Emperor dragonfly  (Anax imperator)

The emperor is Britain's largest dragonfly; they are a spectacular sight with their broad wings and powerful flight. After emerging, both sexes are pale green with brownish markings. The legs are brown, becoming yellowish towards the base; the wings have black veins, and take on a yellowish-brown tinge with age. Males develop a bright blue abdomen with a black 'fish-bone' line passing down the centre; the thorax and head are green and the prominent eyes are blue. Females have similar markings to males, but are mainly green in colour, becoming brownish on the last few segments of the abdomen. Both sexes possess appendages at the tip of the abdomen known as 'claspers', which are used in mating; in males these claspers are more robust than in females (2). The larvae or 'nymphs' are brownish in colour and have stocky bodies with rounded heads that feature very large eyes (3).

This dragonfly has a broad global distribution; it is found in Europe from Portugal to Germany in the north, and extends eastwards to central Asia (1). It is also known from North Africa and the Middle East (2). In Britain, it is fairly widespread in southern England and south Wales, but becomes quite scarce in the north Midlands, although there are signs that the species is currently extending northwards (1).

Breeds in a range of aquatic habitats including large ponds, canals, slow-flowing rivers, lakes, flooded gravel pits, and dykes, but in all cases there must be a plentiful supply of marginal vegetation that emerges from the water (1).

Dragonflies undergo a type of development known as incomplete metamorphosis in which the aquatic larvae (sometime called nymphs) undergo a series of moults; the stages between moults are known as instars or 'stadia' (4). After hatching from eggs, the larvae develop quickly through the summer; they enter their final instar during the autumn of the following year, and then enter 'diapause', a form of hibernation, before emerging as adults early the next summer (2). In the first few instars, the larvae swim by undulating the body from side-to-side; later on they develop a system of jet-propulsion which enables them to easily escape from predators such as water bugs, fish, other dragonfly larvae and beetles. The larvae of the emperor dragonfly are themselves voracious predators, armed with fearsome mouthparts known as a 'mask'; the mask is normally tucked under the head, but is rapidly extended in under 25 milliseconds (4), piercing prey as large as small fish (2).

After diapause, final instar larvae leave the water and crawl up vegetation. The adult emerges, leaving the discarded skin of the nymph attached to the plant (5). The new adults undergo a period of feeding and maturation before starting to reproduce (4). Males set up territories, which they defend fiercely against other males; they fly rapidly at two to six meters over the water, and very rarely come to a rest. During mating, males and females form a typical 'wheel' mating posture, in which the male grabs the female behind her head using the claspers at the tip of his abdomen. After mating the female lays her eggs in floating vegetation, keeping low to avoid encounters with territorial males (2).

This species is not threatened at present, however many dragonflies are vulnerable to water pollution and loss of habitat by infilling of ponds, and drainage of water bodies (4).

The profile of dragonflies has been raised in recent years, and many landowners build ponds in order to encourage them (4). The British Dragonfly Society aims 'to promote and encourage the study and conservation of dragonflies and their natural habitats, especially in the United Kingdom' (6).

For more on British dragonflies see: the British Dragonfly Society:
http://www.dragonflysoc.org.uk/
For more on invertebrates and their conservation see Buglife, the Invertebrate Conservation Trust at:
http://www.buglife.org.uk/

This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:
arkive@wildscreen.org.uk