CH. VIII 



MAY-FLIES 325 



aquatic Beetle (Georyssus pygmcxus) which is found 

 adult under stones in running water. Helophorus 

 and Elmis also disguise themselves with mud,^ and I 

 suspect that Nepa is yet another example. 



Such groups as these are merely short descriptions 

 of the adaptive modifications of particular species or 

 small groups of species. It would be easy to add to 

 the number by adopting a different principle of forma- 

 tion. Vayssiere, for example, defines five groups of 

 Ephemera according to the modifications which the 

 larval respiratory organs may assume."' 



His group I. includes those larvae in which the 

 tracheal gills consist of both leaflets and filaments. 

 (This nearly agrees with Pictet's group of Burrowing 

 Larvae). 



II. receives those in which the tracheal gills consist 

 of leaflets onlv. 



III. has the filaments of the tracheal gills protected 

 by an overlying leaflet. 



IV. has the hinder tracheal gills overlaid and pro- 

 tected by the enlarged leaflet of the second pair, the 

 first pair of gills becoming practically suppresse,d. 



V. contains some very peculiar and interesting 

 larvae in which the mid-thoracic segment extends 

 backwards over all the tracheal gills, forming a special 

 respiratory chamber, which opens behind or by three 

 special apertures of small size. In this last group 

 the arrangement becomes similar in principle to that 

 adopted in Decapod Crustaceans (Lobster, Crayfish, 



^ Kirby and Spence, \"ol. 11., p. 258. 



- Org. des Larves des Ephdm^rines, Ann. Set. Nat. Zoo/.., 1882, 



P- 33- 



