Fig. 54.—Eggs of the Meat-fly
(Calliphora vomitoria).
If we shut up a blue meat-fly in a glass vase, as Réaumur did, and place near the insect a piece of fresh meat, before half a day is passed, the fly will have deposited its eggs thereon one after the other, in irregular heaps, of various sizes. The whole of these heaps consist of about two hundred eggs, which are of an iridescent white colour, and four or five times as long as they are broad. In less than twenty-four hours after the egg is laid the larvæ is hatched. It is no sooner born than it thinks of feeding, and buries itself in the meat with the aid of the hooks and lancets with which it is provided.
These worms do not appear to discharge any solid excrement, but they produce a sticky liquid, which keeps the meat in a moist state and hastens its putrefaction. The larvæ eat voraciously and continually; so much so, that in four or five days they arrive at their full growth. They then take no more nourishment until they are transformed into flies. They are now about to assume the pupa state. In this condition it is no longer necessary for them to remain on the tainted meat, which has been alike their cradle and their larder, and where until now they were so well off. They therefore leave it and seek a retreat under ground.
The larva then assumes a globular form and reddish colour, loses all motion, and cannot any longer either lengthen or shorten, or dilate or contract itself. Life seems to have left it. "It would be considered a miracle," says Réaumur, "if we were told there was any kind of quadruped of the size of a bear, or of an ox, which at a certain time of the year, the beginning of winter for instance, disengages itself completely from its skin, of which it makes a box of an oval form; that it shuts itself up in this box; that it knows how to close it in every part, and besides that it knows how to strengthen it in such a manner as to preserve itself from the effects of the air and the attacks of other animals. This prodigy is presented to us, on a small scale, in the metamorphosis of our larva. It casts its skin to make itself a strong and well-closed dwelling."
If one opens these cocoons only twenty-four hours after the metamorphoses of the worms, no vestige of those parts appertaining to a pupa is to be found. But four or five days afterwards, the cocoon is occupied by a white pupa, provided with all the parts of a fly. The legs and wings, although enclosed in sheaths, are very distinct; these sheaths being so thin that they do not conceal them. The trunk of the fly rests on the thorax; one can discern its lips, and the case which encloses the lancet. The head is large and well formed, its large, compound eyes being very distinct. The wings appear still unformed, because they are folded, and, as it were, packed up. It is a fly, but an immovable and inanimate fly; it is like a mummy enveloped in its cloths.
Nevertheless, it is intended this mummy should awake, and when the time comes it will be strong and vigorous. Indeed, it has need of strength and vigour to accomplish the important work of its life. Although its coverings are thin, it is a considerable work for the insect to emerge, for each of its exterior parts is enclosed in them as in a case, much the same as a glove fits tightly to all the fingers of the hand. But that for which the most strength is necessary is the operation of forming the opening of the cocoon, in which as a mummy it is so tightly enclosed.
The fly always comes out at the same end of the cocoon, that is, at the end where its head is placed, and also where the head of the larva previously was. This end is composed of two parts—of two half cups placed one against the other. These can be detached from each other and from the rest of the cocoon. It is sufficient for the fly that one can be detached, and in order to effect this, it employs a most astonishing means. It expands and contracts its head alternately, as if by dilatation; and thus pushes the two half cups away from the end of the cocoon. This is not long able to resist the battering of the fly's head, and the insect at length comes out triumphant. This fly, which should be blue, is then grey; it, however, comes quickly to perfection, at the end of three hours attaining its ultimate colour; and in a very short space of time every part of the animal becomes of that firmness and consistency which characterises them. At the same time, the wings, which at the moment it came into the world were only stumps, extend and unfold themselves by degrees. The meat-fly is represented below ([Fig. 55]).
