Anatomy of a female dragonfly
Morphology of Odonata
Adult Odonata are medium to large in size. The head is large with flexible neck,
bearing well developed
compound eyes either widely separated (all damselflies, some dragonflies), just
touching, or extensively
fused along the mid-line, three ocelli and a pair of short, bristle-like
antennae. The mouthparts are modified
for biting. The prothorax is small, the mesothorax and metathorax are large and
fused into a single, strong
pterothorax. Their two pairs of wings are long, narrow, and net-veined, and the
legs are short, suited to
perching and to holding prey, not to walking.
Adult Odonata possess two pairs of equal (damselflies) or subequal (dragonflies)
wings. The wing veins
of Odonata are fused at their bases and the wings cannot be folded over the body
at rest. The abdomen is
long, flexible, with 10 visible segments, and terminates in clasping organs in
both sexes. In damselflies, the
abdomen usually thin and cylindrical. Females of all damselflies and several
dragonflies’ families carry a
prominent ovipositor under abdominal segments 9-10. Males always possess
secondary genitalia on the
underside of abdominal segments 2-3.
Dragonflies and damselflies differ in several important ways (Figure 1). The
latter are smaller, more slender
and delicate, slower and more uncertain in flight. In the dragonflies the front
and hind wings are not similar
meanwhile in the damselflies both wings are similar in size and shape.
Dragonflies hold the wings horizontally
outward when at rest, and damselflies hold them folded parallel with the abdomen
or tilted upward
Life cycle
Odonata are hemimetabolus as they only go through three stages in the life cycle
that is egg, nymph and
adult. Mating of the adults usually takes place on the wing in tandem position.
The male then will return to
his territory or perch while the female oviposits her egss alone along the water
surface. In some species,
the pair lay eggs together, maintaining the tandem hold. In others the male
hovers above the female while
she lays her eggs. Eggs are laid into plant tissues on, above, or below the
water surface. In species without
a functional ovipositor, eggs are deposited onto the water surface or inserted
into sand or mud. Egg
development, mostly without diapause, may take several days or months to hatch,
depending on species.
The nymph live underwater and moult up several times before the final instar
crawls out onto overhanging
rocks or vegetation. This final instar will then shed its last nymphal skin and
emerge as an adult. Duration of
nymph development to adult stage depends on the species and where they live.
Some may take one or two
years or utilise temporary water bodies to grow rapidly and develop into adults
after a couple of months.
Adults generally live for just a few weeks.
Feeding
Dragonflies and damselflies are predators as both adults and nymphs. The nymphs
are non-discriminate
hunters that will eat any animal as large as or smaller than themselves,
including their own species. Nymphs
feed on freshwater invertebrates catching them with specialised mouthparts that
are able to spring forward
and grasp the prey. Adults stalk on smaller flying insects by sight, catching
them on the wing with their
modified short legs.
Identification
Adults: Identification is based on wing venation. As for dragonflies, the
hindwing is broader than the
forewing. In both wings a crossvein divides the discoidal cell into a triangle
and supertriangle shapes
(Figure 4A). In damselflies the two pairs of wings are almost exactly equal in
shape, size and venation
with numerous crossveins (Figure 4Bi,ii,iii). In most families pterostigma is
present near the wing tip
(Figure 4B).
Nymph: The most obvious characteristic shared by all odonate nymph is a
conspicuous grasping
labium (lower lip: mask), used for capturing prey. At rest the mask is held
folded underneath head
and thorax, extending back as far or further than the front legs and in some
families far enough
forward to cover the face below the compound eyes. In prey capture the labium is
shot rapidly
forward and prey is grasped with paired, hand-like palps. Even from above and
with the mask
retracted, identification of nymph to order and suborder is very easy, based on
several other features.
Important facts
In most part of the world, Odonata are of little economic importance. Their main
attraction for humans
is aesthetic as they are beautifully coloured . Adult dragonflies are also taken
as a minor food item in
some countries. In certain cases, the presence of odonates may be useful as an
indicator of ecosystem
quality. Changes in water flow, turbidity, or in aquatic or waterside vegetation
are quite influential to
the local faunal composition. The greatest numbers of species are found at sites
that offer a wide
variety of microhabitats. Dragonfly nymphs are beneficial to human because they
can be used as baits
and to assist in the control of aquatic insect pests (such as mosquitoes in
domestic water tanks). In
some countries, such as Japan, Odonata is a popular subject of art and culture,
and similar to butterflies
and birds, it has been a topic of popular scientific interest. In the European
folk tradition, odonates
have a less favorable status where they are called as “horse-stingers” or “devil’’s
darning needles”.
Odonates neither sting nor bite and the entire species are harmless. They are
beneficial to humans
because as voracious aquatic predators they assist in the control of insect
pests.