12 



University of California Puhlications. [Entomology 



FIG. 1. A portion of the abdomen of a mayfly 

 nymph {h'ithogena itiaiiifei^la Eaton), showing 

 tracheal sills and gill covers. 



may be seen in the gill cover of a species of mayfly common 

 in Illinois, Rithogena manifesta Eaton {Baetis debilis Walsh); 

 as shown in Fig. 1. This is not the most wing-like gill found 



in the family, but will 

 show as well as any the 

 tendency of its specializa- 

 tion toward the structure 

 of a wing. 



The shape, while not 

 exactl}^ like any existing 

 wing, still in its general 

 outline suggests very 

 closely the form of wing 

 which several authors 

 have considered the prim- 

 itive shape. 



The ornamentation of 

 the edge by spines and 

 hairs is so exceedingly wing-like that it might almost be 

 duplicated in detail by the wings of some of the loAver Diptera. 

 The surface of this gill cover, while not entirely free from 

 the function of respiration, still has most of its area modified, 

 and in the apical region possesses an arrangement of surface 

 hairs not essentially different from that found on the wings 

 in many groups of insects. 



The most convincing evidence of the relation of such an 

 organ as this gill cover with wings is to be seen in the vein- 

 like stiffening of the membrane. These structures possess all 

 the characteristics of veins. They have, moreover, a definite 

 relation with rows of special spines, such as so often occur with 

 veins. The only matter characteristically different is the want 

 of any relation between these veins and the tracheae of the gill 

 covers. The gill covers are quite well supplied with tracheae, 

 but they do not in any instance correspond with the vein-like 

 structures in these organs. It is possible that we have here a 

 stage in the development of the wing preceding that in Avhich 

 the trachea^ become coordinated with the veins. 



Still another matter making these gill covers comparal)le 

 with wings is the nature of the articulation. The alar articu- 

 lation must have been produced from a structure of this sort. 

 A great many insects living in Avater have a very curious 



