Odonata fast facts
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.
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.
The nymphs (see Aeshna grandis above) 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.
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.
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 above was photographed in Japan. There is is known as aka tombow - red dragonfly, I do not know its scientific name.
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.
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!