THE INSECT HEAD SNODGRASS 477 



in insects, because at an early evolutionary period they were united 

 with each other forming the median, posterior appendicular organ 

 of the head known as the labium, which has since undergone many 

 special modifications in its structure. The labrum and the hypo- 

 pharynx have been least affected in the evolution of the mouth parts, 

 but even these organs in some of the piercing insects have suffered 

 radical changes of form in compliance with special functions they 

 have assumed. 



In the following descriptions there will be discussed only the 

 fundamental modifications of the primitive mouth parts that have 

 given these organs their typical structure in the so-called biting and 

 chewing insects. 



TJie labrum. — The labrum in its typical form, as seen in the 

 cricket (figs. 9 A, Ltn^ 21 A), is a broad flat lobe movable by a 

 transverse line of flection on the lower edge of the clypeus. The 

 muscles of the labrum take their origin on the frons. In general- 

 ized insects there are two pairs of them, one pair (fig. 21 A) inserted 

 anteriorly, the other posteriorly, on the labral base, the posterior 

 pair being usually attached on small bars known as the tormae. 

 In the cricket the anterior labral muscles are united into a single 

 bundle of fibers. The posterior wall of the clypeus is often elevated 

 in the form of a median lobe, of various shapes in different insects, 

 called the efiiiharynx (fig. 4, Ephy). 



The mandibles. — The mandibles are the jaws of ordinary biting 

 insects. Their primary structure as jaws is seen in some of the 

 apterygote insects (fig. 2 B, Md)^ where they closely resemble the 

 mandibles of the more generalized crustaceans, such as the pliyllo- 

 pods (fig. 2 A, Md).^ and Anaspides (fig. 18 A). The insect mandi- 

 bles differ from the crustacean mandibles in that they always lack 

 palpi. 



A generalized mandible of the apterygote insect type of structure 

 is an elongate organ, implanted by the broad inner surface of its 

 base on the membranous latere ventral wall of the head (fig, 2 B, 

 Md), to which it is hinged by a single dorsal point of articulation 

 {a'). The jaw is moved by dorsal and ventral muscles. The dorsal 

 muscles comprise two distinct fiber bundles arising on the dorsal 

 wall of the head, one inserted anteriorly on the base of the mandi- 

 ble (fig. 20 A, /), the other (/) posteriorly. These muscles, there- 

 fore, are the primitive dorsal promotor and the dorsal remotor of 

 the appendage (fig. 14, /, J.) , and probably serve to rotate the mandi- 

 ble on its long axis. The ventral muscles, which are functionally 

 adductors, usually comprise two groups of fibers (fig. 20 A, KL) 

 arising within the hollow of the mandible. Those of one group 

 {KLz) are attached on a median ligament (s), and the fibers from 



