'288 THE MECHANICAL FUNCTIONS. 



tinguish three component segments in the cranium 

 of insects ; while Straus Durckheim considers it 

 as formed by the consolidation of no less than six 

 segments of the vermiform larva. According to 

 this theory, the same elements, which in the tho- 

 racic segments are developed into feet, are here 

 employed to form parts having other destinations. 

 From the segment adjacent to the thorax the an- 

 tennae are supposed to be developed. The two 

 anterior segments belong properly to the face ; 

 the one giving origin to the mandibles (m), to the 

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

 (p) ; the other, producing the articulated joints 

 called the labial palpi (l). 



The mode in which the head is connected with 

 the trunk varies much in different insects. Some- 

 times it is united by a broad basis of attachment, 

 forming a joint between the adjacent surfaces: 

 but usually it is only appended by a narrow fila- 

 ment, or neck ; so that the articulation is effected 

 by ligament alone. Occasionally it is placed at 

 the end of a long pedicle, which removes it to a 

 considerable distance from the trunk. In the 

 Hymenoptera and Diptera, the head moves upon a 

 pivot, so as to admit of its being turned completely 

 round. 



The trunk, or Thorax, is composed, as shown 

 in the figure, of three segments, termed respect- 

 ively the Protliorax (x) ; the Mesotkorax (y) ; and 

 the Metathorax (z).* The first of these, the pro- 



* In these denominations I have followed the nomenclature of 

 Victor Audouin (Annales des Sciences Naturelles, torn. i. p. 119), 

 as being the simplest and the clearest : but other entomologists have 

 applied the same terms to different parts. The first segment is 



