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Do they, then, eat these infusoria?

That is not clear. But minnows and small fry eat them by millions; and so do tadpoles, and perhaps caddis baits and water crickets.

What are they?

Look on the soft muddy bottom. You see numberless bits of stick. Watch awhile, and those sticks are alive, crawling and tumbling over each other. The weed, too, is full of smaller ones. Those live sticks are the larva-cases of the Caperers–Phryganeae–of which one family nearly two hundred species have been already found in Great Britain. Fish up one, and you find, amid sticks and pebbles, a comfortable silk case, tenanted by a goodly grub. Six legs he has, like all insects, and tufts of white horns on each ring of his abdomen, which are his gills. A goodly pair of jaws he has too, and does good service with them: for he is the great water scavenger. Decaying vegetable matter is his food, and with those jaws he will bark a dead stick as neatly as you will with a penknife. But he does not refuse animal matter. A dead brother (his, not yours) makes a savoury meal for him; and a party of those Vorticellae would stand a poor chance if he came across them. You may count these caddis baits by hundreds of thousands; whether the trout eat them case and all, is a question in these streams. In some rivers the trout do so; and what is curious, during the spring, have a regular gizzard, a temporary thickening of the coats of the stomach, to enable them to grind the pebbly cases of the caddises. See! here is one whose house is closed at both ends–‘grille,’ as Pictet calls it, in his unrivalled monograph of the Genevese Phryganeae, on which he spent four years of untiring labour. The grub has stopped the mouth of his case by an open network of silk, defended by small pebbles, through which the water may pass freely, while he changes into his nymph state. Open the case; you find within not a grub, but a strange bird-beaked creature, with long legs and horns laid flat by its sides, and miniature wings on its back. Observe that the sides of the tail, and one pair of legs, are fringed with dark hairs. After a fortnight’s rest in this prison this ‘nymph’ will gnaw her way out and swim through the water on her back, by means of that fringed tail and paddles, till she reaches the bank and the upper air. There, under the genial light of day, her skin will burst, and a four-winged fly emerge, to buzz over the water as a fawn-coloured Caperer– deadliest of trout flies; if she be not snapped up beforehand under water by some spotted monarch in search of supper.

But look again among this tangled mass of weed. Here are more larvae of water-flies. Some have the sides fringed with what look like paddles, but are gills. Of these one part have whisks at the tail, and swim freely. They will change into ephemerae, cock-winged ‘duns,’ with long whisked tails. The larvae of the famous green drake (Ephemera vulgata) are like these: but we shall not find them. They are all changed by now into the perfect fly; and if not, they burrow about the banks, and haunt the crayfish-holes, and are not easily found.

Some, again, have the gills on their sides larger and broader, and no whisks at the tail. These are the larvae of Sialis, the black alder, Lord Stowell’s fly, shorm fly, hunch-back of the Welsh, with which we have caught our best fish to-day.

And here is one of a delicate yellow-green, whose tail is furnished with three broad paddle-blades. These, I believe, are gills again. The larva is probably that of the Yellow Sally–Chrysoperla viridis– a famous fly on hot days in May and June. Among the pebbles there, below the fall, we should have found, a month since, a similar but much larger grub, with two paddles at his tail. He is the ‘creeper’ of the northern streams, and changes to the great crawling stone fly (May-fly of Tweed), Perla bicaudata, an ugly creature, which runs on stones and posts, and kills right well on stormy days, when he is beaten into the stream.