which vary from about nine to fifteen in number. Each stage is marked by the casting-
off of the outgrown chitinous 'skin' or exo-skeleton. and the subsequent expansion and
hardening of the new one. The rate of larval growth is very variable, depending on such
factors as temperature, availability of food, and day-length. Broadly speaking,
development is more rapid in the warmer latitudes and slower in the cooler. Ischnura
elegans (the Blue-tailed Damselfly), for example, may go through three generations in a
year in southern Europe, but takes two years to complete a single generation in the north
(Corbet. 1980).
The nymphs of dragonflies (Anisoptera) are robust creatures, with a segmental
structuring of the body which resembles that of the adult (see Fig. 2:5). However, they
lack the bright colouration of the adult and, since they live under water, they lack wings.
They do, however, have small projections on the thorax under which develop the tissues
which will eventually form the wings. The six fully articulated legs may be used for
walking, and also may help in capture of the prey, but more rapid locomotion is
achieved by a kind of 'jet propulsion". The abdomen of the dragonfly nymph has an
internal hollow which opens out at the hind end. This hollow serves as a breathing
apparatus (gills) and can also be used to squirt water out backwards, so propelling
the insect forwards.
By contrast, the nymphs of damselflies (Zygoptera) are slender, more delicate
creatures, with a set of three feather-shaped external gills attached to the rear end of the
abdomen (see Fig. 2:6). The shape and markings on these are distinctive of each species,
and are used in identification. Lacking the internal gills of the dragonfly nymphs, the
nymphs of damselflies move more slowly by 'wriggling' movements of the abdomen
and external gills.
Fig 2.5 Dragonfly (Anisoptera) nymph (After photograph by A. Chalkley)
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