STRUCTURE OF INSECTS. 289 



thorax, carries the first pair of legs ; the second, 

 or mesothorax, gives origin to the second pair of 

 legs, and also to the first pair of wings, or to the 

 elytra (e), as in the example before us; and the 

 third, or metathorax, supports the third pair of 

 legs, and the second pair of wings (w). These two 

 last segments are closely united together ; but the 

 original distinction into two portions is marked by 

 a transverse line. Each of these three segments 

 is divisible into an upper, a lower, and two lateral 

 portions, which are joined together at the sides of 

 the trunk ; these again admit of further subdi- 

 vision ; but for the names and descriptions of the 

 smaller pieces I must refer the reader to works on 

 Entomology. The parts of the thorax to which 

 the wings are attached indicate the situation of 

 the centre of gravity of the whole insect ; a point, 

 which being in the line of the resultant of all the 

 forces concerned in the great movements of the 

 body, requires to be sustained by the moving 

 powers under all circumstances either of action or 

 repose. 



Victor Audouin, who has made extensive re- 

 searches on the comparative forms of all these parts 

 in a great variety of insects, appears to have satis- 

 factorily established the general proposition that, 

 amidst the endless diversity of forms exhibited by 

 the skeleton of insects, they are invariably com- 

 posed of the same number of elements, disposed in 



termed by Straus Durckheim and other French writers, the Corselet. 

 Mr. Kirby calls it the Manitrunk, and restricts the term Prothorax 

 to its upper portion. The united second and third segments are 

 the Thorax of Straus Durckheim, the Tronc alifere of Chabrier, 

 and the Alitrnnk of Kirby. 



VOL. I. U 



