INSECTA. 373 



which operate upon the solids, and have made the Haustellata and 

 Mandihulata the primary divisions of the class. 



The composite parts of the proboscis or siphon are however fun- 

 damentally the same as those that form the strongest or most 

 formidable apparatus for mastication ; but as they are most con- 

 spicuous and most uniformly developed for the latter office, I shall 

 commence the demonstration of those complex parts of the mouth, — 

 the trophi or cibarial instruments, — as they exist in a Mandibulate 

 insect, {fig. 148.) 



Man has two jaws only, and no Vertebrate animal has more ; they 

 work up and down, or in the direction of the axis of the body. 

 Insects have also their upper and lower jaws — horny edentulous 

 plates, serving in many for little else than to close the mouth, and 

 hence called lips ; the upper one {^fig. 148, a) is the " labrum," the 

 lower one (c?) the "labium:" but they have like- 148 

 wise four more complex jaws, acting upon each 

 other in pairs, from side to side, or transversely 

 to the axis of the body. The upper pair of jaws 

 {h) are called the " mandibles," the lower pair (c) 

 the "maxillae."* The three lower instruments, 

 viz. the two maxillae and the labium, arc pro- 

 vided with the jointed instruments of sensation, 

 called " palpi ;" the maxillse, in some insects, sup- 

 porting each a pair of these appendages, which, 

 besides their sensitive and selective offices, serve Trophi of a Mandibulate 

 also to seize and hold steady the alimentary sub- 

 stances whilst these are divided by the mandibles and maxillae, and 

 represent, in fact, a third pair of lateral jaws. The lower lip has a 

 basal joint {mentiim), supporting a more flexible part (ligula, or 

 labium proper), near to the base of which the palpi are articulated. 

 The upper, or inner integument of the ligula, is usually developed 

 into a kind of tongue, which is a distinct part {lingua) in the lo- 

 custs and Libellulae. The labrum, or upper lip, is generally a simple 

 transverse flattened plate. 



The mandibles are subject to most variety in relation to the habits 

 and kind of food of the insect. In texture they vary from the hardest 

 chitine to soft membrane. In the predatory tiger- beetles they ter- 

 minate in sharp hooked points, like canine teeth, and are hard enough 

 to pierce the firm integument of other insects. In the dragon-fly the 



* In vertebrate anatomy the upper or anterior jaw is called maxilla, the lower 

 or hinder jaw mandibida. It had been well if the analogy had governed the 

 entomological nomenclature. 



BBS 



