in front, the lips and 

 tongue remaining as they are. 



