PHYLUM ARTHROPODA — CLASS IXSECTA 305 



verse plane. They are hard, thick plates of chitin with toothed 

 edges adapted for cutting or crushing food. In some beetles the 

 mandibles become greatly enlarged and apparently worthless. The 

 maxillae or second pair of jaws lie under the mandibles and move 

 in a similar plane. The maxillae, which consist of sclerites, are 

 much more complex. The cardo is the piece which hinges the 

 maxillae to the head; attached to the distal portion of the cardo is 

 the stipes, which bears three sclerites — the lacinia, galea, and palpus. 

 The lacinia is provided with teeth or spines which aid in holding and 

 chewing the food. The palpus is composed of four or five segments 

 and is sensory in function. 



The labium or lower lip is formed by the fusion of what is be- 

 lieved to have been a second pair of maxillae. The labium is hinged 

 to the head by the mentum from which extends one or two pairs of 

 lobes, the ligula. Projecting from the mentum on each side is a 

 palpus which consists of one to four segments ; it functions as a sen- 

 soiy organ, probably detecting senses similar to our own senses of 

 taste and smell. 



The hypopharynx or tongue arises from the labium into the cavity 

 of the mouth, and bears the opening of the salivary duct. 



In the typical sucking insect the mouth parts consist of two pairs 

 of sicklelike or styletlike structures which are modified mandibles and 

 maxillae, as found in the mandibulate orders discussed above. The 

 labium forms a long sheathlike structure in which the styletlike man- 

 dibles and maxillae lie. The upper lip or labrum covers over the 

 proximal portion of the beak. Food is taken in a liquid form by 

 being sucked up through the labial sheath. In the mosquito the hypo- 

 pharynx is long and slender like the mandibles and maxillae; the 

 salivary duct extends throughout the entire length of the food 

 channel. The saliva causes an irritation, and if the mosquito is in- 

 fected with malarial organisms they are introduced into the blood 

 stream of man. 



Thorax 



The thorax is composed of three segments, which are called the 

 prothorax, mesothorax, and metathorax. A pair of jointed legs is 

 attached to each segment and most adult insects bear a pair of wings 

 on the mesothorax and metathorax. The dorsal or back surface of 



