IV 



not necessary from a fishing point of view since 

 two flies, so nearly alike that they have deceived 

 anglers and entomologists for generations, are 

 hardly likely to be separable by the trout, and the 

 same artificial pattern will serve for either. 



For the entomologist, the distinctions lie in the 

 genitalia and the neuration of the wings. In 

 Ecdyurus venosus, the lobes of the penis are very 

 broad and there are numerous cross-veinlets in 

 the. wings, more particularly towards the lower 

 margin of the hindwing, so that this portion 

 appears heavily shaded. In the imago or 

 spinner, the numerous veinlets on the upper 

 margin of the fore wing towards the apex are 

 generally forked. The margin of the last ventral 

 segment is somewhat rounded in the male, 

 pointed in the female. 



In Rhithrogena haarupi, the lobes of the penis 

 are narrow and cylindrical ; the cross-veinlets 

 of the wings are less numerous ; in the imago the 

 veinlets on the upper and apical margin of the 

 forewing are generally simple, not forked ; and, 

 finally, the margin of the terminal ventral seg- 

 ment takes, in the male, the form of two rounded 

 lobes with a rounded excision between and in the 

 female it is excised and not pointed as in Ecdyurus 

 venosus. 



In the plates Fig. i is the true March Brown, 

 Rhithrogena haarupi, and Fig. 2, Ecdyurus 

 venosus, both female subimagines or duns. 



