SYNOPSIS OF THE BRITISH EPHEMERID.E. / 



colour of which only varies very little ; unfortiinate]y these 

 seldom afford characters easily intelligible, yet the transverse 

 veins in the apical part of the marginal field are not iin- 

 frequently very serviceable. The legs and caudal filaments 

 differ in length and colouring, but are rarely so striking as 

 to furnish decided characters for dead specimens. The eyes 

 lose their form and colour almost entirely, the prothorax 

 and head likewise, the thorax loses at least its colour, the 

 abdomen generally loses entirely both form and colour ; so 

 that only very few characters are retained by which the 

 species can be recognised. The external sexual organs, that 

 is, the anal forceps and penis of the male, the egg-valve and 

 the last abdominal segment, appear to me in a number of 

 species to furnish differences rather easily recognisable, 

 althous^h these orc^ans likewise lose some of their form in 

 drying. Besides this uncertainty of the characters, the^'e is 

 still an ao'ofravatino; circumstance to be mentioned. The 

 sub-imago sometimes undergoes a partial metamorphosis 

 after being pinned, and hence specimens actually occur in 

 which the head and thorax, or else only the abdomen and the 

 caudal filaments, are freed from the skin of the sub-imago. 

 In the larger species such a condition is easily perceived, 

 but in the smaller species, which shrink more in drying, 

 attention must be paid to this, in order to avoid mistakes. 



At any rate the species of Ephemerid<^ are considerably 

 more numerous than is generally supposed. I possess about 

 150 species, and in the collections which I have examined 

 there are many species which I do not possess, so that 

 probably at the present time there are about 250 species in 

 collections. AVe know comparatively few of the exotic 

 species, but wherever insects have been eagerly collected, a 

 large number of species has turned up, so that we may with 

 certainty calculate that at the present time we only know a 



