Photo by W. P. Dando, F.Z.S., Regent's Park.
LARGE CADDIS-FLY.
The largest British species measures about an inch and a half across the wings.
The grubs which hatch out from these eggs feed upon plant-lice, of which they devour vast numbers, draining the juices by means of their hollow jaws, and then fastening the empty skins on their own backs, as an American Indian might decorate himself with the scalps of his victims. Owing to this singular habit, the grub becomes perfectly unrecognisable after the first few days of its life, only the jaws and feet being visible beneath the pile of dry skins. When fully fed, it changes to the pupal condition in a silken cocoon, which it attaches to a leaf, and the perfect insect makes its appearance in the course of a few days.
The Alder-flies, in general appearance, are not unlike caddis-flies, but may easily be distinguished by the fact that the wings are not longitudinally folded while at rest. They are very abundant in the neighbourhood of ponds and small streams, where they may be seen flying slowly and heavily, or resting on low herbage or the foliage of trees and bushes. The female insect lays her eggs in clusters of 300 or 400 on the leaves of water-plants, and the little grubs make their way down into the water immediately on hatching out, where they creep about on the mud at the bottom in search of the tiny creatures on which they feed. When full-grown, they are about an inch in length. They then leave the water and bury themselves in the earth, where they change to pupæ, the perfect insects emerging in June or July.
The Caddis-flies, of which there are many British representatives, belong to the Hairy-winged group. The larvæ of these insects are entirely aquatic, and remind one of hermit-crabs, the front part of the body being clothed with horny armour, while the hinder part is entirely unprotected. In order to escape the attacks of predaceous insects these grubs construct cases round their bodies, which they drag about wherever they go. In one or two instances, however, the case is attached to the lower surface of a stone.
The materials of which these cases are made vary in accordance with the species. In one group, for instance, they consist of pieces of twigs and leaves, cut into short lengths, and arranged side by side in such a manner as to form a spiral band. The larva of another kind uses entire leaves, gluing them firmly together and living between them. A third species employs grains of sand and tiny stones, which it arranges in the form of a cow's horn. Most curious of all, however, is the case of a caddis-fly which is made entirely of the shells of water-snails. As these shells are, as a rule, still tenanted by their owners, the snails may sometimes be seen attempting to crawl simultaneously in half a dozen different directions, while the grub is dragging them in a seventh.
All the grubs retain tight hold of their cases by means of a pincer-like organ at the end of the body. When fully fed, they close the aperture at each end of the tube, and assume the chrysalis state, the perfect insects emerging a few weeks later. Although the wings are large and broad, they fly very slowly, and never seem to take more than a short journey through the air. They may often be seen in numbers resting upon the herbage on the banks of streams and ponds, or crawling down into the water in order to deposit their eggs.
STINGING FOUR-WINGED INSECTS, OR ANTS, BEES AND WASPS, AND THEIR ALLIES
BY W. F. KIRBY, F.L.S.