Fabulous acrobats, dragonflies are formidable predators. Five thousand six hundred species of dragonflies have been flying on our planet for three hundred and fifty million years. They had time to inspire humans. Their hovering flights, their agility in the air, their speed make them formidable predators. Predators, dragonflies are already predatory during their aquatic larval stages. Some throw themselves directly at their prey, shredding them with powerful mandibles, others catch them with a chameleon tongue. The dragonfly larvae are ugly, terrifying. They must have inspired more than one science fiction film director. Fortunately the adults are graceful, elegant and of disconcerting skill. You have to watch the dragonfly ballet on summer evenings when the manna emerge from the water film. Carnivorous helicopters sink on their prey a few millimeters from the bubbling surface of the torrent. The dragonflies loop, catch themselves and attack in a dive. What show !
Odonates (so called because of their powerful toothed mandibles) are divided into two suborders: damselflies (Zygoptera) and dragonflies (Anisoptera) The science that studies odonates is odonatology, and the specialists are odonatologists. Odonata are an order of aquatic insects with elongated bodies furnished with two pairs of membranous wings, three pairs of legs and two large eyes composed of facets which allow them to easily locate their prey. They are up to one hundred and ten millimeters in wingspan, but fossil specimens with a wingspan of seven hundred millimeters have been found.
The odonate males wait for the females to pass near them or go to look for them while exploring their territory. Fertilization of the female is not immediate during mating which can last an hour. This can store sperm. The fertilized eggs are then laid in a wetland or underwater. The shape of the eggs depends on the laying support and when laying on the aerial plants bordering the banks they will tend to be more rounded oblong for laying in the water. In some species the spawning is done with the male still mated. In others, the male remains close to the female so that others do not fertilize it, it is then equipped with an organ allowing the elimination of the sperm of the previous partners or quite simply leaves a first female to find one other.
Dragonfly eggs slide through aquatic substrates. The prolarves undergo up to twelve moults in a few weeks to five years before becoming carnivorous larvae therefore hunters who capture their prey with their lower lip which is very enlarged and which can be projected. This lip is called the mask and it is a formidable weapon. Bloodworms, weakened small fish, and other insects are part of his diet. You can recognize the larvae of anisopters and zygopters. The general shape is very elongated in zygopters (damselflies) with three lamellae at the end of the abdomen which allow breathing. The anisopteran larvae are larger and shorter, they are also often larger because the adults are often larger.
Dragonfly larvae do not undergo total metamorphosis but they transform regularly. The wings develop in wing sheaths which cover almost the entire abdomen. During the last moult, the larva mounts on reeds or on waterfront plants. In a short time, the skin opens at the top of the thorax, and the adult dragonfly emerges. The imago emerges on a hard support and leaves an exuviae. The insect dries in several minutes and remains very vulnerable before it can fly away. The graceful and colorful young lady will fly away in a few minutes or a few hours for the anisoptera. An immature dragonfly has not yet acquired the specific characteristics of an adult. The evolution of colors, the appearance or accentuation of colored patterns makes it possible to characterize a sexually mature subject. The natural predators of dragonflies are certain insectivorous birds such as the gray flycatcher, the European bee-eater or the Eurasian hawk. Web weaving spiders, like spheres, often catch them. Odonata can live from ten days to several months from April to October in the aerial world.
Odonates are very skilled hunters. Their flight is extraordinary of speed and virtuosity, they are able to glide, to perform: a turn on the wing, a hover, a reverse, or a vertical climb to catch a fly. Ninety species populate France. Their presence at the water's edge signals the good health of rivers. Odonate populations are threatened by the disappearance of their aquatic and flying prey linked to eutrophication, pollution of watercourses and drying up of wetlands. Insecticides, pesticides, detergents, heavy metals, hydrocarbons are wreaking havoc on the populations of odonates. These being placed high in organic food chains magnify these pollutants by integrating them into their structures. Odonates are real bioindicators of aquatic ecosystems. They illustrate significant biodiversity.
Global warming favors the migration of species from the south to the northern territories invading ecological niches occupied by local species. New balances or new competitions are being formed to sculpt mixed populations.