Dragonflies (Anisoptera)

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 Hawker dragonfly

 Close-up of the head of a hawker dragonfly

 Hawker dragonfly emerging from nymph stage, on reed; note the position of the legs, as they would be held in flight for catching prey
 Close-up of the head of a hawker dragonfly; note the dominant compound eyes but small antennae as well as huge crushing jaws

IDENTIFICATION: Adults are distinct, medium to large predatory insects found around all types of water. The eyes are large and dominate the head. Typically, the eyes meet on top fo the head but in the Club-tailed Dragonflies they do not. The thorax is large due to the large volume of flight muscle present. Two pairs of wings but the veins show a primitive form, showing that they are from an ancient stock. Three pairs of legs, with the hind ones being longest and the fore-legs shortest. They are covered in stiff hairs. Held out in front this is a formidable "web" for catching flying insects. Males have two sets of genitalia. The first consists of merely an aperture at the end of the abdomen from which the sperm is transferred to the second set, located under the first few segments of the abdomen (visible protruding in the emerging insect above).

 Hawker nymph  Cannibals

 Hawker nymph eating tadpole: it being held by the mask while the upper jaws break it down. This nymph is very near emerging, note the well developed wing buds

 Cannibals: the larger rear nymph is eating the one in the foreground. The prey is upside down and the lower "lip" called the mask is visible.

The larval stage is found in water whilst the adult is aerial. Often in the mud or debris at the bottom they may also climb vegetation. Again, large compound eyes on the head.

There are several types of dragonfly:

1. the Hawkers that are the largest, with long bodies that get their name from flying back and forth in relatively straight lines until food is found

2. the Darters that are smaller and get their name from the more random flight, often alighting on vegetation, leaving it to catch food or chase off rivals

ECOLOGY: Eggs are laid in the water. Darters wash the eggs from the end of the abdomen whilst in flight, usually with the male holding the back of the females head. Hawkers on the other hand land on the vegetation in the water and the female inserts individual egg into plant material or in the bank. Hatching larvae are immediately predatory and may eat their brothers and sisters as they hatch. Normally when young they will eat water fleas (Daphnia) or other small, suitably sized animals. As they grow so the diet changes. However, they may well take prey that is bigger than themselves. The lower labium or lip is developed into a large, jointed structure called the mask which shots out rapidly to catch prey. It is then drawn in and held so that the prey can be crushed by the mandibles (jaws). By placing caught larvae in a dish of clear water their droppings can be collected. The faeces is contained within a clear bag (the lining of the gut) and when opened and viewed under a microscope can be dissected to see what they have been eating, just like the pellets regurgitated by owls! The larvae may stay in water for up to 2 years or more, depending on temperature and food availablity. Often the larval stages are called nymphs. This term is used for insects displaying incomplete metamorphosis. Dragonflies do not have a pupal stage. Instead they cease feeding towards the end of the final instar (an instar is a stage between shedding the exoskeleton and dragonflies typically have between 10-12 instars). After a few days the nymph climbs vegetation under the cover of darkness to emerge into an adult several hours before dawn. By the time dawn comes they are ready for flight. To avoid being individually picked off by predatory birds dragonflies will tend to synchronise their emergence so that many will emerge out of the water at the same time, increasing the chance of some surviving. Mass emergence is common in several insect groups.

 Male Emperor Dragonfly (Anax imperator)

 Common Darter (Sympetrum striolatum)

 Male Emperor Dragonfly (Anax imperator): the largest European Dragonfly

 Common Darter (Sympetrum striolatum) at rest; male displaying itself on the edge of its territory

OTHER ADAPTATIONS: Although large the adults weigh very little (around a gram) as the body has numerous air bags. Nymphs breathe by extracting dissolved oxygen from the water. The "gill" for this is a basket of tracheal breathing tubes arranged around the end of the hind gut. So the end of the gut is ventilated by drawing water in through the anus. This body of water can be shot out of the gut rapidly, if necessary, giving it jet propulsion for quickly moving through water to escape or pounce on prey.

Approximately 40 species live in the UK.

These are primitive insects that have survived through their extreme and superb adaption to life in water. See also Damselfies (Zygoptera) a similar group

 Classification:

 

 Kingdom

Animal

 Phylum

Arthropoda

 Class

Insecta

 Order

Odonata