EEV. A. E. EATON ON EECENT EPHEMEEID.E OE MAYFLIES. 17 



In some genera the body and limbs are partially clothed with hair. This is generally 

 simple ; but EphemereUa and some kindred nymphs have clavate hairs in certain regions, 

 closely resembling in theii- spinnlose structure the thickened hairs of simdry Trombidiufe ; 

 and some of the pubescence on the head of a Cingalese PaUngenia is microscopically 

 plumose. The figure of the mandible of Frosojnsfomn (PI. XLIII.) is not on a scale of 

 enlargement sufficient to show the plumose structure of the bristles adjacent to the 

 endopodite. 



The term ' Subimago ' is used to denote the penultimate stage in the life of such of the 

 Ephemeridse (the large majority of them) as moult once after direct respiration through 

 the stigmata has been established, and their wings have become fully expanded. The 

 chief points whereby insects in this condition can generally be distinguished from adult 

 examples are — the duluess of the integuments, especially that of the wings ; the ciliolate 

 terminal margin of the wings in many genera ; the brevity of the fore legs ; the greater 

 hairiness and shortness of the caudal seta?; the less protuberant and less brightly 

 coloured oculi ; and, in the male, the marked shortness and stoutness of the limbs of the 

 foi'ceps. Where (as in Caads and Trlcorytlms) the wings are ciliolate both in the 

 subimago and in the adult, and where (as in Polynutarcys) the wings are opaque in both 

 stages, account has to be taken of the remaining criteria. In some genera the thorax of 

 the subimago differs in its markings from that of the adult fly, and the wings undergo 

 changes in their coloration before the last moult. "When the subimaginal slough is 

 retained vxpon the wings of the adult fly (as in OUgoneurla) they remain dull and 

 subopaque during life. 



The term ' Pseudimago,' employed by a few authors instead of Subimago, is an 

 etymological solecism derived from two words belonging to different languages. ' Pseud- 

 idolum,' or ' Proidolum,' would be the Latinized Greek equivalent of the Latin Sub- 

 imago, if any were needed. 



Characters which have been used as Bases of Classification, but which 



ARE NOT FUNDxVMENTAL. 



Many of the characters upon which the classification of the Ephemeridaj was formerly 

 based have proved to be unsuitable for the purpose. Originally the number of the caudal 

 setcB was deemed a matter of primary importance ; and when forms were discovered with 

 the median seta abbreviated, they were ranked between those with three long equal setaj 

 and those with two only. Subsequently, in addition to the setse, the number of the wings 

 was employed as a leading clue to the arrangement of the genera. But it is now well 

 known that these criteria are serviceable at the most for nothing more than the distin- 

 guishing of genera very intimately related to each other, belonging to various subordinate 

 alliances comprised within the family; while one of them, (the number of the setre) is 

 not always available for even this purpose, varying as it does in some forms with the sex 

 [Polymitarcys), or with the individual {Atalophlebia australasica). In more modern times 

 the comparatice scarcity or abundance of cross veinlets in the wings, and especially in the 

 marginal area of the anterior wing before the nodus, has been supposed to furnish a trust- 



SECOND SERIES. — ZOOLOGY, VOL. IIJ 3 



