7 ° 
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 littlfe 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. to6), and not flat plates attached to the sides 
