EEV. A. E. EATOX ON KECEXT EPIIEMEEID^ OR MAYFLIES. 251 



dissimilar. Penis-lobes narrow, often extremely so when dried and apt to shrink to mere 



peg-like projections (compare PI. XXIV. 43^). — Fore leg of c? l^-li the length of the 



body : tarsus about 1| as long as the tibia, and this ly the length of the femur ; the 



joints in order of their shortening rank 3 subeqnal to 2, 4, 5, 1, and the first is about ^ as 



long as the second joint. Fore leg of ? about as long as the body : tarsus little more than 



J the length of the tibia, and this nearly 1^ that of the femur : the joints, in the order of 



their shortening, rank variously in diiferent species, the second or the fifth being the 



longest and sometimes of the same length, the third usually shorter (rarely as long as 



the fifth), the first usually equal in length to the fourth (rarely a little longer) and about 



^ as long as the second joint. Hind tarsus S about f the length of the tibia, which 



is little shorter than the femur ; its joints, in order of their shortening, rank 5, 1 equal to 



2, 3, 4 : those of the intermediate tarsus rank 5, 2, 1 equal to 3, 4. Ungues each unlike 



the other in every tarsus. Hind wings of the usual form; axillary region narrow. The 



relative lengths of the dorsal segments of the dried $ abdomen may be formulated 



thus : — 5, 9, 12, 14, 13, 10, 9, 5, 4 ; ventral lobe of the ninth segment either obtusely 



rounded or slightly retuse at the tip. Forceps-basis of the d usually concave in the 



middle behind, rarely saHently curved. Penis-lobes divergent from each other during 



life, often contiguous when dried: inferior stimuli weU developed, usually closely 



appressed to the lobes : apertures of the seminal ducts terminal. Outer caudal setae of 



d usually 2-2|, seldom 3 times, the length of the body, those of ? about twice its length. 



Median ocellus of 2 prominent, contiguous with the anterior border of the upper surface 



of the head : the occipital border very slightly (if at all) convex in the middle, almost 



even with the posterior orbits of the oculi. Pronotum of ? somewhat cordately excised 



behind ; the lateral lobes not extensively reflexed. 



SiMmago. — Wings tinted with grey [yellowish in a single species from Arizona]: 

 neuration not conspicuously darker than the membrane, nor bordered with pigment 

 especially. The ordinary length of this period is 24-36 hours : the insect dm-ing repose 

 stands upon all its feet with wings erect and setae divergent. 



Nymph. — In the pose of the laminae of the abdominal tracheal brancliiae — the foremost 



pair attaining mutual contact at the tips of the laminae beneath the metasternum, the 



intermediate pairs reclining outwards (figs. 1, 2) when not deflected out of sight (fig. 3), 



and the hindermost pair transversed lengthwise by an eccentric open fold and brought 



together beneath the penultimate segment — this nymph resembles that of Iron. But the 



iaminse are slightly different in form from those of that nymph : the foremost are very 



arge, transverse, oblong, and curved : the other laminae exhibit successive gradations in 



brm from subrotund-oblong, through oblong-obovate and obovate to oval : again, 



dthough their inferior margins are ciliate thev are not thickened. Moreover the 



asciculated fibrils are longer in proportion to the laminae and more lax than in Iron ; 



md the branchial tracheae are invisible, or become obliterated after death. Median 



■audal setce subeqnal to the others, nearly as long as the body, minutely plumose for 



ome distance beyond its basal ^ : the other setae ciliated correspondingly on then- inner 



ides. Pronotum transverse, very short, undilated, and either curved or nearly straight 



t the sides : its posterior margin well defined, saliently rounded off on each side of a 



