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abdomen is relatively shorter and broader. The larval head has, like
that of the imago, short feelers, and the eyes are somewhat large,
though far from attaining the size of the great globular eyes of the
dragon-fly. But the third pair of jaws, forming the labium, are most
remarkably modified into a 'mask,' the distal central portion (mentum)
being hinged to the basal piece (sub-mentum) which is itself jointed
below the head. The mentum carries at its extremity a pair of lobes with
sharp fangs. Thus the mask can be folded under the head when the larva
lurks in its hiding place, or be suddenly darted out so as to secure any
unwary small insect that may pass close enough for capture. Dragon-fly
larvae walk, and also swim by movements of the abdomen or by expelling a
jet of water from the hind-gut. The walls of this terminal region of the
intestine have areas lined with delicate cuticle and traversed by
numerous air-tubes, so that gaseous exchange can take place between the
air in the tubes and that dissolved in the water. The larvae of the
larger and heavier dragon-flies (Libellulidae and Aeschnidae) breathe
mostly in this way. Those of the slender and delicate 'Demoiselles'
(Agrionidae) are provided with three leaf-like gill-plates at the tail,
between whose delicate surfaces numerous air-tubes ramify. These
gill-plates are at times used for propulsion. Thus air supply is ensured
during aquatic life. But occasionally, when the water in which the
larva lives is foul and poor in oxygen, the tail is thrust out of the
water so that air can be admitted directly into the intestinal chamber.
The aquatic life of these insects lasts for more than a year, and F.
Balfour-Browne (1909) has observed from ten to fourteen moults in
Agrion. Outward wing-rudiments are early visible on the thoracic
segments; when these have become conspicuous the insect, beginning in
some respects to approach the adult condition, is often called a nymph.
In an advanced dragon-fly nymph, H. Dewitz (1891) has shown that the
thoracic spiracles are open, and, as the time for its final moult draws
near, the insect may thrust the front part of its body out of the water,
and breathe atmospheric air through these. Thus before the great change
takes place the nymph has foretastes of the aerial mode of breathing
which it will practise when the perfect stage shall have been attained.
The emergence of the dragon-fly from its nymph-cuticle has been
described by many