Hawks of the insect world
By Nutan Shukla

FLYING insects that catch other flying insects need a good speed to catch their prey. The fastest appear to be the dragonflies, the hawks of the insect world, which can fly at speeds as fast as 36 mph (58 km/h). Indeed, one group is called the ‘hawker’ dragonflies, and like their relatives the ‘darters’, they have large, effective eyes that can spot prey from 33 feet away.

The hawkers attack on the wings, while the darters launch out from a resting place. Both catch flying insects that inadvertently fly into their territory, and they do it with the legs. The thorax, to which the legs are attached, is angled forwards so that the legs can form a spiny basket in which the prey is caught in mid-air. The victim is taken back to a favourite perch and eaten. The crunching sounds are audible from several yards away. With large prey items, such as butterflies (and sometimes other smaller dragonflies), only the body is eaten, and the wings discarded. Small insects, such as midges and mosquitoes, may be caught and eaten without the dragonfly touching the ground.

These skilled fliers eat, drink and mate in flight. They have two pairs of wings which not only facilitate high speed but also help the insect in hovering and flying backwards.

A dragonfly’s two large and bulging compound eyes play a major part in its hunting skill. They cover almost the whole of its head and give it complete all-round vision.

Each eye is composed of more than 1000 tiny eyes — six sided facets, each with its own lens and retina. On the whole they have about 30,000 separate lens-like facets — more than any other insect — allowing it to see as well as most mammals. Through these compound eyes the dragonfly sees a mass of images that together resemble a speckled newspaper photograph.

Each facet is stimulated successively by movement through its field of vision. This ‘flicker vision’ through many facets enables a dragonfly to spot even the smallest movement of prey. The number of facets dictates an insect’s quality of vision. An ant, with only nine facets, gets a very unclear picture.

The dragonfly is an effective predator even during the earlier stages of its life. The nymph stage is aquatic, and it is one of the most voracious freshwater killers. Its secret, like the John Dory, is in its jaws. John Dory is a group of compressed, marine fish belonging to the family ‘Zeidae’ and is found in Europe, having long spines in the dorsal fin. The nymph possesses a ‘mask’ (so-called because it masks the parts of the mouth) which consists of a lower jaw hinged to an upper jaw tipped with movable hooks. The whole structure can be extended in front of the nymph’s face and can capture anything from small fish to large tadpoles.

Dragonflies belong to the order of Odonata. They have two pairs of net-veined wings, short inconspicuous antennae, but truly enormous eyes, specialised for spotting prey.

They rest with the wings spread out, but the damselflies, who also belong to the same order, fold their wings while resting. The legs are not meant for walking or running or crawling, but are spined and form a sort of open meshwork basket, for holding the victims, snapped when flying, to be eaten at leisure. Often as many as several dozen gnats may be found in this basket. During larval stage or nymph stage they breathe by means of gills, inside their rectum, from which a jet of water is often so forcibly thrown backward that the body rushes forward — true jet-propulsion locomotion!

This feature was published on September 19, 1999