THE THORACIC EXO-SKELETON. 163 



The posterior part of the base remains membranous, and 

 forms an axillary fold which checks the forward movement 

 of the wing. I shall term the dorsal nervures costal, sub- 

 costal, and patagial; and those of the ventral ridges hypo- 

 costal and median nervures. The costal nervure forms the 

 anterior thick margin of the wing. The wing membrane at 

 its attachment to the thoracic wall dips downwards and 

 backwards between the costal and hypocostal, or second 

 nervure ; it then ascends nearly perpendicularly to the sub- 

 costal. It descends backwards to the median, and reascends 

 to the patagial nervure. 



The three superior nervures terminate on the dorsal aspect 

 of the thorax in three tubercles, which articulate with the edge 

 of the dorsal plate. 



The anterior tubercle supports the marginal nervure. The 

 median tubercle is a complex sclerite, very narrow above 

 where it bears the subcostal nervure, but broad below where 

 it supports the hypocostal ; the posterior tubercle terminates 

 in the median and patagial nervures. 



The wing joint, therefore, consists of three parts ; each 

 capable of independent movement, and acted upon by special 

 muscles. I shall term them respectively the pro-, meso-, and 

 metapterygium. 



These are present in the wings of every insect I have 

 examined. The pro- and mesopterygium are generally more 

 largely developed in the anterior wing, whilst the metapterygium 

 is more developed in the posterior wing. 



Movements of the Wings, — The wings move in the horizontal 

 and vertical planes ; the forward movement is extension, the 

 backward flexion. The upward and downward movements are 

 elevation and depression. 



The wings are further capable of rotation on their own long 

 axis ; this action is similar to the feathering of an oar, and is 

 of the utmost importance in the mechanism of flight. I shall 

 term it rotation. 



The Wing Joint admits, therefore, of flexion, extension, 

 elevation, depression, and rotation, although the extent 



II 



