196 ANATOMY AND PHYSIOLOGY 



Muscles analogous to those just noticed exist in 

 insects whose prothorax is connate, that is, intimately 

 united to the succeeding segment ; but, in these the 

 mesothorax being most highly developed, it is there 

 that they acquire their greatest dimensions, and one 

 pair is generally enlarged at the expense of another, 

 Thus, the dorsal pair is most voluminous in the Hy. 

 menoptera and Lepidoptera, while it is the lateral 

 pair in the Diptera. 



Many of the muscles already noticed contribute, 

 in a greater or less degree, to promote the act of flight, 

 by contracting or dilating the walls of the thoracic 

 cavity, but there are a few to which the office of 

 moving the wings is exclusively assigned. . These 

 originate from the lateral parts of the sternum, and 

 are attached by pointed tendons to the principal ner 

 vures of the wing. Their development is always i 

 proportion to that of the wing which they are destine( 

 to move. If the anterior wings be largest, as state( 

 by Burmeister, the dorsal muscle of the anterio 

 wing is likewise the largest ; if the posterior wings 

 are wanting, their extensor is also wanting, and 

 if both are of equal size, their extensors also are 

 of equal size; but, if the posterior wings are the 

 largest, this is likewise the case with their extensor 

 as may be seen in the Coleoptera, while the extenso 

 of the elytra in that order is very small. A sma 

 extensor, flexor muscles, and a series of smaller ones, 

 which, when in action, cause the relaxation of the 

 extensors, are the other motive instruments of the 

 wings. 



