jo The May-flies and Stone-flies 



separate; that is, paired and bilateral for their whole course. This is taken 

 to be an indication of the primitiveness and antiquity of the order. 



If the May-flies are an ancient group of insects, and there is little doubt 

 of this, we have in them another example (we have previously noted one 

 in the case of Campodea, see p. 60) of primitive insects of excessively 

 frail and defenceless character persisting in the face of the strenuous struggle 

 for existence and of the competition, in this struggle, of highly developed, 

 specialized insect forms. Perhaps the solution of this problem in the case 

 of the May-flies is to be found in their extreme prolificness and in the 

 ephemeral character of their adult lives. It is only in the adult condition 

 that May- flies are so ill-fitted to defend themselves; so they simply make no 

 attempt to do so. They lay their eggs immediately on coming of age, and 

 thus accomplish the purpose of their adult stage. In their immature form 

 they are not so handicapped in the struggle for existence, although they 

 seem by no means in position to compete with some of their neighbors, like 

 the nymphs of the stone-fly and dragon-fly. 



About 300 species of Ephemerida are known, of which 85 occur in 

 North America. Their classification has been comparatively little studied 

 and is a difficult matter for beginners. The differences among the adults 

 are so slight, and the preserved specimens are so uniformly misshapen 

 and dried up, that most of us will have to be satisfied with knowing that 

 we have in hand a May-fly, without being able to assign it to its genus. 

 Keys to the North American tribes and genera of May-flies may be found 

 by the student who may wish to attempt the generic determination of his 

 specimens, in a paper by Banks in the Transactions of the American Ento- 

 mological Society, v. 26, 1894, pp. 239-259. 



There are better defined differences among the nymphs than among 

 the adults, but unfortunately the nymphs have been as yet too little studied 

 for the making out of a comprehensive key to the genera. Needham and 

 Betten give an analytical table of genera of Ephemerid nymphs as far as 

 known in the Eastern United States, in Bulletin 47 of the New York State 

 Museum, 1901. 



ON THE under side of the same stones in the brook "riffles" where 

 the May-fly nymphs may be found, one can almost certainly find the very 

 similar nymphs (Fig. 106) of the stone-flies, an order of insects called 

 Plecoptera. More flattened and usually darker, or tiger-striped with black 

 and white, the stone-fly nymphs live side by side with the young May-flies. 

 But they are only to be certainly distinguished from them by careful exam- 

 ination. The gills of the immature stone-flies usually consist of single short 

 filaments or tufts of short filaments rising from the thoracic segments, one 

 tuft just behind each leg (Fig. 106), and not flat plates attached to the sides 



