Phylum Arthropoda
Subphylum Hexapoda
Class Insecta
Order Odonata
Common name: dragonflies and damselflies
Overview
Odonata, or damselflies and dragonflies, are well known, commonly seen insects with terrestrial adults and aquatic nymphs. They are closely related to the Ephemeroptera (mayflies). The highly mobile, fast-flying adults are generally found near freshwater, although some species disperse widely and may be found far from their breeding sites. Adults have two pairs of narrow wings of similar shape and size, with complex wing venation and many cells, a large head with large compound eyes, short, bristle-like antennae and a long, slender abdomen with short cerci. Males have modified second and third abdominal segments (secondary genitalia) where sperm are stored prior to insemination of the female. Dragonflies (suborder Anisoptera) tend to be larger and stouter than damselflies, their hindwings slightly broader than the forewings; the wings are held horizontal to the body at rest. The smaller and more delicate damselflies (Zygoptera) have both wings similar in size and held vertically when at rest. Many species are territorial and can display complex behaviours. Adults range from 2�15 cm in length and can be bright blue, red, yellow or green; some have striking metallic colour patterns and banded wings. Odonate larvae live in freshwater. Damselfly larvae have gills in the form of three long processes extending from the end of the abdomen; dragonflies lack these structures, but have internal rectal gills.
Distribution and diversity
Dragonflies and damselflies are widely distributed and common components of most freshwater and riparian ecosystems across Australia. Just over 600 genera and about 6,500 species of Odonata are recognised worldwide. In Australia they are relatively well known taxonomically. Around 320 described species in 110 genera, assigned to 30 families, are recorded, with estimates of only another 10 or so species as yet undescribed.
Life cycle
Elaborate courtship rituals often precede mating which usually takes place in the air. After transferring sperm to his secondary genitalia, the male clasps the female by the head or prothorax with his anal claspers. The female bends her abdomen forward and connects her genitalia with the male�s secondary genitalia to collect the sperm. Eggs are deposited on the water surface, into plant tissue, sand or mud. The nymphs, which differ considerably from the adults, moult up to 15 times before they are ready to emerge from the water. When fully mature the final instar crawls out onto overhanging rocks or vegetation where it completes its metamorphosis, sheds the last nymphal skin and emerges as an adult. For most species, development takes 1�2 years but some species that inhabit temporary water bodies can grow rapidly and develop into adults after a couple of months. Adults generally live for only a few weeks.
Feeding
Adults are aerial predators that hunt aided by their extremely acute eyesight. They catch flying insects with their legs, or glean sedentary insects, and use their mandibles to break apart their prey. The aquatic nymphs are also predaceous and are generally ambush predators that remain concealed in silt, or under rocks and plants, waiting for slow-moving prey. The larvae have a unique, hinged, prehensile lower mouthpart (labium) that they shoot forward to capture their prey and bring it to the mandibles. Odonate nymphs are relatively indiscriminate predators and will eat anything up to and as large as themselves, including other odonate nymphs and small vertebrates such as tadpoles and fish fry.
Ecology
Although dragonflies and damselflies need water to breed, individuals can be seen flying many kilometres from freshwater. Males tend to be territorial and stay close to water to guard their hunting and mating grounds. They can often be seen perched on waterside vegetation, rocks or logs, hovering over the water or flying up and down in search of females. Females often roam further from water in search of prey. The nymphs are fully aquatic, although one species in known to inhabit wet leaf litter in northern Queensland. Species can be found in most freshwater habitats, both permanent and temporary, including rivers, creeks, lakes, ponds, wetlands and estuaries. As significant predators, both adult and nymphs are ecologically important. Odonate nymph abundance, diversity and community composition are often used as ecological indicators for monitoring changes in water quality, flow, turbidity, or aquatic or riparian vegetation. Freshwater fishermen know odonate larvae as �mud-eyes� and use them as bait. In Australia a large number of species are known to be endangered or threatened due to habitat change and loss.
Nymph from Mount Glorious, Queensland, Australia
Image credit: Photographer: E.S. Volschenk
� Western Australian Museum
Nymph from Brisbane, Queensland, Australia
Image credit: Photographer: E.S. Volschenk
� Western Australian Museum
Nymph with extended jaw from Brisbane, Queensland, Australia
Image credit: Photographer: E.S. Volschenk
� Western Australian Museum
