222 ^ THE WINGS OF EPHEMERIDA 



been the chief lines along which specialization has extended. The former 

 has doubtless added much to the efficiency of the wings; but a too close 

 adherence to the latter method of specialization has resulted in the forma- 

 tion of a rather indifferent organ; although it is the most perfect develop- 

 ment of its peculiar type. 



(e) THE HOMOLOGIES OF THE WING-VEINS OF THE HIND WINGS OF THE 



EPHEMERIDA 



The hind wings of the Ephemerida are always reduced in size and in 

 venation; even in the most generalized forms some of the wing-veins are 

 wanting. This fact has made it difficult to determine the correspondence 

 of the veins of the fore and hind wings. 



Fig. 222. — Fore wing of Chirotonetes albomanicatus (After Morgan). 



In the course of the preparation of this account, it occurred to me to 

 make use, for this purpose, of a criterion that had not been previously 

 employed, that is the corrugations of the wings. 



It is probable that in the evolution of the fan-type of wing in this order 

 the two pairs of wings became corrugated in a similar manner ; this we know 

 to be the case in the Odonata ; that is, the same veins become convex or 

 concave, as the case may be, in the two pairs of wings. 



After the corrugations had become established, it is not at all probable 

 that in the course of the reduction of the hind wings concave veins should 

 become convex or that convex should become concave. 



In the table on page 220 the nature of the veins, whether concave or 

 convex is indicated; and in Figure 222, which represents a fore wing, and 

 Figure 223, which represents a hind wing, both of Chirotonetes albomanicatus 

 the convex veins are designated by plus signs and the concave veins by 

 minus signs. 



A study of these two figures shows that there is a quite close corres- 

 pondence between the venation of the two wings. The more striking 



