Odonata

Odonata is an order of insects, encompassing dragonflies (Anisoptera) and damselflies (Zygoptera). The word dragonfly is also sometimes used to refer to all Odonata, but the back-formation odonate is a more correct English name for the group as a whole. Odonata enthusiasts avoid ambiguity by using the term true dragonfly, or simply Anisopteran, when referring to just the Anisoptera.

The largest living odonates are the giant Central American damselfly Megaloprepus coerulatus, and the Giant Hawaiian Darner (Anax strenuus), a dragonfly endemic to the Hawaiian Islands. The prehistoric "giant dragonflies" belonged to the Protodonata (or Meganisoptera), closely related to true dragonflies but not part of the Odonata in the restricted sense.

Etymology
Fabricius coined the term Odonata from the Greek οδόντoς (οδούς), odontos (tooth) apparently because they have teeth on their mandibles, even though most insects also have toothed mandibles.

Systematics and taxonomy
This order has traditionally been grouped together with the mayflies and several extinct orders in a group called the "Paleoptera", but this grouping appears to be paraphyletic. What they do share with mayflies is the nature of how the wings are articulated and controlled (see insect flight for a detailed discussion).

In some treatments, the Odonata are understood in an expanded sense, essentially synonymous with the superorder Odonatoptera but not including the prehistoric Protodonata. In this approach, instead of Odonatoptera, the term Odonatoidea is used. The systematics of the "Palaeoptera" are by no means resolved; what can be said however is that regardless of whether they are called "Odonatoidea" or "Odonatoptera", the Odonata and their extinct relatives do form a clade.

It was long believed that the Anisoptera were a suborder and that there existed a third one, the "Anisozygoptera" (ancient dragonflies). However, they were combined in the suborder Epiprocta (in which Anisoptera is an infraorder) after it was revealed that the "Anisozygoptera" are a paraphyletic group composed of mostly extinct offshoots of dragonfly evolution. The two living species placed in that group are thus placed in the infraorder Epiophlebioptera, whereas the fossil taxa formerly placed therein are now strewn about the Odonatoptera (or Odonata sensu lato).

The Tarsophlebiidae are a prehistoric family of Odonatoptera that can be considered either a basal lineage of Odonata or their immediate sister taxon.

Description
These insects characteristically have large rounded heads covered mostly by well-developed, compound eyes, legs that facilitate catching prey (other insects) in flight, two pairs of long, transparent wings that move independently, and elongated abdomens. They have two ocelli and short antennae. The mouthparts are on the underside of the head and include simple chewing mandibles in the adult.

In most families there is a structure on the leading edge near the tip of the wing called the pterostigma. This is a thickened, blood–filled and often colorful area bounded by veins. The functions of the pterostigma are not fully known, but it most probably has an aerodynamic effect and also a visual function. More mass at the end of the wing may also reduce the energy needed to move the wings up and down. The right combination of wing stiffness and wing mass could reduce the energy consumption of flying. A pterostigma is also found among other insects, such as bees.

The nymphs have stockier, shorter, bodies than the adults. In addition to lacking wings, their eyes are smaller, their antennae longer, and their heads are less mobile than in the adult. Their mouthparts are modified, with the labium being adapted into a unique prehensile organ for grasping prey. Damselfly nymphs breathe through external gills on the abdomen, while dragonfly nymphs respire through an organ in their rectum.

Although generally fairly similar, dragonflies differ from damselflies in several, easily recognizable traits. Dragonflies are strong fliers with fairly robust bodies and at rest hold their wings either out to the side or out and downward (or even somewhat forward). Damselflies tend to be less robust, even rather weak appearing in flight, and when at rest most species hold their wings folded back over the abdomen (see photograph below, left). Dragonfly eyes occupy much of the animal's head, touching (or nearly touching) each other across the face. In damselflies, there is typically a gap between the eyes.

Ecology and life cycle
Odonates are aquatic or semi-aquatic as juveniles. Thus, adults are most often seen near bodies of water and are frequently described as aquatic insects. However, many species range far from water. They are carnivorous throughout their life, mostly feeding on smaller insects.

Male Odonata have complex genitalia, different to those found in other insects. These include grasping cerci for holding the female and a secondary set of copulatory organs on the abdomen in which the sperm are held after being produced by the primary genitals. To mate, the male grasps the female by the thorax or head and bends her abdomen so that her own genitalia can be grasped by the copulatory organs holding the sperm.

Eggs are laid in water or on vegetation near water or wet places, and hatch to produce pronymphs which live off the nutrients that were in the egg. They then develop into instars with approximately 9–14 molts that are (in most species) voracious predators on other aquatic organisms, including small fishes. The nymphs grow and molt, usually in dusk or dawn, into the flying teneral immature adults, whose color is not yet developed. These insects later transform into reproductive adults. Male odonates have a copulatory organ on the ventral side of abdominal segment 2 in which they store spermatozoa; they mate by holding the female's head (Anisoptera) or thorax (Zygoptera) with claspers located at the tip of the male abdomen; the female bends her abdomen forward to touch the male organ and receive sperm. This is called the "wheel" position, and resembles a loveheart.

Photo galleries

 * Dragonflies and Damselflies
 * Photos of odonates from Asia, Africa and America
 * Photos of odonates from Russia

Journals

 * Journal of the Entomological Research Society