THE THORACIC EXO-SKELETON. 203 



The Parapteron, seen from within (PI. VIII., Fig. 9, 77), exhibits four pro- 

 cesses : an ascending process orpivot, which rests inasocketinthedorso-pleural 

 rosta, on which the sclerite turns ; an anterior process (a), which rests against 

 the lateral plate ; and an inferior or long process (<), which joins the apodeme 

 of the pre-epaulet. The closure of the meso-pleural syndesmosis causes the 

 short anterior process to turn outwards ; this movement carries the pre-epaulet 

 forwards in a semicircle, throwing the anterior margin of the wing forwards. 

 A fourth process, behind the body of the sclerite, gives insertion to a muscle 

 which opens the meso-pleural syndesmosis and so drives the anterior margin 

 of the wing backwards, flexing the wing. 



The Metapterygium (PL X ,mp) consists of a single sclerite, composed of 

 two limbs united at an obtuse angle. The posterior limb or shaft articulates 

 by a sub-cylindrical head with the crutch-shaped extremity of the posterior 

 alar apophysis. The shaft somewhat resembles a humerus, and Jurine 

 called it after this bone ; the anterior limb is deltoid in form I term it the 

 'deltoid.' 



The shaft of the metapterygium is concave on its under and convex on 

 its upper surface ; its outer edge supports the squamula ; several muscles 

 are inserted into its concave surface. A strong apodeme projects below its 

 head into the thoracic cavity. 



The deltoid articulates with a T-shaped nervure, the tau (Mihi), which 

 supports the patagial nervures. The proximal edge of the deltoid presents 

 an anterior and a posterior angle ; the former is united with the extremity 

 of the shaft the latter articulates with a conoid sclerite, the conus (k], which 

 forms a kind of locking plate between the pro- and metapterygium. 



The shaft of the metapterygium is capable of flexion and extension on the 

 posterior alar apophysis, and of rotation on its own axis, as well as of abduc- 

 tion and adduction to a limited extent. 



A compound movement, which throws the apex of the deltoid backwards 

 and upwards, raises and opens the patagium ; the reverse movement closes 

 and lowers it. Abduciion extends and adduction closes the patagial portion 

 of the wing. A simultaneous abduction of the metapterygium and extension 

 of the propterygium extends the wing disc. 



An ascent of the anterior angle of the deltoid accompanies an upward 

 movement of the metapterygium, and causes the anterior margin of the wing 

 to descend, by its action on the conus being transmitted to the rernigium. 

 This gives the wing a screw surface, with the anterior margin of the wing 

 depressed and its posterior margin raised. The descent of the apex of the 

 deltoid produces an opposite rotation of the remigium, so that the anterior 

 margin of the wing rises and its posterior margin is depressed. 



The Movements of the Wing in Flight. If a fly is held by its 

 legs, the wings are frequently seen to vibrate as in flight. The 

 movements of each wing, when seen obliquely from below, 

 produce the optical appearance of a double cone (Fig. 36). 

 The line abcdefghis the locus of successive positions of 



