The maggot is aware of the trials that await it when, as a fly, it will have to come up from under ground; it knows beforehand how difficult the ascent will be with the feeble instrument at its disposal, so difficult, in fact, as to become fatal should the journey be at all prolonged. It foresees the dangers ahead of it and averts them as well as it can. Gifted with two iron shod sticks in its throat, it can easily descend to such depths as it pleases. The need for greater quiet and a less trying temperature calls for the deepest possible home: the lower down it is, the better for the welfare of the worm and the pupa, on condition that descent be practicable. It is, perfectly; and yet, though free to obey its inspiration, the grub refrains. I rear it in a deep pan, full of fine, dry sand, easy to excavate. The interment never goes very far. About a hand's breadth is all that the most progressive digger ventures upon. Most of the interred remain nearer still to the surface. Here, under a thin layer of sand, the grub's skin hardens and becomes a coffin, a casket, wherein the transformation sleep is slept. A few weeks later, the buried one awakes, transfigured but weak, having naught wherewith to unearth herself but the throbbing hernia of her open forehead.
What the maggot denies itself it is open to me to realize, should I care to know the depth whence the fly is able to mount. I place fifteen bluebottle pupae, obtained in winter, at the bottom of a wide tube closed at one end. Above the pupae is a perpendicular column of fine, dry sand, the height of which varies in different tubes. April comes and the hatching begins.
A tube with six centimeters of sand, the shallowest of the columns under experiment, yields the best result. Of the fifteen subjects interred in the pupa stage, fourteen easily reach the surface when they become flies. Only one of them perishes, one who has not even attempted the ascent. With twelve centimeters of sand, four emerge. With twenty centimeters, two, no more. The other flies, jaded with their exertions, have died at a higher or lower stage of the road. Lastly, with yet another tube wherein the column of sand measured sixty centimeters, I obtained the liberation of only a single fly. The plucky creature must have had a hard struggle to mount from so great a depth, for the other fourteen did not even manage to burst the lid of their caskets.
I presume that the looseness of the sand and the consequent pressure in every direction, similar to that exercised by fluids, have a certain bearing on the difficulties of the exhumation. Two more tubes are prepared, but this time supplied with fresh mould, lightly heaped up, which has not the incoherence of sand, with the attendant drawback of pressure. Six centimeters of mould give me eight flies for fifteen pupae buried; twenty centimeters give me only one. There is less success than with the sandy column. My device has diminished the pressure, but, at the same time, increased the passive resistance. The sand falls of itself under the impact of the frontal rammer; the unyielding mould demands the cutting of a gallery. In fact, I perceive, on the road followed, a shaft which continues indefinitely such as it is. The fly has bored it with the temporary blister that throbs between her eyes.
In every medium, therefore, whether sand, mould or any earthy combination, great are the sufferings that attend the exhumation of the fly. And so the maggot shuns the depths which a desire for additional security might seem to recommend. The worm has its own prudence: foreseeing the dangers ahead, it refrains from making great descents that might promote the welfare of the moment. It neglects the present for the sake of the future.
CHAPTER XVI. A PARASITE OF THE MAGGOT
The dangers of the exhumation are not the only ones; the Bluebottle must be acquainted with others. Life, when all is said, is a knacker's yard wherein the devourer of today becomes the devoured of tomorrow; and the robber of the dead cannot fail to be robbed of her own life when the time comes. I know that she has one exterminator in the person of the tiny Saprinus beetle, a fisher of fat sausages on the edge of the pools formed by liquescent corpses. Here swarm in common the grubs of the greenbottle, the flesh fly and the bluebottle. The Saprinus draws them to him from the bank and gobbles them indiscriminately. They represent to him morsels of equal value.
This banquet can be observed only in the open country, under the rays of a hot sun. Saprini and greenbottles never enter our houses; the flesh fly visits us but discreetly, does not feel at home with us; the only one who comes fussing along is the bluebottle, who thus escapes the tribute due to the consumer of plump sausages. But, in the fields, where she readily lays her eggs upon any carcass that she finds, she, as well as the others, sees her vermin swept away by the gluttonous Saprinus.
In addition, graver disasters decimate her family, if, as I do not doubt, we can apply to the bluebottle what I have seen happen in the case of her rival, the flesh fly. So far, I have had no opportunity of actually perceiving with the first what I have to tell of the second; still, I do not hesitate to repeat about the one what observation has taught me about the other, for the larval analogies between the two flies are very close.