Dragonflies are among the most popular insects living near water sources. Adults are indeed often admired for their colors. But we easily forget that this appearance is not permanent and only corresponds to a moment in the life of the insect. Let’s take a closer look and follow it throughout its lifecycle.
What do we mean by “dragonfly”?
First of all, we would like to stress the fact that, scientifically, there is an order which is called the Odonates and which is divided into two sub-orders:
- Zygoptera, also called “damselflies”, whose main characteristic is to fold the wings above them when they land,
- And the Anisoptera. These are the large dragonflies which, at rest, keep the wings unfolded to the sides.
In everyday life, the term “dragonfly” indiscriminately designates the two suborders. Their life cycle is organized in the same way: one part of aquatic life and the other aerial.
Beyond their beauty, it is little known that these animals already existed over 200 million years ago, with anatomy similar to what we know of them today. Dragonflies therefore rubbed shoulders with dinosaurs, were present when the first birds appeared and knew the first primates. These insects are found almost everywhere on the planet, with the exception of the polar regions. More than 6,000 species of dragonflies are recorded worldwide, including around 130 in central Europe and 90 in mainland France.
The dragonfly spends a large part of its life in water
Scientists have identified as many as 15 different stages for an egg to grow into the adult dragonfly everyone knows. It all starts in the water. And the species vary depending on the type of water point. In stagnant water (lakes, ponds, ponds, marshes), we will find Orthetrum. In slow-flowing rivers, gumphs can be found. In small meadow streams, we will find Agrions de Mercury, in peat bogs, Arctic Cordulia, in garden ponds, blue Aeschnes.
Most of their life is time spent in the water. This duration varies considerably depending on the species and ranges from a few weeks to several years.
Some species lay their eggs directly above water, others above dry land which will then be submerged. Eggs are often very resistant to cold, pollution, heat and drought. It can be a release of eggs while the female flies several tens of centimeters above the water, or a deposit on the surface of the water. Some dragonflies can also insert their eggs in dead plants or mosses.
The number of eggs varies depending on the species. But it is usually large, given the large losses due to predators. They are a little over 1mm in height. The larva emerges from the egg around May. For a scientist, it is not actually a primary larva whose appearance is close to a fry. It is already 3 mm long.
It is only after that comes the larval stage proper. This is what the larva of an Anisoptera (“real” dragonflies) looks like:
- The head is not very mobile and has antennae, the shape and size of which can help identify the species,
- The eyes allow the insect to see in all directions,
- Under the head, it has a mask or labium, an organ that is used to capture prey. It can also be called “chin arm”,
- The thorax has 3 pairs of legs and wing sheaths,
- The abdomen, massive, is composed of 10 segments and extended by an anal pyramid.
The body of Zygoptera is much longer and thinner, and has 3 caudal lamellae in the shape of feathers in place of the anal pyramid. However, they also have a mask.
The movement of larvae is generally slow when walking. But they can suddenly empty the water from their rectal bulb to move around more quickly.
The dragonfly larva is reputed to be a ravenous predator : it devours worms, rotifers, crustaceans, larvae of aquatic insects including congeners if they are smaller in size, batrachians, from the tadpole stage to that of the small frog, and small fry. It burrows into the sediment and waits for the passage of a prey, motionless.
The aerial life of adults
The “puberty metamorphosis” then takes place, starting with a pupal moult and ending with a imaginal moult. It is the last larval stage which is called “nymph”. The dragonfly begins to inflate its wings in the wing sheaths, the lip mask regresses and the eyes grow larger. The dragonfly comes out of the water and leaves its old envelope. With patience and keen observation, it is possible to find exuviae in the vegetation that borders sites where the larvae have lived.
The imago, the flying insect, will mature and its main mission will be to reproduce, usually between June and October. Adult longevity varies from two weeks to several months depending on the species. The Striated Sympetrum and the Bright Calopteryx can thus expect to live for more than 6 months.
The adults are just as carnivorous as the larvae and feed on flies, mosquitoes, grasshoppers, butterflies, flying ants, and even other dragonflies. The natural predators of dragonflies are insectivorous birds such as the gray flycatcher. Orbital spider webs often trap them.