7 August 2010


Anax imperator

The dragonfly is the heraldic emblem of these August afternoons. It is a creature of the heat, its darting activity and purposefulness a direct contrast to the lethargy that overtakes the canal, the river, the weed-choked ponds and streams. Its colouring belongs more to the tropics than to the English lowlands; and in its ferocity the dragonfly reminds us of the merciless insect-war going on, mostly unseen, everywhere in the dense vegetation of summer at its height.

Throughout the whole animal kingdom, indeed, there are no more brilliant colours than those of certain dragonflies. Insect colours are produced in various ways and for various reasons. Some pigments are mere by-products of other bodily processes, with no special significance, but most have some function, however obscure.

The bright yellow laid down in the cuticle of, say, wasps or bees, gives notice that the animal is not to be trifled with. Reds and oranges often signify that the owner is unpleasant to eat. Many other colours and patterns are important in camouflage, courtship, mating, or the defence of territory.

In some insects, colours are produced by intricate structural effects as well as by simple pigments. Iridescence in the more dazzling hues of many butterflies, and the metallic bronzes and greens of beetles, for example, results from an interference effect – involving the reflection of certain wavelengths of light from successive layers in the insect’s scales or cuticle. These colours are brilliant enough, but the dragonflies have gone one stage further.

Besides having a wide complement of pigments, they make use of a phenomenon called Tyndall scattering. In this, light is dispersed in all directions by irregularities in the cuticle surface or by granules laid down just beneath it. The size of the irregularities or granules is minutely adjusted to the wavelength of the light which is to be reflected.

Tyndall scattering is responsible for the incandescent blues and greens of many of our dragonflies. It depends for its success on a dark masking layer below the granules, provided by a brown-violet pigment, an ommochrome.

Ommochromes are widespread in insects. As well as providing brown, red, and yellow body-colours, they have a more specialized use as masking pigments in vision, isolating the individual elements of the compound eyes and enabling them to function.

This leads us to another remarkable feature of the dragonfly’s life, the refinement of its eyesight. Dragonflies are normally day-flying insects. They rely almost entirely on their eyes: the antennae, the seat of taste and smell, are poorly developed. In some insects the compound eyes may contain only a few widely spaced elements; in the dragonfly there may be ten thousand or more in each eye, closely packed together in a hexagonal formation.

Each element in the compound eye sends a signal to the brain, forming a sort of mosaic image. Compared with the human eye, focusing ability and acuity are not very impressive, but the dragonfly’s eye is supremely well adapted to detecting movement. There is very rapid recovery of bleached visual pigments, a high so-called “flicker rate”, enabling the animal to make sense of the landscape as it races past, and enabling it to see and catch its prey with lightning speed.

The prey consists mainly of flying insects, sometimes other dragonflies. It is caught with either the jaws or legs. The legs are found well forward on the thorax, which is itself tilted in such a way that they form a basket to scoop an insect from the air and hold it steady while the mouthparts are brought into action.

There are two sub-orders of dragonflies. The first contains the damsel-flies, and the second the larger, more powerful dragonflies such as the fearsomely named Anax imperator. The eyes in this second group reach an astounding peak of size and development, covering most of the surface of the head.

Much time is expended by the insect in cleaning the eyes with special movements of the forelegs. The head is balanced on a delicate suspension mechanism which allows it to swivel, giving all round vision. So delicate is this mechanism that if the dragonfly hits head-on even a lightweight obstruction – such as the folds of a collector’s net – irreparable damage will be done.

Set beside the technical achievement of constructing just one dragonfly, human efforts to date in the fields of cybernetics and microelectronics look distinctly clumsy. To be fair, the craft of dragonfly-making has had some three hundred million years in which to be perfected: the dragonflies were among the first of the insects to evolve.

The individual adult dragonfly also has a long time in which to develop. Most of a dragonfly’s life is spent underwater, as a nymph, a fierce carnivore which repeatedly moults its skin as it grows in size. There may be a dozen moults in all, and the underwater period varies from one year in the damselflies to a dozen in the largest species. When the time comes for the final moult – usually in June or July – the nymph crawls up a plant-stem, often during the early hours, and, with frequent pauses for rest, breaks out of first one part of the skin and then another.

By dawn the newly emerged, or teneral, dragonfly resembles a mature adult in all respects except for its pale colouring and the flaccidity of its wings and body. As the sun rises the wings harden and become ready for flight. The cuticle hardens also, and within a few hours the insect is ready to fly: the colours develop over a period of days.

In the few weeks of life remaining to it the dragonfly must mate. Females are actively sought out by the males, which frequently are strongly territorial, maintaining one stretch of water as their exclusive property. The eggs are laid in vegetation, or simply scattered in the water to sink to the bottom.

With the first frosts the remaining adults are ruthlessly cut down. Together with hundreds of millions of other insects, they have betrayed themselves by leaving eggs to perpetuate the race. They are no longer required. Thus does Nature treat the most intricate fruits of its creation: and thus vanishes the magical dragonfly, emblem of this brief English summer.

(Introduction to these pieces; see all)

No comments: