PSE UDONE UROP TERA. 67 



of one of the lines of development of winged insects. This posi- 

 tion is indicated by fan-like form of the wings, which is characteristic 

 of the primordial insects, and by the paired openings of the repro- 

 ductive organs, a feature not found in highly developed animals. 

 Our representatives of this family have not been thoroughly 

 :icd. Eaton in his monograph* enumerates less than one hun- 

 dred species from the whole of North America. But these he dis- 

 tributes among more than twenty genera. The following key is 

 based on the characters given for the genera and groups of genera 

 by this author. It will aid the student in classifying his specimens ; 

 but it should be regarded as merely provisional. 



In distinguishing subordinate groups in the Ephemeridae much use is made 

 of the venation of the wings. It is necessary here to describe only that of the 

 fore wings. The system adopted is that of Eaton. 



The principal nerves of the fore wing show a tendency to be segregated into 

 three groups, as follows: (Fig. 66.) 



c*i- y. 



FIG. 66. Fore wing of May-fly. 



Group I. This group consists of three veins, which communicate directly 

 with the thorax. These are named the cdsta (c), subcdsta (sc), and radius (r). 

 The costa is coincident witli the anterior margin of the wing; and the sub- 

 costa and radius are nearly parallel with it ; the three veins are united m-ar tin- 

 base of the wing by the great cross vein. 



Group II. This group also consists of three principal veins; but the i 

 these gives off a prominent branch before the middle of tin- wini:. which is 

 termed the slctor (s). The sector is a branch nf the cubitus (///) ; just behind 

 the cubitus is the prabrdchial ( f<> \. and behind that the postbrdchial 

 This group of veins is annexed to the first group, or terminates in the wing 



brane adjacent to it. close to the base of the wing. 



Group III. The remaining veins constitute the third proup. This is asso- 

 ciated with the prominent curved or angulated crease in the membrane of the 

 I the boundary of a depression near the base of the \\ 



: the remaining ones are tc 



etc). The anal vein as a rule subtends directly the 



anal angle of the wing; in one group of genera it is joined at its base to the 

 postbrachial vein. 



* A Re visional Monogra; -u Ephcmcridse or May-flics, by the Rev. A. E. 



Eaton. This work Is to f the Trans, of the I. inn. Soc. of London. 



Only four parts have appeared at the present wrr 



