EEV. A. E. EATON ON RECENT EPHEMERID^ OR MAYFLIES. 13 



Adolescence is evidenced by the advancement towards maturity of the reproductive 

 organs internally, and externally by the outgrowth of rudimentary wings from the hind 

 borders of the proper segments. The forceps of the male also begin to bud forth, and in 

 certain genera an extension of the apical integument of the penultimate ventral segment 

 becomes perceptible in the female. 



Characters and FeettUarities of the Nymjih. — In their general form most nymphs nearly 

 resemble the adult. Prosopistoma is exceptional in having the body oval in outline, 

 convex above and flattened beneath ; and it possesses the faculty of adhering firmly by 

 suction, like a limpet, to stones. A N. -American ally of Ephcmcrella (PL XXXIX.), 

 longer in the body than Prosopistoma, is furnished with concavities on the pectus and 

 venter that are densely pilose, which appear to aiTord it a similar power. Many other 

 genera have the body of the nymph dilated and flattened beneath more than it is in the 

 imago, but not Avith any view to its employment as a means of adhesion. 



Head diversiform, prominent, usually about as wide as the thorax, vertical or else 

 pretense; cranial sutures mostly distinct. Labrum (when present) transverse, emar- 

 ginate in front, and rounded off at the anterior corners ; when it is absent, a velvety fold 

 of the palate-membrane completes the enclosure of the mouth opening immediately 

 above the mandibles at a distance from the edge of the epistoma. Frons usually even, 

 but sometimes (e. g. in Ephemera) produced into short projecting points. Ocelli three, 

 small. Oculi moderate, becoming large in the adolescent male. Antenna! slender and 

 tapering (their first two joints the stoutest), usually many-jointed and long, but some- 

 times few-jointed, very short and subulate; in many genera the joints are nearly bald, 

 whilst in others (e. g. Epihemera) they arc Ijcset near their tips each with a whorl of long 

 spreading hair. Mandibles strong, with more or less asymmetrical dentition, which com- 

 prises usually a molar surface in addition to fang-like lobes ; but in some predatory 

 genera (Pis. XLIII. & LIII.) the molar region is wanting. The cndopodite is often 

 represented by a slender jointless movable appendage attached to the inner base of the 

 inferior lobe (as in Sii)]ilurii.s, PI. L.), or sometimes by a tuft of hair [Ecdijurus, 

 PI. LXIL). The lobes are remarkably abrupt in Bactls (PI. XLIV.). The outer lateral 

 region of the mandible in a Palinfjeuia from Ceylon (PL XXV.) is produced into a 

 massive enlargement continuous with the crown ; in Potamanthus it is armed with a 

 tooth-like tubercle (PI. XXXI.); in Polijmitarcijs, Ephemera, &c. it is extended into a 

 strong tapering tusk, distinct from the crown, variously furnished outside with tubercles 

 (Pis. XXVIII. & XXX.); this prolongation is more slender in Euthi/plocia and densely 

 hairy (PI. XXIX.). First maxilla unaccompanied by a galea, its inner edge generally 

 either hairy or rigidly setulose, but sometimes spinose, the point often pungent, the 

 crown variously armed, sometimes with long fine hair distributed evenly upon it 

 (Oliffoiwuria, Poli/mitarci/s, Pis. XXVI.-XXVIII.), sometimes with a dense beard of 

 harsh hair, as in Ealrophlehia (PL XXXVL), at other times with pectinate spinules, as 

 in Chirotonetes, MhUhrogena, &c. (Pis. XLIX., LIV., &c.). Palpus of first maxilla 

 usually 2-, 3-, or 4.-jointed, according to the affinities of the genus [but in one instance 

 multi-articulate (PL LIII.)], and of various relative length,— extremely long in Eiithj- 

 plocia (PL XXIX.), very short in Ep)hemerella and its kindred (Pis. XXXVII.-XL.). 



