Odonata

(′ōd·ən′ad·ə)

(invertebrate zoology) The dragonflies, an order of the class Insecta, characterized by a head with large compound eyes, and wings with clear or transparent membranes traversed by networks of veins.

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(Dragonflies and damselflies)

Class: Insecta

Order: Odonata

Number of families: 28

Evolution and systematics

Odonates appeared by the early Permian period, and lineages corresponding to the three extant suborders flourished in the Mesozoic—Zygoptera and Anisozygoptera in the Triassic and Anisoptera in the Jurassic. Debate exists about the relationship of the three suborders; conventional classification fails to match the true evolutionary relationships of the groups, with "Anisozygoptera" being an artificial grouping. Zygoptera (damselflies) comprise four superfamilies and 18 families, the so-called Anisozygoptera has only one family and genus, and Anisoptera (dragonflies) contains three superfamilies and four families.

Physical characteristics

Wingspans range from 6.5 in (162 mm) in the Australian dragonfly, Petalura ingentissima, to 0.8 in (20 mm) in the Southeast Asian damselfly, Agriocnemis femina. They have large compound eyes and chewing mouthparts. The two posterior segments of the thorax are fused together. The legs are well developed for seizing prey and for perching; locomotion is almost solely by flight. The large, strong, multiveined wings usually have an opaque pterostigma near the wing tip. The tensegmented abdomen is long and slender. In males unique secondary genitalia evolved on the underside of the second and third abdominal segments, separated from the actual genital opening near the abdomen tip. Damselfly and several dragonfly females have well-developed ovipositors used to insert eggs into plant tissue; in some dragonflies the ovipositor valves are reduced, and eggs are dropped into water. Both sexes have caudal appendages at the tip of the abdomen, which in males work like claspers to grasp the female during mating. Larvae are aquatic and have a unique lower jaw specialized for grasping prey. Damselfly larvae are long and narrow and have three caudal lamellae used for breathing. Dragonfly larvae have broad bodies and breathe through tracheal gills located in the rectum.

Distribution

Dragonflies are found worldwide, except in frozen polar areas. Their greatest diversity is in the tropics.

Habitat

Larvae are mostly aquatic and are found under stones, buried in mud or detritus, or clinging to vegetation in stagnant and running freshwater. A few inhabit small water reservoirs in plants; others live in moist terrestrial burrows in forests. Adults occur over almost any kind of freshwater, where they mate and oviposit (lay eggs).

Behavior

Odonates regulate their body temperature by assuming different postures and selecting perching sites. In cool weather they engage in wing whirring and land on sun-facing perches, whereas in hot weather they avoid overheating by assuming an "obelisk" position, with the abdomen exposing the least possible area to the sun. Many males are territorial and patrol an area of water, chasing rival males from it. Females cruise through territories, attracted to possible egg-laying sites. In some species, males perform threatening displays for other males or courtship displays for females by exposing color patches on the head, legs, abdomen, or wings. After emergence, some species undertake long-distance migrations; others disperse short distances when mature, searching for suitable sites to oviposit.

Feeding ecology and diet

Larvae and adults are active or ambush predators. Adults capture and eat insects on the wing, and larvae eat mosquito larvae, other aquatic invertebrates, and even fish and tadpoles. Adult adaptations for feeding include large eyes, which allow them to see in virtually all directions; legs forming a "basket" to scoop up prey; and strong wings, providing amazing flight maneuverability. Larvae capture prey by rapidly extending the labium and seizing prey between the two movable hooks at its tip.

Reproductive biology

Mating is unique. The male caudal appendages grasp a female at the back of the head (dragonflies) or anterior part of the thorax (damselflies), forming the "tandem position." Before copulation, the male arches his abdomen, transferring sperm from near the tip to the secondary genitalia at the base. Copulation ensues when the female arches her abdomen to bring her genital opening into contact with the accessory male genitalia, forming the "wheel position." After copulation, the female oviposits either alone or guarded by the male, who continues to hold her in the tandem position or flies near her. Eggs are laid in aquatic plant tissue, mud, or water. The growing larva sheds its skin several times before metamorphosing into an adult. Larvae live from six months to five years, depending on water temperature and food supply. The adult is the dispersal stage and lives from one to two months in temperate areas to a full year in the tropics.

