EEV. A. E. EATON OK RECENT EPHEMEEID.E OR MAYFLIES. 3 



with ten pairs of stigmata, two tlioracic and eight abdomiual. Ventral nervous tract 

 slightly abbreviated, and posteriorly somewhat concentrated, in P rosopistoma ex- 

 tremely so. 



Peculiarities in structural detail are often noticeable in both or one of the sexes, and 

 are chiefly presented by the ocelli, wings, legs, and caudal setiB, and in the male by the 

 oculi and forceps. 



The foremost ocellus is sometimes as large as the others, sometimes much smaller. 



The oculi, always much larger in the male than in the other sex, are in him, in some 

 genera, divided each into two parts transversely ; the upper portion has larger facets 

 than the lower, and is sometimes coloured differently from it. The division between 

 these segments of the oculus may amount to nothing more than a mere superticial 

 furrow or impressed line traversing the faceted surface horizontally ; but when it is 

 deeper, the upper part of the oculus (always much the larger of the two) assumes a short, 

 subcylindrical or turbinate form, faceted only on its summit, and supports on its outer 

 base the smaller division, which is oval, and is faceted all over. 



The fore wings, seldom ovate or oblong, are usually trilateral, ample, and rounded off 

 at the extremities. Their margins are unequal in extent, the costal or anterior margin 

 being slightly (sometimes not much) longer than the terminal or outer margin (measured 

 along the curve), and seldom less than double the length of the inner margin. The 

 costal margin is somewhat sinuous as a rule ; nearly straight at the base of the wing, it 

 generally presents a shallow sinus midway towards the apex, and then proceeds in a 

 gradual salient curve to its extremity. The terminal margin is arched ; its curvature is 

 sometimes stronger towards its extremities than in its midst, where it is occasionally 

 almost straight. In the greater part of its course the inner margin is usually straight or 

 incurved ; but there are cases in which it describes a salient curve continuous with that 

 of the terminal margin. The wings are relatively longer in the female than in the male. 



The hind lobiys in some of the genera are not developed ; in others they are very 

 minute ; at the most they are not particularly large. Their usual form is triangular, 

 ovate, or oblong-ovate, with a salient prominence in front, either close to the wing-roots 

 or midway towards the apex, in which last case the prominence is sometimes {e. g. in 

 Rabr-ophlebia) followed by a deep sinus ; their neuration is fairly plentiful. When they 

 are very minute their nervures are very scanty, and their form is usually oblong or 

 linear-lanceolate. 



The inner margin of the fore wing and the anterior margin of the hind wing hitch 

 together automatically to a larger or smaller extent when the wings are spread open. 



The iciiig-rnembrane is usually glassy and iridescent in the adult. In OUgoneuria 

 and some other genera, however, the subimaginal pellicle of the wings is not shed, and 

 these are dim in consequence (viewed as transparencies) ; their reflection too differs from 

 that of the wings of other Mayflies in being either glossy instead of iridescent, or else (as 

 in Lachkmia &c.) of uniform azure glow. Pigment is often deposited in the marginal 

 and submarginal areas of the fore wings, and occasionally in all of the wings beside some 

 of the nervures and cross veinlets as well as at the wing-roots ; by the confluence of 



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