Dragonflies 1, 2, 3, 4

Dragonfly nymphs

Aeshna grandis, brown hawker larva

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.

dragonfly larva mask

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.

Adult emergence

dragonfly adult emerging

The drawings above and below show the emergence of Aeschna cyanea as an adult. The happens in five main stages:

  1. The larva climbs out of the water and up a reed or some other structure it feels it can fasten its legs securely to.
  2. The exoskeleton splits open at the back of the neck, and the adult thorax and abdomen emerges and hangs down backwards with the legs facing outwards.
  3. The dragonfly rests in this position a while to allow the legs to harden. Then it heaves its body up, and using its newly hardened legs grabs hold of the larval exoskeleton, still tightly fixed to the reed.
  4. Using its legs it pulls the rest of its abdomen free from the now empty larval exoskeleton.
  5. Blood is pumped into the abdomen and wings till they reach their full size, and the adult rests until the whole exoskeleton hardens and dries. In the drawing below note how small the newly emerged wings are.

dragionfly emerging as adult

Dragonfly adults

aka tombow, red dragonfly, Japanese dragonfly

When adults emerge from the nymphal state their adult colours are not fully developed. Females and males of the same species are often different colours, and younger adults are usually a paler colour than older ones. 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.

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.

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.

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

Dragonfly feeding

The adults are strong fliers and eat other insects, especially mosquitoes, black flies and other small flies, that they catch on the wing. The prey is detected by sight, and some species can detect suitable prey up to twenty metres away. They have a row of stiff bristles on either side of their long legs, and the three pairs of legs are held slightly forward to form a basket to scoop flying prey. However because of the positioning of the front legs Odonata cannot walk or crawl. Once in the basket the prey is held by the mandibles and dismembered. Larger odonata may do all of this while still flying, and may also eat on the wing. Small species, and those with prey that is awkward to handle will return to a perch to dismember it. Males are highly territorial, and will chase off other males.

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!

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