THE PTERYGOTA. THE EPHEMERIDA 67 



The adults live only a few hours not more than a day or two at 

 most but during this time the eggs are dropped into the water. The 

 nymphs which hatch from them feed, probably mainly on vegetable 

 matter at the bottom, though some are doubtless partly carnivorous. 

 They live for one, two or three years, according to the species concerned 

 (some have two generations each year) , feeding, and molting with unusual 

 frequency for insects (Lubbock observed 21 molts in one species), until 

 they are full-grown. During this time the mouth parts are well developed 

 and of the chewing type, but in the adult they become practically useless. 



These insects are of no economic importance except perhaps to a 

 very slight degree as scavengers in the water, feeding on matter that 

 might otherwise decay and become objectionable, but their value for 

 this is probably small at best. They are fed upon as larvae and to 

 some extent as adults, by fish and some carnivorous insects of other 

 groups, and for this reason also may be rated as slightly beneficial. 

 At present about 500 kinds are known, but the group has not been very 

 thoroughly studied. Many fossil Ephemerids have been found, which 

 suggests that the insects are possibly less abundant now than was once 

 the case. 



