Odonata (dragonflies and damselflies)

Odonata fast facts

  • Large, with long bodies.
  • Large eyes with up to 30,000 ommatidia (facets).
  • Minute antennae.
  • Specialised mouthparts with strong teeth.
  • Legs end in a powerful pair of claws.
  • Two pairs of similar wings finely veined.
  • Both adult and nymph are predators. Nymphs aquatic and long lived.
  • 5 600 species worldwide, 130 European, 49 British.
  • Hemimetabolous
  • Fossil record goes back over 300 million years.
dragonfly larva mask

Odonata, dragonflies, damselflies, are also sometimes known as horse stingers, devil's darning needle, which according to superstition will be used to sew up the lips of liars. They are divided into two sub-orders: Zygoptera, the damselflies, and Ansioptera, the true dragonflies.

Damsel flies usually have slimmer abdomens in both adults and nymphs. Both sub-orders usually fly during the day.

During the Permian there were huge dragonflies (with wingspans as large as that of a seagull today) to be found in what is now England. Meganeuropsis schusteri had a wingspan of 71 cm. The atmospheric oxygen concentration was 35% (today it is 21%), which may have allowed more oxygen to reach further down the tracheoles (breathing tubes). Also the oxygen-rich atmosphere would have made flying easier. At that time insects were the only flying animals. It is believed that the huge dragonflies were slower fliers that those of today.

Odonata Wings and flight

The 2 pairs of wings of the damselflies are alike, whereas the hind wings of the dragonflies are broader than the front wings (see Libellula depressa below).

At rest damselflies hold their wings vertically over their body, whereas true dragonflies rest with their wings spread out.

In flight the wings operate independently, unlike those of other strong fliers, e.g. wasps and bees who link both wings together. Wind tunnel studies, partly financed by the U S Navy and Air Force, have found that they twist their wings on the downstroke. This creates miniature whirlwinds that move the air much faster over the upper wing surface, and so reduces air pressure and increase lift. Over short distances they can reach speeds of 70 kph/45 mph. They can hover, and fly backwards and forwards.

The venation of the wings is used in identification to species level, however there is disagreement between entomologists in naming the veins, so identification for a beginner is usually easiest using illustrations.

Aeshna grandis, brown hawker larva

Dragonfly nymphs

The nymphs (see Aeshna grandis left) are aquatic, carnivorous and sluggish, and are usually found in slow moving or still water. On hatching from the eggs the larvae start feeding at once.

They have a prehensile lower lip (called the mask) which is folded under the head at rest (see the drawing above and the photograph left). This has moveable teeth, and the whole thing can shoot out at great speed to grab prey. The mask is shot forward by a rapid increase in blood pressure. In the photograph on the left the mask is partially extended.

The larvae breathe underwater through tracheal gills. These gills are located in a rectal respiratory chamber. Water is drawn into the rectum to the gills. This water-filled chamber can also function as a jet by the rapid expulsion of the water enabling the nymph to move at great speed for a short distance and escape danger or capture prey.

They tend to creep around the bottom of ponds and streams searching for prey. When a nymph is about to moult into an adult it crawls out of the water. Dawn or dusk during fine weather are the most common times for moulting. There are usually 10 - 15 moults as nymphs. It can take nymphs from 1 to 5 years to mature depending on species.

Dragonfly adults

When adults emerge from the nymphal state their adult colours are not fully developed. The adults are strong fliers and eat other insects, especially mosquitoes, black flies and other small flies, that they catch on the wing. They have a row of stiff bristles on either side of their legs, so the three pairs form a basket to scoop flying prey. However because of the positioning of the front legs Odonata cannot walk or crawl. Males are highly territorial, and will chase off other males.

Dragonfly flight speed has been recorded as 7.0 metres per second with a wing beat of 38 per second. Compare this with other insects.

Females and males of the same species are often different colours, and younger adults are usually a paler colour than older ones.

aka tombow, red dragonfly, Japanese dragonfly

They have 10 or 11 abdominal segments. All males have a pair of claspers on segment 10, and their reproductive organs on segment 2 or 3. In females the ovipositor is in segment 8 or 9. Some females may have a pair of appendages on segment 10.

