18 EEV. A. E. EATON ON RECENT EPHEMEEIDJi OE MATELIES. 



worthy guide to the succession of the genera ; but it is unequal to the actual requirements 

 of the case ; compare, for example, the fore wing of Baetis Salvini (PL XVI. 29 a) with 

 that of Cosmetogenia (PI. XXIII. 42). The most recent suggestion as to the classitication 

 of the genera was to group them according to the structure of the tracheal branches of 

 the nymphs. According to this scheme Haljrojihlchia (PI. XXXVI.) and Thraulus 

 (PI. XXXV.) would fail to come into the same group as Choroterpes (PI. XXXIV.) and 

 Blasturus (PL XXXIII.). Indeed if dependence be placed implicitly upon the modifica- 

 tions to which this organ or that is subjected in the adult, or in the junior conditions of 

 the insect, the scheme of classification arrived at can hardly fail to be unnatural and 

 arbitrary. It is only by taking cognisance of points of difference and agreement in many 

 details, in the anatomy and the mode of development and the habit, of leading repre- 

 sentatives of the various alliances of genera, at different periods of their lives, before and 

 after their exclusion from the egg, that the mutual affinities of the several associations 

 of genera to one another can be demonstrated adequately. Until such comparisons can 

 be and shall have been carried out, the whole question of their arrangement can only be 

 dealt with in a tentative and experimental manner ; and it will be fortunate if error be 

 avoided in the necessary grouping of the genera into provisional alliances of apparently 

 kiucked forms, preparatory to the study of their affinities. It is far more easy to demon- 

 strate defects in proposed methods of classification than to devise a trustworthy system 

 in their stead ; and possibly extended observation in the futu.re may eventually show that 

 some of the bases of arrangement adopted in this present work are mere temporary 

 expedients worthy of mention in this paragraph. 



History of the Classification of the EPHEMERiDiE. 



The species of Ephemeridae known to Linn6 were arranged by him in two sections of 

 one genus Ephemera, according to the number of their caudal setae — species with three, 

 and with two sette, respectively. 



No further subdivision of the family was attempted until the year 1815, when Leach 

 separated the latter of Linne's sections into two genera, Baetis with four wings, and 

 Cloeoii with two. In the hands of subsequent entomologists these two genera became 

 ' obscured by the interpolation of extraneous forms. To Baetis, Say referred sundry species 

 of Hexagenia and ILeptageiiia, as well as a Bwtisca ; and eventually this name was 

 diverted altogether from the original type, and was misapplied by general consent to 

 species of Heptagenia and Siphlurus. 



Curtis, in 1834, proposed another genus named Brachycercus, like Cloeon deficient in 

 hind wings, but distinguishable from it by its having three setae. This name was suitable 

 for the female insect only, with which sex alone he was acquainted. 



Stephens, in 1835-6, possessing males of Brachycercus, finding them differ in very 

 obvious particulars from Curtis's definition of the genus, and failing to perceive that their 

 points of difference were of merely sexual character, established a genus Ccenis with two 

 sections, species with three long setse, and species with three short setse, a section for 

 each sex. He also transferred the genus from the position after Cloeon previously 

 assi-^ned to it, to the end of the section of Ephemeridae with three setse. 



