CH. XIV.] EPHEMERA OR MAYFLY. 207 



hibits a semicircular expanse, in which the air-tubes 

 ramify like the veins in a rose-leaf (fig. 3). These 

 veins or nervures are hollow and cartilaginous. 

 The insect, like the other already described, moves 

 the fins constantly ; not, however, in a circular di- 

 rection, but from before backwards, and the reverse. 

 FIG. 1. 



FIG. 2. FIG. 3. 



In a third species of ephemera the fins are laid 

 flat along the back, and in these the organs of res- 

 piration consist of two distinct leaves. Each leaf 

 is divided into two nearly equal parts by a large 

 hollow tube, which traverses it, and thus probably 

 conveys the air, by means of little thread-like ves 

 sels which arise from its sides, to the minute emi 



