- Written by Nadia O’Carroll
Dragonflies are members of the order Odonata, they are ancient and successful insects, one species of dragonfly that existed 250 million years ago is the largest insect recorded; its wingspan was 70 cm. There are approximately 6,000 species of dragonflies worldwide and 320 in Australia.
Their appearance is characterised by small antennae, large mandibles, enormous eyes plus three small ocelli, tilted thorax, long slender abdomen and two pairs of veined membranous wings of similar shape and size. They have excellent sight, their compound eyes may have up to 30,000 individual lenses and their head can rotate almost 360 degrees. Many species have brightly coloured bodies and wings. Modern dragonflies are smaller than their ancestors, the largest species have wingspans of 19cms and bodylength of 12 cms and the smallest species has a wingspan of 1.7 cm.
There are three stages in the dragonfly’s life cycle. Adult dragonflies mate in the air and the female lays eggs on a water plant or in the water. The egg hatches into a nymph (larva). The nymph is a flattened chunky creature with wing pads and six legs located near its head. The nymph breathes by drawing water over its internal gills, so it is well adapted to life underwater. Growth and development occurs by moulting and the nymph may have 15 moults before it climbs out of the water for a final nymphal moult and emerges as a fully developed adult. Dragonflies spend most of their life as nymphs; most species spend an average of 1-2 years as nymphs and only a few weeks as adults.
Dragonflies are found in a wide range of freshwater environments – bogs, ponds, fast and slow flowing streams. They are sensitive to pollution and habitat disturbance, so their presence can be an indication of water quality.
Nymphs and adults do share some common characteristics – they are voracious carnivores and highly effective hunters. Nymphs stalk and ambush their prey, they use a hinged toothed underlip to swiftly strike out at their victim and then draw it into their mouth where it is devoured. Adult dragonflies are extremely agile flyers with excellent vision; they use their spiny legs like a basket to grab insects in midair. Nymph and adult dragonflies catch insect pests such as midges and mosquitoes.
Both nymphs and adults use highly sophisticated locomotion. Nymphs have a hollow bladder, which is lined with tracheal gills; water is drawn in across the gills then expelled as a powerful column of water which jet propels the nymph through the water. The adult dragonfly is unable to walk but it has prodigious flying ability – manoeuvrability (acceleration recorded as 4g in straight lines and 9g in turns), endurance and speed (it can flap its wings independently at 1600 times per minute and reach speeds of over 60 km per hour) are achieved by complex aerodynamic processes.