154 EEV. A. E. EATOX OX EECENT EPHEMEEID.E OE MAYFLIES. 



edge. Tracheal brancliise borne by segments 1-7 of tlie abdomen, all exposed, foliaceous, 

 fringeless, and without fasciculated fibrils ; those of segment 1 smaller than the inter- 

 mediate pairs, sometimes minute and erect. Hinder angles of the abdominal pleurae 

 hardly produced at all. Setas natatorial ; the outer setae ciliate along their inner side, 

 and the median seta plumose, either throughout or in the greater part of their length. 

 Natation agile, effected solely by undulations of the body and seta% the legs trailing 

 through the water extended at full length close to the sides. 



The genera of this type have near relations with the Leptophlehia section of the second 

 group of the Ejihemeridas. A common, though not universal, feature in the neuratiou 

 of the fore wing in this group is noticeable in the order of shortening of the sectorial 

 intercalary norvures, which are usually five in number. Their ordinary rank in diminish- 

 ing length, when numbered successively from the foremost, is 5, 1, 4, 2, 3 ; or occasion- 

 ally 5, 1 subequal to 4, 2, 3. In the third group of the genera, and also in some genera 

 of the first group, their most frequent gradation is 5, 4, 1, 2, 3. Differences in the 

 tarsi, and in the oculi of the males, disguise their affinities with the section of SipMurus 

 when adult ; but their essential approximation to that type is manifest in the nymphs. 

 The number of joints in the antenna; of nym^jhs of the Baetis type largely exceeds the 

 maximum hitherto observed in those of that section. 



Comjjsoiiciirla, a genus of the Ecdijurus section of the third group of the Ephemeridse, 

 has the cross veinlets of the fore wings arranged very much in the same manner as in 

 those of most of the genera of the Baetis type. But, without looking at its hind wings 

 and tarsi, and ajiart from other important differences, the forked prsebrachial nervure (6) 

 in the fore wing is sufficient to distinguish it from every genus of this type. 



Summary of Generical Characteristics. — Type of Baetis. 



Among miscellaneous representatives of the genera, adult and in good condition, Cloeon 

 is easily distinguished by the absence of hind wings ; Callibcetis by the large rounded 

 costal projection and numerous cross veinlets of its broad, oblong, obtuse hind wings ; 

 Baetis by hind wings as broad and obtuse as those of the preceding genus, but with the 

 costal projection (if any) small and acute, and with scarcely a cross veinlet at all ; 

 Centroptilum by the extreme narrowness of its very small hind wings, and usually by 

 the slenderness of their costal projection. 



But to discriminate from Cloeon defective specimens of the other genera deprived 

 accidentally of their hind wings is a task attended often with insurmountable difficulty. 

 Satisfactory determinations of the genera of such examples may, however, be arrived at 

 sometimes by means of the following indications afforded by the fore wings ; or, when 

 absolute determinations are impossible, a near approach may be made to their identifica- 

 tion sufficient to facilitate the comparison of the defective specimens with better examples 

 of the same insects. In the first place, it may be noted that in genera of the present 

 type the rudimentary intercalar veinlets of the terminal margin of the mesothoracic 

 wing are disposed either singly or in pairs. In a large majority of the species of Cloeon 

 ■ and Centroptilum, as well as in many of Callibcetis, these veinlets are single. As a rule, 

 the species of this last genus have more numerous cross veinlets than the others 



