378 Annals Entomological Society of America [Vol. VI, 



are very active and nearly all are voracious herbivores. The 

 nymphal period is one of extreme competition and during 

 it the nymph must find safety, and get food sufficient for 

 its entire life. With the exception of the Diptera, May- 

 flies are the dominant aquatic insect herbivores. They have 

 attained this position by utilization of a vegetable diet and 

 by remarkable adjustments to particular situations. The 

 population about them is divided into two classes, competitors 

 and . enemies. Their competitors are mostly insects, which, 

 like the May-flies, live upon herbivorous or nearly herbivorous 

 diet; among them are the larvcC of Caddis-flies (except the 

 Hydrosychidas) , Crane-flies and most of the smaller Diptera. 

 Their enemies are wholly or in part carnivorous. Important 

 among them are the nymphs of Dragon-flies, Stone-flies, many 

 beetles and the Hemiptera and Neuroptera. The adolescence 

 of the nymph is evidenced internally by the development of the 

 reproductive organs, and externally by the growth of rudi- 

 mentary wings. This stage is terminated by a gradual change 

 in organs of locomotion, respiration and digestion and by 

 the final casting off of the nymphal skin. 



The first winged or aerial stage is known as the sub-imago. 

 The general form of body differs little from that of the actual 

 adult insect. The wings are fully expanded and direct respira- 

 tion through open spiracles is established. All surfaces are 

 dull and in most cases the wings have a prominent marginal 

 fringe of hairs. A few May-flies (females of Palingenia and 

 Cam'psurus, Eaton '83) never lose the sub-imago skin, but 

 in nearly all it is shed. The duration of the sub-imago stage 

 varies from a few minutes in the most ephemereal species to 

 several days. Needham,* '08, has given this account of Caenis 

 diminuta. "It is the most ephemeral of all Ephemera. It 

 emerges from the water at nightfall, leaving its nymphal 

 skin floating on the surface, and, alighting on the flrst support 

 that offers, sheds its skin again, and the sub-imago stage is 

 ended." 



*N. Y. State Bull. 124, p. 178. 