When adults die the colours fade very quickly, and in many species the powdery blue colouration easily rubs off. This type of colouration is called pruinescence.

The dragonfly on the left was photographed in Japan. There is is known as aka tombow - red dragonfly, I do not know its scientific name.

Dragonfly mating

Before mating the male must transfer sperm from the genital opening on segment 9 to the reproductive organs on segments and 3. Then on finding a female he grabs her by the neck with his claspers. She curves her body around until the tip of her abdomen touches his reproductive organs in segment 2 and 3 to collect the sperm. This is known as the copulation wheel. After mating the pair may fly in tandem with the male leading.

Females mate with more than one male and store the sperm from the matings, although she tends to use the sperm from the last mating.

The male mating organ contains a structure that allows him to scrape or push aside the sperm from previous matings before depositing his own in the most favourable spot. The length of time he holds on to the female (the copulation wheel) will also prevent her mating with another.

The eggs are either dropped into the water, or laid inside plant tissue cut by the female's ovipositor.

Dragonfly migration

It has recently been found that dragonflies from India migrate to southern Africa, stopping off at the Maldives and Seychelles. This is a distance of 14,000 - 18,000 km!

Aeshnidae family - the Hawkers

Anax imperator, the Emperor dragonfly

These are the hawkers, and they are the largest and swiftest European dragonflies. There are 11 British species. The adult body length ranges from 50 - 120 mm, and the wingspan is up to 150 mm. Above, near the top of the page, is the nymph of Aeshna grandis, the brown hawker. The adult flies from July - October in England and Ireland. It is fairly easily recognised as it has amber-tinted wings and a brown body. The adult wingspan is around 102 mm, and body length 73 mm. Adults tend to hawk around pond edges and down the middle of streams. The males are fiercely territorial and will attack other males. The females deposit their eggs in plants or floating wood.

The preserved specimen above is an adult male Anax imperator, Emperor dragonfly.

It is usually bright blue with a green thorax, and has a thick dark line down the abdomen (the colours fade in preserved specimens).

It is one of the largest British species with a wingspan of 106 mm and a body length of 78 mm.

The adult flies in June, July and August, and it is a hawker. It is usually found flying over ponds and canals in the south of England. And it catches its prey around reed beds.

After hatching as an adult and feeding it returns to the waterside about a week later to mate. The female lays her eggs in floating debris and plants in weedy ponds and canals.

It spends 2 years as a larva. The nymphs can grow to over 5 cm long.

Libellula quadrimaculata

Libellulidae family, commonly known as the Darters, Chasers and Skimmers

This family has 1300 species worldwide, and 14 native British species. Libellulids are shorter than most other Ansioptera, and a flattening of the abdomen makes them appear stout.

In Europe the adult body length ranges from 18 - 75 mm.

Unlike other Ansioptera they do not patrol a territory, but tend to perch in the same spot and dart out to capture any passing prey. In the UK the adults fly from May until September.

On the left is the nymph of Libellula quadrimaculata, the Four-spotted chaser. It is found in all types of still water. The adults fly from May - August, and have a body length of 39 - 48 mm, and a wingspan of 70 - 80 mm.

Libellula depressa, below in the Libelludidae family, is also known as the broad-bodied chaser and broad-bodied Libelulla.

The males have a blue abdomen except for brown spots at the side. The females have yellow spots down the side of a brown abdomen. Both have dark brown patches at the base of the wings. The wingspan is around 74 mm and body length around 44 mm.

The males are territorial and use the same perch for days on end. Females tend to rest on vegetation near to ponds.

Libellula depressa, broad-bodies chaser, dragonfly adult

They are fairly common in southern England and Wales. Adults fly from late May - September. They fly from dawn to dusk, and prefer habitats that are open.

The nymphs are 40 - 46 mm long when fully grown. Their body is fat and rounded. They are tolerant of semi-polluted waters, so can breed in canals, and are also found in large ponds and the weedy margins of lakes.

The adult female lays her eggs in floating timber or vegetation close to the water's edge.

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