THE WINGS OF EPHEMERIDA 



219 



wing-pads showed this connection, while half of them showed no sign of it. 

 Figure 217 shows the tracheation of a wing of a nymph in which the connec- 



Fig. 219. — Wing of a nymph of Hexagenia (After Morgan). 



tion of trachea Rg with trachea R was retained; and Figure 218 represents 

 the venation of the adult of the same species. 



It is evident that in Heptagenia interpunciata we have a species that 

 illustrates a transitional stage in the switching of the basal connection of the 

 trachea Rg. In some individuals the primitive connection of trachea Rg 

 with trachea R is retained; in others, trachea Rg has been transferred to 

 trachea Mi. With this fact in view I have no doubt that the vein which 

 Miss Morgan designated as vein Rg? is really vein Rj; and I suggest the 

 omission of the note of interrogation. 



The series of wings of nymphs figured by Miss Morgan shows a remark- 

 able evolution of the tracheation, which consists of a continuous reduction 

 of large tracheae and a replacement of them by small tracheal branches. 



Fig. 220. — Wing of a nymph of Leptophlebia (After Morgan). 



Three figures selected from the series given in her paper will serve to 

 illustrate this, (Figs. 219, 220, and 221). 



(g?) the corrugations of the wings of the ephe.merida 



The fan-like structure of the ephmerid wings has been referred to by 

 many writers. But it is worth while to point out in this place the degree 



