described for the dragon-flies, but there are some suggestive
differences. In the first place, we notice a wider divergence between
the imago and the larva. An adult may-fly is one of the most delicate
of insects; the head has elaborate compound eyes, but the feelers are
very short, and the jaws are reduced to such tiny vestiges that the
insect is unable to feed. Its aquatic larva is fairly robust, with a
large head which is provided with well-developed jaws, as the larval and
nymphal stages extend over one or two years, and the insects browse on
water-weeds or devour creatures smaller and weaker than themselves. They
breathe dissolved air by means of thread-like or plate-like gills
traversed by branching air-tubes, somewhat resembling those of the
demoiselle dragon-fly larva. But in the may-fly larva, there is a series
of these gills (fig. 9_b_) arranged laterally in pairs on the abdominal
segments, and C. Börner (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 Réaumur (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