56 



THE GENERAL FEATURES OF WINGS 



The above generalization is true of all of the orders of winged insects 

 except the Ephemerida and the Odonata. In these two orders the flexible 

 bases of wing-veins merge into the lateral margins of the notum, and only 

 one distinct axillar}^ sclerite is present at the base of each wing. 



The axillary sclerites. — There are typically four axillary sclerites; but 

 occasionally some of these are subdivided and sometimes there occur small 

 extra chitinizations in the axillary membrane. 



The four principal axillary sclerites are designated as the first, second, 

 third, and fourth axillar}^, respectively. Their relative positions are indi- 

 cated in Figure 38. "Two of these, the first {lAx) and \he fourth {4AX), 

 form a hinge with the anterior and the posterior notal wing processes, 

 respectively, while the second {2 Ax) articulates below with the wing process 

 of the pleurum, constituting thus a sort of pivotal element. The third 



Tg — 



Fig. 38. — Diagram of a wing showing the axillary sclerites 

 (From Snodgrass). 



axillary {sAx) inteiTnediates between the bases of the anal veins and the 

 fourth axillary — except when the latter is absent ( as it is in nearly all insects 

 except Orthoptera and Hymenoptera), in which case it articulates directly 

 with the posterior notal process. 



"The base of the costa is not directly associated with any of the axil- 

 laries, but is specially connected by tough membrane below with the 

 episternal paraptera. The subcosta abuts against the end of the curved 

 neck of the first axillary. The radius is either attached to or touches ui)on 

 the anterior end of the second. The media and the cubitus are usually 

 associated with each other at their bases and also more or less closely with 

 one or two median plates (ni) in the wing base. These plates, however, are 

 not of constant shape and occurrence as are the articulating axillarics. The 

 anals are generally attached to the outer end of the third axillary, which 

 acts as a lever in the folding of the wing. 



"A few insects have a generalized wing almost identical with the 

 diagram (Fig. 38), but most of them depart from it in varying degrees." 

 (Snodgrass, '10). 



