Dragonflies

Dragonflies are a well known and fascinating order of insects; you will likely see plenty of them as you go out into the field in late summer, normally near water. They are more common in warmer parts of the world than in temperate areas. There are 5 300 named species world-wide. They are conveniently divided up into two groups Anisoptera the true Dragonflies which rest with their wings out from their body in a cross shape and Zygopteran or Damselflies who hold their wings above their body. In this article when I say Dragonflies I will mean both Anisoptera and Zygoptera, but I will use these terms separately when talking about the individual groups.

Dragonflies have strongly biting mouthparts and are active and aggressive carnivores, both as adults and as young (called nymphs), preying mostly on other insects. The adults have massively large eyes, often meeting at the top of the head in Anisopterans. These eyes may each contain as many as 30 000 individual lenses or ommatidia (your eyes have only one lens each). Because of this Dragonflies have exceptionally good eyesight and have been known to respond to stimuli from more than 40 feet away. They have very small and poorly developed antennae though.

They have two pairs of almost equally sized long thin membranous wings; both pairs of wings usually have a stigma (a dark or coloured patch near the middle of the leading edge) and a mass of cross veins giving them the appearance of being a mesh. Unlike most insects, which either flap both pairs of wings in unison (i.e. Bees and Butterflies), or only flap the hind pair (i.e. Beetles), or only have one pair (i.e. Flies), Dragonflies can flap or beat their wings independently. This means the front wings can be going down while the back ones are coming up. You can see this happening if you watch closely. Dragonflies are excellent fliers, particularly the Anisopterans and can loop-the-loop, hover and fly backwards quite easily. It is not unusual for the larger species to reach 30kph and the Australian Austrophlebia costalis has been clocked in at an impressive 58kph or 36 mph for short bursts. They flap their wings relatively slowly though, at less than 30 beats per second. Compare this with 200 bps for a hoverfly or 300 bps for a honey bee.

Dragonflies are a very ancient order of insects and fossils exist from more than 300 million years ago. Dragonflies are also relatively large insects, even now, but in they past they were much larger. Fossil remains of some of the largest flying insects to have ever existed are Dragonflies, one species Meganeura monyi had a wingspan up to 75 cms. The largest Dragonfly in the world now is actually a Damselfly (Zygoptera) Megaloprepus caerulata from Costa Rica with a wingspan of 19.1 cm or 7.52 ins and a body length of 12 cm or 4.72 ins. Tetracanthagyna plagiata from Borneo is the largest Anisopteran or true Dragonfly. The smallest is probably Agriocnemis naia from Burma with a wingspan of just 1.76 cm or .69 ins.