THE THOKAX AND ITS APPENDAGES. 



65 



WP, 2Ax sAx 



connected with the anterior part of the base of the wing so that a 

 contraction of the muscle turns the wing forward and at the same 

 time depresses its anterior margin. For this reason the parapterum 

 and the extensor muscle have been called the pronator apparatus, and 

 the muscle is known also as the pronator muscle. In some insects 

 which fold the wings back against the body this muscle is a great 

 deal larger than in the bee. The -flexor muscle {RMcl) consists of 

 three parts situated upon the anterior half of the pleurum and in- 

 serted upon the third axillary {3Ax) by long tendonlike necks. 

 These muscles are antagonistic 

 to the extensor and by their 

 contraction pull the wing 

 back toward the body. 



The mechanism which pro- 

 duces the wing motion thus 

 seems to be a very simple one 

 and may be summarized as 

 follows: Each wing rests and 

 turns upon the wing process 

 of the pleurum (figs. 24 arid 

 28, WP) by means of the 

 pivotal sclerite or second axil- 

 lary in its base (figs. 26 F and 

 '2,^, 2 Ax). It is hinged to the 

 back by the first and fourth 

 axillaries (fig. 26 F, lAx and 

 Ji-Ax) which articulate with 

 the anterior and posterior 

 notal wing processes (fig. 23 

 A, r^, ANP and PNP), re- 

 spectively. The large vertical 

 muscles (fig. 27, VMcl) of 

 the thorax depress the ter- 

 gum, which pulls down with 

 it the base of the wing and 

 hence elevates the distal part — 

 the fulcrum being the pleural wing process. The dorsal longitudinal 

 muscle {LMcl) restores the shape of the thorax, elevates the tergum, 

 and consequently depresses the wing. Extension and flexion of the 

 wing are produced by special muscles (fig. 28, PMcl and RMcl) acting 

 upon its base before and behind the pleural wing process, respectively. 

 Besides these muscles there are several others (fig. 28) associated 

 with the wing whose functions are less evident. Most conspicuous 

 of these is a muscle occupying the posterior half of the mesopleurum 

 {aa) and inserted upon the outer end of the scutellum. This may 

 22181— No. 18—10 5 





X^* 



Fig. 28. — luternal view ot right yleurum of 

 mesothorax of drone, showing muscles in- 

 serted upon parapteral plates (2P and 3/') 

 and upon third axillary (SAx). The wing 

 rests upon wing process of pleurum {'WP2) 

 by second axillary (gAi') ; it is turned for- 

 ward and downward by the pronator muscle 

 {PMcl), inserted upon anterior parapterum 

 (iP) which is attached to costal head of 

 wing, and is turned back toward body by 

 flexor muscle (RUcl) inserted upon third 

 axillary (SAx). 



