Emperor dragonfly (Anax imperator)
|Size||Average length: 78 mm (2)|
Average wingspan: 106 mm (2)
Length of larvae: 45-56 mm (2)
The emperor dragonfly is classified as Least Concern (LC) on the IUCN Red List (1). It is common and widespread in the UK (3).
The emperor dragonfly (Anax imperator) 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).
The emperor 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).
The emperor dragonfly 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 metres 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).
The emperor dragonfly 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:
The British Dragonfly Society:
For more on invertebrates and their conservation:
Buglife, the Invertebrate Conservation Trust:
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- Abdomen: in arthropods (crustaceans, insects and arachnids) the abdomen is the hind region of the body, which is usually segmented to a degree. In crustacea (e.g. crabs) the limbs attach to the abdomen; in insects the limbs are attached to the thorax (the part of the body nearest to the head) and not the abdomen. In vertebrates the abdomen is the part of the body that contains the internal organs (except the heart and lungs).
- Diapause: in insects, a temporary pause in development and growth. Any stage of the lifecycle (eggs, larvae, pupae or adults) may enter diapause, which is typically associated with winter.
- Hibernation: a winter survival strategy characteristic of some mammals in which an animal's metabolic rate slows down and a state of deep sleep is attained. Whilst hibernating, animals survive on stored reserves of fat that they have accumulated in summer. In insects, the correct term for hibernation is 'diapause', a temporary pause in development and growth. Any stage of the lifecycle (eggs, larvae, pupae or adults) may enter diapause, which is typically associated with winter.
- Incomplete metamorphosis: Type of insect development (also known as hemimetabolous development) in which the adult form is reached via a series of moults. The larva (nymph) resembles a miniature wingless adult; the wings develop externally as the nymph grows.
- Larvae: stage in an animal's lifecycle after it hatches from the egg. Larvae are typically very different in appearance to adults; they are able to feed and move around but usually are unable to reproduce.
- Territorial: describes an animal, a pair of animals or a colony that occupies and defends an area.
- Territory: an area occupied and defended by an animal, a pair of animals or a colony.
- Thorax: part of the body located near the head in animals. In insects, the three segments between the head and the abdomen, each of which has a pair of legs.
IUCN Red List (March, 2011)
- McGeeny, A. (1986) A complete guide to British dragonflies. Jonathan Cape, London.
- Sterry, P. (1997) Complete British Wildlife photo guide. Harper Collins Publishers, London.
- O' Toole, C. (2002) The new encyclopedia of insects and their allies. Oxford University Press, Oxford
- Buczacki, S. (2002) Fauna Britannica. Hamlyn, London.
British Dragonfly Society (March 2003):