Stone-flies — Part 1

(Order Plecoptera.)

This order is not a large one and contains only the single family Perlidae, of which thirteen genera are represented in North America and less than a hundred species. The stone-flies have mouth-parts formed for biting; the body is long and soft and flat; the wings are four in number and are membranous, the hind wings being much larger than the fore wings, folded in plates, and lie upon the abdomen when at rest. The antennae are long and thread-like. The larvae are aquatic and are usually found under stones in running water. The flies are commonly seen about water courses in the first warm days of spring, and the cast skins of the nymphs, or pupae, sticking to stones and logs on the banks of streams, are very common objects. The eggs are produced in enormous numbers. They are small and are probably dropped on the surface of the water, as with the May-flies, but some of them, even after they issue from the abdomen enclosed in a kind of capsule, are carried about by the female. One female may deposit 5,000 or 6,000 eggs. The larvae when hatched are very active and are carnivorous in habits, feeding upon the young of the May-flies and other soft-bodied aquatic animals. They are as a rule very flat in form, which enables them to crawl under heavy stones at the bottom of swift running streams. The head is large and flat and the eyes are large and compound. They resemble the adults in general structure except that they lack the wings. There are as a rule two long protruding filaments at the anal end of the body, and they breathe by means of tracheal gills. The legs are flattened and are fringed with hairs, fitting the insect admirably for swimming. The long antennae are present in the larvae also and when the larva is hidden beneath a stone frequently the antennae curve around upwards, apparently to give it warning of the approach of prey. The jaws are strong and toothed but are hidden by the upper lip. The full-grown nymph is not at all pupa-like and is active.

The enormous number of eggs laid by stone-flies indicates that the chances of successful larval growth are rather small and as a matter of fact these larvae are extensively eaten by fishes. It makes no difference to the average fish whether he eats a carnivorous stone-fly larva or a vegetable-feeding May-fly larva. If there were economic entomologists among the fishes these would devote themselves to the destruction of the stone-fly larvae in order that the general food supply of May-flies might be left undisturbed for fish food!

We have said that the larvae breathe by means of tracheal gills. As a matter of fact, however, with some species there are no such gills and no spiracles. These larvae must take their oxygen from the water through the skin, the ultimate tracheae being especially numerous in the spots where the spiracles would be if there were any and the skin at this point is especially thin. This method of breathing has been already mentioned in our accounts of the larvae of the Chironomidae and of the larvae of the internal feeding parasitic Hymenoptera.

An aquarium student with an aquarium so constructed as to maintain a constant current of water will do good service in fully elaborating the life history of any of our common species.