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The Life-Story of Insects by George H. (George Herbert) Carpenter
page 28 of 132 (21%)
of the fly on many lakes ushers in a welcome season to the angler.

The nymph-cuticle opens and the winged insect emerges. But this is not
the final instar; may-flies are exceptional among insects in undergoing
yet another moult after they have acquired wings which they can use for
flight. The instar that emerges from the nymph-cuticle is a sub-imago,
dull in hue, with a curious immature aspect about it. A few hours later
the final moult takes place, a very delicate cuticle being shed and
revealing the true imago. Then follow the dancing flight over the calm
waters, the mating and egg-laying, the rapid death. The whole winged
existence prepared for by the long aquatic life may be over in a single
evening; at most it lasts but for a few days.

[Illustration: Fig. 9. Nymph of May-fly (_Chloeon dipterum_) showing on
right side wing-rudiment (_a_), on left tracheal gills (_b_). Magnified
4 times. [Feelers and legs are cut short.] From Miall and Denny after
Vayssière.]

In the development of the may-flies, then, we notice not only a
considerable divergence between larva and imago, both in habitat and
structure; we see also what is to be observed often in more highly
organised insects--a feeding stage prolonged through the years of larval
and nymphal life, while the winged imago takes no food and devotes its
energies through its short existence to the task of reproduction. Such
division of the life-history into a long feeding, and a short breeding
period has, as will be seen later, an important bearing on the question
of insect transformation generally, and the dragon-flies and may-flies
afford examples of two stages in its specialisation. The sub-imaginal
instar of the may-fly furnishes also a noteworthy fact for comparison
with other insect histories. In two points, however, the life-story of
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