THE GENUS DENDROCTONUS. 49 



dorsal or inner edge of the head of the subcosta and with the clav- 

 icle condyle. 



Subscapular plate (b) . — The subscapular plate is more complicated in 

 its structure than the scapular plate, to which it is a direct accessory, 

 the two being closely joined by articulating ligament. It functions 

 as an intermediate patella-like connection of the system of tendons 

 which connect the pleural disk at the head of the pleural muscle with 

 the flexor, head of scapular plate, head of coracoidal process, head of 

 subcosta, etc. Therefore it must be of fundamental importance in 

 wing motion. 



Radial plate (d) . — The radial plate is represented by a thin chitinous 

 piece connecting the radius with the subscapula. 



Medial plate (e) . — The medial plate is of flexible chitin connecting 

 the media with the flexor and subscapula. 



Both the radial plate and flexor plate evidently function as articu- 

 lating accessories. 



Flexor plate (c) . — The flexor plate comes next to the scapular plate 

 in its fundamental importance in wing connection and articulation, and 

 is especially fitted in structure and muscular connection for its pri- 

 mary function of flexing and longitudinally folding the wing, as well 

 as in the reverse action of contributing to its outward extension and 

 rigidity during flight. 



Mesothoracic Wings or Elytra. 



The form and general structure of the elytra are shown in figures 1 

 and 31. They are oblong, rigid shields, with a subacute apex and a 

 truncate declivous base and a produced articulating head. The 

 structure, like that of the metathoracic wing, consists of two layers 

 of integument inclosing the tracheal and circulatory system, but 

 instead of the dorsal and ventral layers being partiall}^ composed of 

 flexible membrane, they are chitinous throughout. The ventral 

 layer is thin and smooth, while the dorsal one is thick and deeply 

 sculptured. 



Tracheation. — The six primary tracheae (figs. 31, 34) occupy the 

 marginal and the alternating longitudinal spaces between the rows 

 of punctures. Each has numerous fine laterpJ branches passing be- 

 tween the punctures into the intervening interspaces, producing a 

 network of fine tracheae, with the punctures representing the mesh. 

 Thus we have a probable explanation of the primary cause of the 

 system of punctures in the elytra and the longitudinal and tranverse 

 thickened spaces between them. The thickened and elevated areas 

 are due to a concentration of chitin over the tracheal and circulatory 

 canals, while the punctures and grooves are the points of adhesion or 

 junction of the two layers to form the walls between the canals. 



