3*22 THE MECHANICAL FUNCTIONS. 



different insects, and is in general proportion ably 

 larger than it is in the larva state. Its integu- 

 ment, which, from analogy with vertebrated ani- 

 mals, has been called the skull, or cranium, 

 (c, Fig, 150), is usually the hardest part of the 

 general crust. Although it may appear, a on 

 superficial examination, to consist of a single un- 

 divided piece, yet, on tracing its gradual forma- 

 tion, it is found to be in reality composed of a 

 union of several of the segments of the larva. 

 Audouin and Carus distinguish three component 

 segments in the cranium of insects ; while Straus 

 Durckheim considers it as formed by the con- 

 solidation of no less than six segments of the 

 vermiform larva. According to this theory, the 

 same elements which in the thoracic segments 

 are developed into feet, are here employed to 

 form parts having other destinations. From the 

 segment adjacent to the thorax the antennae are 

 supposed to be developed. The two anterior 

 segments belong properly to the face ; the one 

 giving origin to the mandibles (M), to the 

 maxilla?, or proper jaws, (j), and also to the palpi 

 (p) ; the other producing the processes called the 

 labial palpi (L). 



The mode in which the head is connected with 

 the trunk varies much in different insects. 

 Sometimes it is united by a broad basis of at- 

 tachment, forming a joint between the adjacent 

 surfaces : but usually it is only appended by a 



