THE NATURAL HISTORY OF DRAGONFLIES
Dragonflies are among the most colourful and striking of all groups of insects. They.
along with butterflies, crickets and some groups of beetles, have a place in folk stories,
popular mythology, art and ornamentation world-wide. It was not until the 19th century
that any systematic study of dragonflies was undertaken. The dragonflies themselves, of
course, long pre-date the evolution of their human observers and students. Fossil
remains of close relatives (the Protodonata) of our modern dragonflies date from
Carboniferous times (some three hundred million years ago). Some of the dragonflies of
those times were much larger than those we encounter today - as much as 75 cm. (two
feet six inches) across the wings. Dragonflies are thus amongst the oldest and most
primitive of all the insect orders.
The Structure of Dragonflies
Like other insects, the bodies of dragonflies have a segmental structure, the various
segments being organised into three connected structures - head, thorax and abdomen
(see Fig. 2:1). The head carries the mouth-parts (adapted for grasping and biting), as
well as the antennae (organs of chemical sense) and small, simple eyes, the ocelli. The
most prominent feature of the head, however, is the pair of huge compound eyes, which
give a wide field of vision necessary for the avoidance of predators and for the location
of the dragonfly's prey. Behind the head is the thorax which bears the six legs, and two
pairs of wings. At the front end of the thorax (nearest the head) is a distinct structure
known as the prothorax, the shape of which is important in the identification of some
species. The legs are not very useful for walking, but are adequate for clinging to
vegetation, and climbing. Their distinctive use is to form a kind of 'cage' for the aerial
capture and holding of the dragonfly's insect prey. The main parts of a dragonfly's legs
(the shape or colouration of which can be helpful in the identification of some species)
are shown and named in Fig. 2:2.
Dragonflies have two fully formed pairs of wings, both used for flight, but they differ
from other insects in the method of muscular control of wing movements. In dragonflies
this control is direct, whereas in other insects the muscles move the parts of the thorax to
which the wings are attached. Because of this unique feature, dragonflies are capable of
independently moving the fore- and hind-wings, and exceptional manoeuvrability in
the air, including the ability to fly backwards! In their method of seeking and capturing
prey, dragonflies are comparable to swallows and bats, rather than to most other insects.
The tough, membranous wings are kept in shape by a network of 'veins', the pattern of
which is distinctive for each species. Wing-venation was established by the great student
of dragonflies, De Selys Longchamps and his co-workers as the basis of their scientific
classification in the middle decades of the last century. Also useful in identifying some
species of dragonflies is a small dark marking on the leading edge of each wing, and
towards the tip. This is known as the pterostigma. Some species have other areas of dark
markings or "clouding" on the wing which may also be useful in identifying them. The
abdomen of dragonflies is usually long and cylindrical, being made up of a 'string' of
ten segments.
11