ranged laterally in pairs on the abdominal
segments, and C. Boerner (1909) has recently given reasons, from the
position and muscular attachments of these organs, for believing that
they show a true correspondence to (in technical phraseology are
homologous with) the thoracic legs. One feature in which the larva often
agrees with the imago is the possession on the terminal abdominal
segment of a pair of long jointed cerci, and in many genera a median
jointed tail-process (see fig. 9) is also present, in some cases both in
the larva and the imago, in others in the larva during its later stages
only. The prolonged larval life in may-flies often involves a large
series of moults; Lubbock (1863) has enumerated twenty-one in the
life-history of Chloeon. In the second year of aquatic life
wing-rudiments (fig. 9 _a_) are visible, and the larva becomes a nymph.
When the time for the winged condition approaches the nymphs leave the
water in large swarms. The vivid accounts of these swarms given by
Swammerdam (1675), de Reaumur (1742) and other old-time observers are
available in summarised form for English readers in Miall's admirable
book (1895). May-flies are eagerly sought as food by trout, and the rise
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
Vayssiere.]

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 o