AQUATIC INSECTS

Evening Star, Issue 21240, 22 October 1932, Page 2

AQUATIC INSECTS

[Written by L.AV.M'C., for the ‘Evening Star.’] At this time of the year anyone walking up the Leith may chance on man or boy on the edge of the stream carefully turning over the larger stones. Preliminary visions of a gold rush are dispelled on closer acquaintance; the object of the search is found to be the larval stage of an insect, and the searcher an angler collecting a supply of bait for his next indulgence in the piscatorial art. The insect is familiarly known as the black creeper, the larva of the alder fly. At the time it is found under the stones it may be as much as 2in long, with a black vicious-looking head carrying formidable mandibles with strong, sharp teeth for seizing prey. Since most of the larval life is spent in the water, special organs for breathing dissolved air arc needed. These are to be found on the abdomen as long jointed appendages which give the insect much the appearance of one of those tropical centipedes found in cases of bananas. At all times of the year some creepers may be found under the stones in the bed of the stream, but, for the completion of the life history, a drier habitat is required. Thus in the early summer most leave the water, and under the stones construct oval cells in which they pupate. Later on will emerge large moth-like creatures, pale-brown in colour, with a wing span of some 3in. They are rarely observed during the daytime, but in December and January evenings, quite close to the Leith Valley road, they may be seen in flight characteristic in its slow, laborious movements.

If almost any large stone is lifted out of the Leith anywhere in its course there will bo found, on the underside, curious little masses of pebbles joined together in an irregular but nevertheless expert manner. The stone-mason responsible for the structure is usually found inside, and acquaintance with him will arouse surprise that so small an insect should be able to move and cement together such comparatively large pebbles. The inhabitant of the case is known as a caddis-worm, another favourite bait of anglers. Sometimes portable cases are made of leaves, grains of sand, or hollow pieces of stick. Hooks along the body attach the insect to its movable dwelling so firmly that it usually refuses to be withdrawn for the purpose of providing bait for trout. The angler, with equal ingenuity, inserts a straw into the lower end oi the case, and tickles the “ worm ” until it releases its hold. Breathing is performed by means of finger-like gills on the abdomen. As the pupa stam; is passed in the case under water these gills keep moving, even daring the transformation period, to supply the necessary oxygen for the developing insect. in due time the pupa bites its way out of the case and rises to tho surface. Then emerges the perfect insect, or caddis-fly, which resembles a moth in its dull colouring and general shape- It is rarely seen by the passer-by, but during summer it may readily be disturbed from its hiding place by beating or shaking thick foliage near the river bank. The short, gay, dancing life of the adult may-fly is often used as the type of a “ brief and ineffective life,” scarcely with justice. An adult insect is concerned mainly with reproducing its kind, and, judged by the countless numbers of larval forms to be found in any local stream, the may-fly must bo considered exceedingly successful. Even the smallest stone lifted from the water will have at least one young mayfly on the underside; larger ones may have a hundred or more. They are easily recognised by the long tails and the almost complete transparency of the body. So thin is the skin in some that in’ an individual placed in water under a microscope the circulation of tho blood may be seen with ease. Aeration of tho blood is performed by means of leaf-like abdominal gills; some in addition use the tail feelers as breathing organs. By a series of moults, sometimes as many as twenty, the larva approaches maturity.' It then leaves the water before divesting itself of its last disposable garment. After this is discarded we sec a fragile, elegant, graceful insect, with two or three elongated slender tails and membranous wings with delicate colour and markings. With the jaws reduced to mere vestiges feeding is impossible. Although a complete digestive tract is present, its sole function seems to be to absorb air so as to render the insect more buoyant for its dance of love and death. For the only occupation of the adult is to mate and lay eggs. The mating dance may be seen near bush creeks on warm summer evenings. Thousands of insects rise and fall in the air in perpendicular lines in a dance which is the consummation of months, even years, of labour, growth, and change in the water, a dance swift, joyous, dramatic. As the twilight fades the exhausted females fall on the surface of the water but, before the spark of life departs, countless eggs have been deposited in the water to stock the stream and perpetuate the species. Dragon flies, those voracious hunters of tho air, are among the most easily recognised of insects because of the elegant structure, brilliant colours, and wonderful powers of flight. “ Horsestingers ’’ is a common name of obscure origin ; they do not sting, nor do they go near horses. The legs are unusual in that they are placed too close to the head to be of any use for walking; but they have considerable strength, and are strongly armed for the capture of prey. The sense of sight is not excelled in the whole insect tribe; it shares perfection with that of the birds. If a dragon 11 y is caught and Held in the hand tho eyes are seen to glow with a most beautiful light. Sandflies and mosquitoes caught on the wing seem to form the major part of the food. Dr Tilyard caught a large specimen after watching it hunt for ten minutes. Over 100 mosquitoes were so tightly packed in the gape that the mouth could not shut to enable them to be swallowed. The eggs are placed in the soft tissues of water plants in ponds such as that in Woodhaugh or in the still hackwaters of streams. Sometimes they are laid above the waterline; at others the female will actually go below the surface to perform the operation. As she submerges she takes with her a film of water over tho whole body, which then has tho appearance of being silverplated. From the egg comes a strange fierce larva, with mouth parts so specialised as to make an organ for the capture of prey unique in the animal kingdom. It is known as the “ mask,” because it hides much of tho mouth and sometimes the whole face. To feed tho larva either remains motionless until the victim, a larval mayfly or mosquito or a tadpole, comes within reach, or it stalks it slowly and stealthily until near enough to strike. Then with lightning rapidity the mask is thrown out, and the cruel books are

fixed in the body of the hapless victim. This career of underwater savagery lasts for a year or more. Then, after a succession of moults, the nymph climbs the stem of a plant into the air and undergoes that final process immortalised by Tennyson: To-day I saw the dragon fly Come from the wells where he did lie. An inner impulse rent the veil Of his old husk; from head to tail Came out clear plates of sapphire mail. He dried his wings: like gauze they grew; Thro’ crofts and pastures wet with dew A living flash of light he flew.