EPHEMEROPTERA . 



685 



The larvae differ remarkably from the adults and are more 

 peculiarly adapted to an aquatic existence than are those of any 

 other Insects. The form which issues from the egg seems to 

 be but little developed ; the number of moults is large, more than 

 twenty in Cloeon, and each results in some change of form. At 

 first the larvae show no respiratory organs, but after a few moults 

 plate-like gills traversed by tracheae arise from the upper angles of 



FIG. 432. Ephemerid larva, k tra- 

 cheal gills ; t principal trunks of 

 the tracheal system. 



FIG. 433. Ephemera danica, male, 

 Britain. From Sharp. 



the abdominal segments. The larvae may either dig or burrow 

 in the banks and are then thought to eat mud ; or they may live 

 on stones, in which case they are flattened and are carnivorous ; 

 or they may swim freely about, or they may inhabit still waters, 

 in which case they cover themselves with mud (Fig. 432). The 

 later stages, when the wings begin to appear, are termed the 

 nymphs, and these vary greatly as regards the arrangement of 

 their tracheal gills. The tracheae are closed except momen- 

 tarily at the time of ecdysis. The caudal appendages seem 



