28 



GENERAL ZOOLOGY 



at dusk in swarms so dense that the atmosphere seems one 

 mass of moving forms, and, after laying their eggs, perish 

 with the day, forming a great food supply for fishes and birds. 



The eggs are laid in the water 

 and hatch into nymphs, which 

 do not at all resemble the adult 

 (Ephem'era, Fig. 14), but are 

 adapted to an aquatic existence 

 by the presence, along the sides of 

 the abdomen, of outgrowths of the 

 body wall penetrated by tracheae. 

 These outgrowths are called tra- 

 cheal gills. The delicate skin of 

 which the gills are formed permits 

 the passage of oxygen from 

 the surrounding water in- 

 ward, and allows the escape 



Fig.„14. Nymph and imago of May fly. (Natural size) 



of carbon dioxide. The young feed on small aquatic ani- 

 mals or on plants. After a year or more of this life beneath 

 the surface, during that time undergoing many molts, the 

 nymph develops rudimentary wings. From the nymph 

 issues a winged form which may be called a subimago. 



