No. 1774. 



THORAX OF IIYMEN0PTERA—SN0DGRA8S. 



51 



the third articulates more or less directly with the posterior process. 

 Distad to the second axillary, and associated with the bases of the 

 media (M), the cubitus (Cu) and the first anal (lA), when these 

 veins are distinct at their bases, are one or two small median plates (m) 

 which are not of constant occurrence, and which vary much in differ- 

 ent forms when they are present. It will be observed that the base 

 of the costa is not associated with any of the axiilaries. The mem- 

 brane of the wing base directly connects this vein with one or both 

 of the episternal paraptera, upon which is inserted the extensor muscle 

 of the wing, called also the "pronator" because, while it turns the 

 wing forward, it at the same time depresses the anterior edge. 



The diagram, figure 8, is constructed from a study of all the orders, 

 for no one form shows all the parts of the wing so generalized. Many, 

 however, approximate it. The stonefly shown in figure 1 is very 

 simple. In it the subcosta {Sc) articulates with the first axillary 

 {lAx), the raidus {R) is continuous with the second {2 Ax). The 

 media ( M) fuses basally with the radius, 

 but a distinct median basal plate (m) is 

 present. The cubitus {Cu) does not reach 

 the wing base, and there is no separate 

 first anal. The other anals {A) are con- 

 nected with the third axillary {3 Ax) , which 

 is articulated directly to the posterior wing 

 process (PNP), the fourth axillary being 

 absent. The wing of the caddicefly, shown 

 in figure 9, is likewise very generalized, 

 though it not only lacks a fourth axillary, 

 but has also no median plates. The 

 media (M) is here, again, fused with the 

 base of the radius (^), which is continuous 

 with the second axillary (2 Ax). The sub- 

 costa (Sc) articulates, by a prominent basal knob, with the first axil- 

 lary (lAx). The cubitus (Cu) and the first anal (lA) reach the 

 base of the wing as separate veins, while the other anals are associated 

 with the third axillary (3 Ax). 



The first, third, and fourth axiilaries are developed in the dorsal 

 wall of the wing sac. The second is formed from united dorsal and 

 ventral elements. The latter is clearly shown as a separate piece in 

 the wing of Tseniopteryx frigida (fig. 10, SAx), and has attached to 

 its posterior end the large internal chitinous axillary disc (AxD). The 

 ventral part articulates with the pleural wing process, while the 

 dorsal part (fig. 1, 2 Ax) carries the radius (R), which is but 

 weakly developed below. The axillary disk (fig. 10, AxD) is of very 

 general occurrence and bears the dorsal insertion of the coxo-axillary 

 muscle, whose lower end is attached to the rim of the coxa of the 

 same segment. The disk is very commonly attached to the second 



AxD" 



Av.C 



Fig. 10.— Ventral surface of base 

 of front wing of t^niopteryx 

 frigid a (see fig. 1): 2ax, ventral 

 element of second axillary; 

 AxC, AXILLARY cord; AxD, axil- 

 lary disk; C, costa; R+M, basal 

 trunk of radius and media; tg, 



TEGULA. 



