ALTHOUGH there are many insects which have taken to water, comparatively few of them are entirely aquatic; they have to come to the surface at intervals to breathe atmospheric air. When we speak of an aquatic insect, we do not necessarily mean one which spends its whole life in the water; in fact there are very few which do so, although a bug and a fly which live entirely in the sea are known. No, we speak of an insect as aquatic if any part at all of its life is spent in water. In the case of an aquatic insect whose wings develop externally, it is usually the nymph, but in the higher aquatic insects the larva is the stage which lives in water, and it is these stages which are adapted for a water existence.
The eggs are laid or dropped into the water, or else they are laid on vegetation verging the selected pond or stream. The female of certain species pierces the stems of aquatic plants with her egg-laying organ, or ovipositor, and lays her eggs in the internal tissues. When eggs are laid beneath the surface of the water, they resemble frog spawn in that they are cemented together by a gelatinous, water-absorbing material or mucilage.
The nymph is the aquatic stage in stone-flies, mayflies and dragon-flies. The larva is the aquatic stage in alder flies. In caddis flies, mosquitoes and gnats, all stages except the adult live in water. The big carnivorous water beetles live in water as adults and yet are dependent on atmospheric air for breathing. They carry a supply of air to the bottom of the pond in the form of a bubble at their tails, and draw on this supply by means of their abdominal spiracles. The grown larva leaves the water and undergoes metamorphosis under the soil and is therefore terrestrial. Even the adult is not confined to water, for it can leave it to fly from one pond to another. Space forbids us to discuss each of these examples, so let us look more closely at the life-histories of the mayfly and the gnat.
GROWING UP IN TIME TO DIE: THE MAYFLY’S CURIOUS HISTORY THE female mayfly drops her eggs into the water and the subsequent development extends over a long period. The nymphs which ultimately hatch out are quite different from their parents in that they possess large biting mouth-parts and are entirely aquatic. The nymphal phase is a prolonged one, and may last for three years. The nymphs feed on small
aquatic creatures and vegetation. Their principal adaptation to aquatic life lies, as one would expect, in their breathing mechanism. They breathe by filamentous or thread-like gills which are penetrated by the air-tubes; these gills are paired and are arranged along the sides of the abdomen.
As we have seen, the nymph is unlike its parents, but contrary to the usual sequence of events in such a case, the metamorphosis from nymph to adult is incomplete. When fully-grown, the nymph leaves the water; its skin splits down the back and a winged insect emerges. This is not the adult, but a peculiar and unique condition called the sub-imago which has been interpolated between the usual stages of nymph and imago. The sub-imago flies away and moults once before reaching maturity. These discarded sub-imaginal skins are often found on fences near streams. The adult mayfly is easily identified by the three long streamers which project from the abdomen. An adult which may have taken three years to develop may only live long enough to lay eggs—a matter of a few hours.
The gnat lays masses of eggs on the surface of ponds, and these eggs are so placed that on hatching the larva; can immediately take to the water. The larva; or ‘wrigglers ‘are commonly found in outdoor water butts and troughs in the summer. The larva is dependent upon atmospheric and not dissolved air, and a special breathing structure is developed near the tail. This takes the form of a tube which is thrust through the surface film, the larva itself hanging head downwards from the surface film during the process of breathing. From the air tube, the air is drawn into the trachea; (internal air tubes) as in other insects. The larva of the allied mosquito Anopheles, which differs from the gnat larva, suspends itself horizontally and not vertically from the surface film. In the pupal, which follows the larval stage, the breathing is carried out by means of thoracic trumpets which connect up with the air tubes (trachea;), and in the act of breathing the pupa floats at the surface with its breathing trumpet erect above the water.
In countries where malaria is widespread, the mosquitoes of the Anopheles type carry the parasites of the disease from one human being to another, and the mosquito itself is a necessary host for part of the life-cycle of the malarial parasite. As the parasites only occur in other animals, the best way of checking them is to destroy one of the hosts, and in this case, as we are men, we destroy the mosquito in preference to man ! Now,
it is easier to destroy localised larvae than widely-spread adults, and, in view of their breathing mechanism, it is possible to drown them by preventing their breathing tubes from reaching the air; this can be done by spreading oil over the surface of the water. The oil forms an effective barrier between the air-tubes and the atmospheric air, with the result that hundreds of larva; are drowned.
The stone-flies have aquatic nymphs which may or may not have gills. If they have not, they breathe through the skin at certain points. The dragon-fly nymph carries gills and is quite unlike its parents. A characteristic contrivance called the mask is formed by the considerable development of the lower lip. The mask is arm-like and jointed so that it can be folded below the head or extended. At the end of the mask, two curved spines are carried for securing prey. The aquatic lame of the caddis fly build cases of sand or shells and debris which are carried about with them. These are so well known to collectors of pond life that further description is unnecessary,