6 EEV. A. E. EATON ON RECENT EPHEMEEID^ OE MAYFLIES. 



angle directly, receiving some or all of the adventitious nervures that may happen to 

 orio-inate in the interval between itself and the first axillary nervure. But there are 

 many deviations from this rule. In genera related to Folymitm-cys and in Falingenia 

 (PI. I., II., V. & VI.), from one to five adventitious nervures come between the anal 

 nervure and the anal angle ; while in Bcetisca (PL XXL), where no nervure worthy of 

 mention intervenes before the asillarics, the first and second nervures of this last group 

 extend to the terminal margin between the said angle and the anal nervure. The usual 

 interj)olated nervures in other instances are occasionally intercej)ted by the first axillary 

 instead of by the anal nervure (PI. XI. 18, &c.). In several genera the last of these 

 adventitious nervures sometimes assumes the aspect of a main branch of the anal nervure 

 (PI. I.-III. &c.). 



The axillary nervures seldom extend beyond the middle of the inner margin ; but in 

 Cloeon and its allies (PI. XVI. & XVII.) the first of them reaches to where the anal angle 

 would be in wings of a more distinctly trilateral form, and in Bcetisca (as has been 

 stated just above) both it and the second axillary nervure terminate beyond this angle. 

 In OUffoneiiria and kindred genera the axillary nervures are either suppressed, or are 

 represented only by a very few short obsolescent rudiments at the commencement of 

 the inner margin. 

 • By careful inspection of the third group of nervures, observing especially the disposi- 

 tion of the proximal extremities of the main nervures along the prominent curved fold 

 of the membrane, the form of the area contained by the fii-st axillarv nervure and the 

 inner margin, or of that enclosed between the first and the second of the axillary nervures, 

 and lastly, the general aspect of the adventitious and other nervures, the approximate 

 afiinities of Ephemeridae to one another can be ascertained very easily. 



Cross veinlets, speaking generally, are of very small account in classification, though 

 the contrary was formerly supposd. Their relative abundance or scarcity in the marginal 

 area used to be considered as an item of prime importance ; but the sexes of the same 

 species sometimes {e. g. certain species of Cloeon) difi'er from one another, in respect of 

 this very particular, more than, in other instances, the species of different genera. They 

 occasionally are serviceable in the distinction of species, more especially the veinlets in 

 the pterostigmatic portion of the marginal area : in some genera these are indifferently 

 simple or branched in individual examples of the same species, and their branches are 

 apt to anastomose with one another. The nature of the series of anastomosing branches 

 is obvious enough in actual specimens of the insects, but in figures of wings it is liable 

 to be mistaken for an adventitious longitudinal nervure, as has recently been done by a 

 distinguished entomologist. Several of the genera related to OUgoneuria have a peculiar 

 arrangement of elevated folds and cross veinlets forming communications between the 

 main nervures close to their proximal extremities, to which attention was first directed 

 by Dr. Hagen in 1855. They are indicated in only one of my figures (PI. III. 2 o, ? ). 



Nervures of the Hind IFing. First Group. — A noticeable dillerence is perceptible in 

 the composition of the first group of nervures in the hind wing, compared with the corre- 

 sponding group in the other wing, because the cubitus (5) is transferred to it from the 

 second group, and is annexed to the radius (3) either near the base (PI. I. 1 a), or 



