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. The term odonate has been coined to provide an English name for the group as a whole, but is not in common usage; most Odonata enthusiasts avoid ambiguity by using the term true dragonfly when referring to just the Anisoptera.

History
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).

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 (Lohmann 1996, Rehn 2003).

Characteristics
These insects characteristically have large rounded heads covered mostly by well-developed, faceted eyes, legs that facilitate catching prey (other insects) in flight, two pairs of long, transparent wings that move independently, and elongated abdomens. In most families there is a structure on the leading edge near the tip of the wing called the pterostigma, which actually is a thickened, blood–filled and often colorful area called a cell. Cell in this case means a closed area of an insect wing 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, like bees. 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.

The largest odonates extant in the world today are the giant Central American damselfly, Megaloprepus coerulatus and Anax strenuus, a Hawaiian endemic dragonfly.



Habitat
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, seeking open fields and hilltops where they prey on smaller insects, catching these in flight.

Reproduction
Eggs are laid in water or on vegetation near water or wet places, and hatch to produce naiads that become (in most species) voracious predators on other aquatic organisms, including small fishes. The nymphs grow and transform into the adult flying insect. Male odonates have an organ near the back of the thorax in which they store spermatozoa; they mate by holding the female behind the head with claspers located at the tip of the male abdomen; the female bends her abdomen forward to touch the male organ and receive sperm.

Photo Galleries

 * Dragonflies and Damselflies