Let us, some day in July, knock away the pebbles that fasten the nests of the Mason-bees to the sloping ground on which they are built. Loosened by the shock, the dome comes off cleanly, all in one piece. Moreover—and this is a great advantage—the cells are all exposed at the base of the nest, for at this point they have no other wall than the surface of the pebble. Without any scraping, which would be wearisome work for us and dangerous to the Bees, we have all the cells before our eyes, together with their contents—a silky, amber-yellow cocoon, as delicate and transparent as the skin of an onion. Let us split the dainty wrappers with the scissors, cell by cell, one after another. If fortune be at all kind, as it always is to the persevering, we shall end by finding cocoons harbouring two larvæ together, one more or less faded in appearance, the other fresh and plump. We shall also find some, no less plentiful, in which the withered larva is accompanied by a family of little grubs wriggling uneasily round it. It is easy to see that a tragedy is happening under the cover of the cocoon. The flabby, faded larva is the Mason-bee’s. A month ago, in June, having finished its ration of honey, it wove itself a silken sheath in which to take the long sleep that precedes its transformation. It was bulging with fat, and was a rich and a defenceless morsel for any enemy that could reach it. And enemies did reach it. In spite of obstacles that might well seem insurmountable, the wall of mortar and dome-shaped cover, the enemy grubs appeared in the secret retreat, and began to eat the sleeper. Three different species take part in this murderous work, often in the same nest, in adjoining cells. We will concern ourselves only with the Anthrax Fly. The grub, when it has eaten its victim and is left alone in the Mason-bee’s cocoon, is a naked worm, smooth, legless, and blind. It is creamy-white, and each of its segments or divisions forms a perfect ring, very much curved when at rest, but almost straight when disturbed. Including the head I can count thirteen segments, well-marked in the middle of the body, but in the fore-part difficult to distinguish. The white, soft head shows no sign of any mouth, and is no bigger than a tiny pin’s head. The grub has four pale red stigmata, or openings through which to breathe, two in front and two behind, as is the rule among Flies. It has no walking-apparatus whatever; it is absolutely incapable of shifting its position. If I disturb its rest, it curves and straightens itself alternately, tossing about violently where it lies; but it does not manage to progress.