VIII MAY-FLIES 305 



pair being considerably the larger. When the Insect 

 is at rest, the wings arc carried vertically, as in most 

 Butterflies. 



" The body ends behind in two or three long fila- 

 ments, which are very fragile. The winged Ephemerae 

 often have these appendages more or less imperfect. 



" Though the duration of the winged Ephemerae is 

 so short, they live a long time as larvae in the water. 

 Swammerdam supposes that the species described by 

 him remains three years in the larval stage. Some 

 species known to me live two years as larvae, and 

 many others about one year. 



" During its aquatic life the Ephemera undergoes 

 no conspicuous change, except that the pupa, or 

 nymph, exhibits on its thorax the sheaths of wings, 

 which are altogether wanting in the larva.^ There 

 are six jointed legs, a head furnished with biting 

 jaws, and slender, many-jointed antennae. The ab- 

 domen is ten-jointed, and ends in two or three long 

 filaments. 



1 The aquatic period of an Ephemera does not admit of 

 definite subdivision into stages. The fresh-hatched larva has 

 commonly neither tracheal gills nor any trace of wings. Rudi- 

 ments of gills shortly appear, and become larger and more complex 

 at each succeeding moult. Before they have attained their com- 

 plete development the rudiments of the wings are often apparent. 

 The gradual acquisition of the imaginal structures is facilitated 

 by the unusual number of moults. In Chloeon dimidiatum as 

 many as twenty-one larval moults have been observed by 

 Lubbock. "In moulting the insect does not split the skin of 

 tlie thorax, as is so generally the case, but merely that on the 

 upper part of the head. It is wonderful, indeed, how it can 

 escape through so small an orifice" (Lubbock, Linn. Traits., 

 Vol. XXIV., p. 68). 



X 



