What determined her, therefore, is not simply the smell, which can easily be perceived even through the uncut paper, but, above all, the crevice, which will provide an entrance for the vermin, hatched outside, near the narrow passage. The maggots' mother has her own logic, her prudent foresight. She knows how feeble her wee grubs will be, how powerless to cut their way through an obstacle of any resistance; and so, despite the temptation of the smell, she refrains from laying, so long as she finds no entrance through which the new-born worms can slip unaided.
I wanted to know whether the colour, the shininess, the degree of hardness and other qualities of the obstacle would influence the decision of a mother obliged to lay her eggs under exceptional conditions. With this object in view, I employed small jars, each baited with a bit of butcher's meat. The respective lids were made of different-coloured paper, of oil-skin, or of some of that tin-foil, with its gold or coppery sheen, which is used for sealing liqueur-bottles. On not one of these covers did the mothers stop, with any desire to deposit their eggs; but, from the moment that the knife had made the narrow slit, all the lids were, sooner or later, visited and all, sooner or later, received the white shower somewhere near the gash. The look of the obstacle, therefore, does not count; dull or brilliant, drab or coloured: these are details of no importance; the thing that matters is that there should be a passage to allow the grubs to enter.
Though hatched outside, at a distance from the coveted morsel, the new-born worms are well able to find their refectory. As they release themselves from the egg, without hesitation, so accurate is their scent, they slip beneath the edge of the ill-joined lid, or through the passage cut by the knife. Behold them entering upon their promised land, their reeking paradise.
Eager to arrive, do they drop from the top of the wall? Not they! Slowly creeping, they make their way down the side of the jar; they use their fore-part, ever in quest of information, as a crutch and grapnel in one. They reach the meat and at once instal themselves upon it.
Let us continue our investigation, varying the conditions. A large test-tube, measuring nine inches high, is baited at the bottom with a lump of butcher's meat. It is closed with wire-gauze, whose meshes, two millimetres wide (.078 inch.—Translator's Note.), do not permit of the Fly's passage. The Bluebottle comes to my apparatus, guided by scent rather than sight. She hastens to the test-tube, whose contents are veiled under an opaque cover, with the same alacrity as to the open tube. The invisible attracts her quite as much as the visible.
She stays awhile on the lattice of the mouth, inspects it attentively; but, whether because circumstances failed to serve me, or because the wire network inspired her with distrust, I never saw her dab her eggs upon it for certain. As her evidence was doubtful, I had recourse to the Flesh-fly (Sarcophaga carnaria).
This Fly is less finicking in her preparations, she has more faith in the strength of her worms, which are born ready-formed and vigorous, and easily shows me what I wish to see. She explores the trellis-work, chooses a mesh through which she inserts the tip of her abdomen, and, undisturbed by my presence, emits, one after the other, a certain number of grubs, about ten or so. True, her visits will be repeated, increasing the family at a rate of which I am ignorant.
The new-born worms, thanks to a slight viscidity, cling for a moment to the wire-gauze; they swarm, wriggle, release themselves and leap into the chasm. It is a nine-inch drop at least. When this is done, the mother makes off, knowing for a certainty that her offspring will shift for themselves. If they fall on the meat, well and good; if they fall elsewhere, they can reach the morsel by crawling.
This confidence in the unknown factor of the precipice, with no indication but that of smell, deserves fuller investigation. From what height will the Flesh-fly dare to let her children drop? I top the test-tube with another tube, the width of the neck of a claret-bottle. The mouth is closed either with wire-gauze or with a paper cover with a slight cut in it. Altogether, the apparatus measures twenty-five inches in height. No matter: the fall is not serious for the lithe backs of the young grubs; and, in a few days, the test-tube is filled with larvae, in which it is easy to recognize the Flesh-fly's family by the fringed coronet that opens and shuts at the maggot's stern like the petals of a little flower. I did not see the mother operating: I was not there at the time; but there is no doubt possible of her coming, nor of the great dive taken by the family: the contents of the test-tube furnish me with a duly authenticated certificate.
I admire the leap and, to obtain one better still, I replace the tube by another, so that the apparatus now stands forty-six inches high. The column is erected at a spot frequented by Flies, in a dim light. Its mouth, closed with a wire-gauze cover, reaches the level of various other appliances, test-tubes and jars, which are already stocked or awaiting their colony of vermin. When the position is well-known to the Flies, I remove the other tubes and leave the column, lest the visitors should turn aside to easier ground.