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Odonata

(Dragonflies and damselflies)

Class Insecta

Order Odonata

Number of families 28

Photo: Male Zenithoptera americana from French Guiana. (Photo by Rosser W. Garrison. Reproduced by permission.)

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

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Order: Odonata

A newly emerged dragonfly (Rhionaesetina variegata) with larval skin. (Photo by Rosser W. Garrison. Reproduced by permission.)

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

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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 Defi-

Dragonfly mating wheel. (Illustration by Kristen Workman)

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A damselfly (Archilestes californica) female (below) ovipositing while guarded by a male (above) in tandem position. (Photo by Rosser W. Garrison. Reproduced by permission.)

cient. Little is known about distribution and habitat preferences for most species, and continuing habitat destruction precludes obtaining essential information necessary to ensure conservation of sensitive species. Nature preserves to conserve dragonflies exist in Japan, Europe, and the United States. Valuable conservation efforts are under way in local areas and for specific problems in South Africa, Australia, and India, but many areas with high numbers of unique species are in tropical regions without habitat protection. Japan,

Order: Odonata

where artificial habitats are created and managed for propagating dragonflies, represents one of the best examples of dragonfly conservation.

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.”

Dragonfly oblisking behavior. (Illustration by Kristen Workman)

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1. Living fossil (Epiophlebia laidlawi); 2. Forest giant (Megaloprepus caerulatus); 3. Wandering glider (Pantala flavescens). (Illustration by Jacqueline Mahannah)

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