Alderfly is a primitive insect whose larvae live in water. It is not a true fly, like the housefly, but belongs to a group of insects which includes the lacewings and ant-lions. This group reached its zenith during the times when the coal measures were being laid down, 300 million years ago.
The adult insect has a body about 1 in, in length, long antennae and dull, heavily wined wings. When the insect is at rest the wings are folded over the back and ridged like a house roof. They are little known, if not wholly unfamiliar to most people, but are well known to the fly fishermen as one of the «flies» used for trout.
The adult insect flies little – almost reluctantly – but when it takes off the flight is direct. More usually it rests or crawls on plants or stones near a water’s edge. Its life is short – merely long enough to ensure the laying of a batch of eggs. The most striking feature of the larvae is its gills. Designed for breathing under water, of which there are seven pairs on the abdomen. They look like extra legs, each being made up of a series of five joints clothed with bristles, except that they are held upwards and backwards. A further unpaired gill is carried at the end of the body. The larva spends much of its time under stones and pebbles but can swim freely by undulating its body. When the oxygen content of the water is low the larva will undulate its body in the same way while remaining stationary, so causing a current of water to flow over the gills. This is of considerable value, especially to those species of alderfly that live in muddy or stagnant water, which lacks the aeration of a running stream.
In March each female lays up to 2,000 brown, cigar-shaped eggs on plants or stones near water. These stand on end in flat masses, cach of 200-500 eggs, like commuters on a station platform. They hatch in two weeks and the larvae make their way to water. The larval stage of life usually lasts for two years, during which the brown larva grows to 1 in. long. During May and June the full-term larvae leave the water and may travel some distance before making an oval cell in mud or vegetable debris in which to pupate. One was seen to travel 6 yd. apparently having climbed over a concrete wall and through a cotoneaster thicket to an open flower bed. Three weeks later the pupa leaves its cell and from it the adult insect emerges.
In many insect pupae the legs and wings are inside the pupal case. In the alder fly pupa they are already free of the body but in special sheaths: as is usual in pupae that have to make their way to the surface before the adult insect emerges, there are spines on the abdominal ridges.
Anglers use the larvae of alder-flies as bait. But they are valuable to fishermen in another way, as well as to the economy of the streams, for they and other insect larvae make possible the existence of fish such as trout. They are an important link in the food chain of water creatures, being converters of protein. Plants manufacture food; small animals feed on plants; the larvae of alder-flies feed on these vegetarian animals such as the larvae of caddis-flies; thus trout and bass, which eat the alder fly larvae, gain protein. So the larvae are the main support of the fishes which the fisherman catches even though he uses the adult as bait. Normally adult alder-flies alight on water only when they fall from overhanging vegetation – when suddenly disturbed by a bird flying near, or when blown down by a gust of wind.
A second point of interest is one that applies to many insects – the adult is shortlived. It serves only for reproducing and dispersing the species. The male, having fertilised the female serves no other purpose except as food for other animals. It is the same with the female once she has laid her eggs. The «real» life of the insect is in the larva, which is longer-lived and is adapted to an entirely different way of life.
The adult does not feed. The larva is carnivorous and seizes any small animals that come its way with its vicious, pincer-like mouthparts, or mandibles.
When authors write of a reluctance on the part of the adult alderfly to take wing, they are referring to the manner in which the insect will, on being disturbed, run quickly up a leaf or a stem, then fly directly to a nearby leaf or stem, and merely repeat this manocuvre whenever disturbed.
It is very likely that this behaviour of the alderfly gives us an important clue to the origin and evolution of flight in insects. The alderfly is primitive in other ways and it is not unreasonable to suppose that its flying behaviour is also primitive. We have seen how the larvae live for a year or two while the adult has only a short-lived existence, taking no food and living just long enough to mate and for the female to lay her eggs. Flight can have no value for food capture, because no food is eaten. There is no question of migration. Consequently, the two primary purposes of flight in the alderfly are to bring the sexes together and to make possible evasive action to escape enemies long enough for the eggs to be fertilised and laid.
In the course of insect evolution other functions have been added. For example, bees, wasps and dragonflies use flight to get food. Many insects migrate – some butterflies migrate from the Sahara to Scotland. Nevertheless, it must be emphasised that the two primary purposes of flight in insects are to ensure mating and to provide protection from enemies, with a third but subsidiary function – that of spreading the species. In the case of the alderfly this spread could only be very slow.
All this contrasts strongly with the function of flight in birds and bats, where it plays such a large part in food capture and security and is often a way to avoid the worst rigours of climate by migration.