Conservation status

Of the more than 5,500 known species of odonates, 137 are included on the IUCN Red List: two as Extinct; 13 as Critically Endangered; 55 as Endangered; 39 as Vulnerable; 17 as Lower Risk/Near Threatened; and 11 as Data Deficient.

Significance to humans

Despite menacing common names (e.g., "devil's darning needles" or "horse-stingers"), odonates are harmless; they have no sting. They consume large numbers of harmful insects (including disease-transmitting mosquitoes) and also are excellent indicators of freshwater quality. For the Navaho Indians they symbolize pure water. Traditionally known as the "invincible insect," the dragonfly was a favorite symbol of strength among Japanese warriors, and the old name for the island of Japan (Akitsushima) means "Island of the Dragonfly."

Species accounts

Living fossil
Wandering glider
Forest giant

Resources

Books:

Corbet, P. S. Dragonflies: Behavior and Ecology of Odonata. New York: Cornell University Press, 1999.

Dunkle, S. W. Dragonflies Through Binoculars: A Field Guide to Dragonflies of North America. New York: Oxford University Press, 2000.

Needham, James C., Minter J. Westfall, and Michael L. May. Dragonflies of North America. Revised edition. Gainesville, FL: Scientific Publishers, 2000.

Silsby, J. Dragonflies of the World. Collingwood, Australia: CSIRO Publishing, 2001.

Westfall, Minter J. Jr., and Michael L. May. Damselflies of North America. Gainesville, FL: Scientific Publishers, 1996.

Organizations:

British Dragonfly Society. Membership Office, 53 Rownhams Road, Maybush, Southampton, SO 16 5DX United Kingdom. Web site:

Dragonfly Society of the Americas. 2091 Partridge Lane, Binghamton, NY 13903 United States. Web site:

Gesellschaft Deutschsprachiger Odonatologen. Web site:

International Odonata Research Institute. E-mail: iori@afn.org Web site:

Societas Internationalis Odonatologica. P.O. Box 256, Bilthoven, NL-3720 AG Netherlands.

Worldwide Dragonfly Association. P.O. Box 321, Leiden, 2300 AH Netherlands. Web site:

Other:

"Dragonfly (Odonata) Biodiversity." [22 Dec. 2002]. .

[Article by: Natalia von Ellenrieder, PhD]

An order of the class Insecta known as dragonflies. The Odonata are a relatively small order of insects. There are probably less than 3000 species known throughout the world. The order is divided into the Anisoptera, or true dragonflies, and the Zygoptera, or damselflies (see illustration).

Typical Odonata. (a) Dragonfly nymph. (b) Adult damselflies; note mites attached.

The young inhabit ponds, streams, and marshes; the adults fly over these localities or adjacent land. The adult structure is unique, characterized by a head with large compound eyes and wings with clear or transparent membranes traversed by networks of veins; the male has accessory genital organs possessed by no other insects.

There are three general stages in the life history: the egg, the nymph (illus. a), sometimes called naiad, and the adult. Development is usually slow, often requiring 3–5 years. Rarely is there more than one generation a year in the northern range. Adults may live for an extended period in summer. Eggs are laid by insertion into plant stems, either beneath the water or just above the surface. Others are dropped directly into the water and sink to the bottom, where they hatch and the nymphs develop.

Enemies include birds and fishes, with frogs and insects being of lesser importance. Mites sometimes attach themselves to dragonflies (illus. b) but do little harm.

Dragonflies constitute one of the oldest insect orders; they can be traced back through fossil records to the Carboniferous and Permian. Surprisingly few changes have occurred since then, although the order has diversified and specialized to counter competition and to avoid enemies.

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IN BRIEF: n. - An order of predacious insects with aquatic larvae; dragonflies and damselflies.

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