171 



4. Gill filaments relatively well developed. . Hcptagenia 

 Gill filaments not so well develoved; la- 

 mellae relatively larger Ecdyiinis 



In the larvæ of Rhithrogena we find the most typical 

 illnstration of special adaption to life in rapid currents 

 amongst the Ephemendæ. The broad head with its 

 strongly developed maxillary palpi, the depressed body, 

 the flattened femora, the rows of spurs at tip of tibiae, 

 the dentated claws make the larvae well fitted for clinging 

 to the surface of stones etc. But this is not all; the gill 

 lamellae and the filaments form a closely overlapping series 

 whose outer border fits the supporting surface to which 

 the nymph clings; the foremost pair of the lamellæ is 

 enlarged, so that their front borders touch each other 

 beneath the first abdominal segment; the hindmost pair 

 of the lamellæ is prolongated, and their tip is curved in- 

 wards in such a way that they also touch each other be- 

 low the 8th abdominal segment; thereby a disc for ad- 

 hesion to the surfaces of the stones is formed so com- 

 .plete that any elevation of the body would cause a par- 

 tial vacuum beneath it. 



