Dragonflies are an extremely old group of insects; they are found as fossils over 300 million years old. Fossils of enormous dragonflies with wingspans of at least 70cm have been found. The closely related dragonflies (Anisoptera) and damselflies (Zygoptera) can be distinguished from each other by observing them at rest. The damselflies are generally smaller and at rest hold their wings vertically above their body, or partly open. The dragonfly will always rest with its wings spread horizontally.
Both the water-dwelling juveniles and adult Odonata are predatory, eating other insects and small vertebrates. They have an interesting sex life; females mate with a number of males and store their sperm in specialised organs. She will tend to use sperm from the most recent mating to fertilise her eggs and this leads to great competition between males. The penis contains structures that allow the male to scrape out or reposition the sperm of rival males. The male will also hold the female in the copulatory position for long periods of time to prevent other males from mating with her.
In some countries, dragonfly nymphs are used to control mosquitoes; dropping a nymph into a water container can remove up to 90% of the larvae.
Some damselfly males will demonstrate the flow rate of water in his territory by floating downstream for a few seconds. This is a risky thing to do since he might be eaten by a fish. This may show his intended mate that he is strong enough to escape.
Dragonflies are strong fliers and can reach speeds of up to 20 miles an hour.