The Insect World. Figuier Louis

Чтение книги онлайн.

СКАЧАТЬ by the mucus secreted by the mucous membrane of the stomach, and that it breathes the air which the horse swallows with its food during the process of deglutition. It must be acknowledged, however, that it is in the midst of a gaseous atmosphere which is very unhealthy, for nearly all the gases generated in the stomach of the horse are fatal to man and to the generality of animals, as they consist of nitrogen, carbonic acid, sulphuretted hydrogen, and carburetted hydrogen. To explain how the insect can live under such circumstances, M. Joly has suggested the following ingenious hypothesis:—

      "When the stomach which the larva inhabits," says this learned naturalist, "contains only oxygen, or air that is nearly pure, the insect opens the two lips of the cavity which contains the spiracles, and breathes at its ease. When the digestion of the alimentary substance generates gas which is unfit for respiration, or when the spiracles run the risk of being obstructed by the solid or liquid substances contained in the stomach, it shuts the lips, and continues to live on the air contained in its numerous tracheæ."

      "Whatever may be the value of this explanation," adds M. Joly, "it is nevertheless very curious to see an insect pass the greater part of its life in an atmosphere which would be instantly fatal to most animals, and in an organ where, under the government of life, chemical processes bring about the most wonderful changes of the food into the substance of the animal itself. But how can the insect itself resist the action of these mysterious powers, and remain alone intact in the midst of all these matters which are unceasingly changing and decomposing? This is another question which it is difficult, or rather impossible, to explain in the present state of science; another enigma which humbles our pride, and of which He who has created both man and the worm alone knows the secret."

      Arrived at a state of complete development, the larva of the Œstrus imprisoned in the stomach of the horse leaves the membrane to which it has been fixed, then directing the anterior part of its body towards the pyloric opening of the stomach, allows itself to be carried away with the excrementitious matter. It traverses, mixed with the excrementary bolus, the whole length of the intestinal canal, leaves it by the anal orifice, and on touching the ground at once seeks a suitable place to go through the last but one of its metamorphoses.

      The skin then gets thick, hardens, and becomes black. All the organs of the animal are composed of a whitish amorphous pulp, which soon assumes its destined form, and the insect becomes perfect. It then lifts a lid at the anterior part of its cocoon, emerges, dries its wings, and flies off.

Fig. 45.—Bot-fly (Œstrus bovis).

      The Bot-fly (Œstrus bovis, Fig. 45) has a very hairy body, large head, the face and forehead covered with light yellow hair, the eyes brown, and the antennæ black. The thorax is yellow, barred with black; the abdomen of a greyish white at the base, covered with black hair on the third segment, and the remainder of an orange yellow; the wings are smoky brown.

II.—A Herd of Cattle attacked by Bot-flies (Œstrus bovis).

      As soon as the cattle are attacked, they may be seen, their heads and necks extended, their tails trembling, and held in a line with the body, to rush to the nearest river or pond, while such as are not attacked disperse (Plate II.). It is asserted that the buzzing alone of the Œstrus terrifies a bullock to such an extent as to render it unmanageable. As for the insect, it simply obeys its maternal instinct, which commands it to deposit its eggs under the skin of our large ruminants.

      Let us now explain how the eggs of the Œstrus, deposited in the skin of the bullock, accommodate themselves to this strange abode. The mother insect makes a certain number of little wounds in the skin of the beast, each of which receives an egg, which the heat of the animal serves to bring forth. It is a natural parallel to the artificial way which the ancient Egyptians invented of hatching the eggs of domestic fowls, and which has been imitated badly enough in our day.

      Directly the larva of the Bot-fly is out of the egg and lodged between the skin and the flesh of its host, the bullock, it finds itself in a place perfectly suitable to its existence. In this happy condition the larva increases in growth, and eventually becomes a fly in its turn. Those parts of the animal's body in which the larvæ are lodged are easily to be recognised, as above each larva may be seen an elevation, a sort of tumour, termed a bot—a bump, as Réaumur calls it, comparing it more or less justly to the bump caused on a man's head by a severe blow.

      Fig. 46, taken from a drawing in Réaumur's Memoirs, represents the bots of which we speak.

      The country people are well aware of the nature and cause of these bots. They know that each one contains a worm, that this worm comes from a fly, and that later it will be transformed into a fly itself. Each of these bots has in its interior a cavity, occupied by a larva, which, as well as the bot, increases in size as the larva becomes developed.

      It is generally on young cows or young bullocks—in fact, on cattle of from two to three years of age—that these tumours exist, and they are rarely to be seen on old animals. The fly, which by piercing the skin occasions these tumours, always chooses those whose skin offers little resistance. Each tumour is provided with a small opening, by which the larva breathes.

      In order to examine the interior of the cavity, Réaumur opened some of these tumours, either with a razor or a pair of scissors. He found them in a most disgusting state. The larva is lodged in a regular festering wound, matter occupying the bottom of the cavity, and the head of the worm is continually, or almost continually, plunged in this liquid. "It is most likely very well off there," says Réaumur; and he adds that this matter appears to be the sole food of the larva.

      

      "The position of a horned beast," observes the great naturalist, "which has thirty or forty of these bumps on its back, would be a very cruel one, and a terrible state of suffering, if his flesh were continually mangled by thirty or forty large worms. But it is probable they cause no suffering, or at least very little, to the large animal. Besides," continues Réaumur, "those cattle whose bodies are the most covered with bumps, not only show no signs of pain, but it does not appear that they are prejudicial to them in any way."

Fig. 46.—Bumps produced on Cattle by the larvæ of the Bot-fly.

      Réaumur tried to discover how the larva, when arrived at its full growth, succeeds in leaving its abode, as the opening is smaller than its own body.

      "Nature," says Réaumur, "has taught this worm the surest, the gentlest, and the most simple of methods, the one to which surgeons often have recourse to hold wounds open, or to enlarge them. They press tents into a wound they wish to enlarge. Two or three days before the worm wishes to come out, it commences to make use of its posterior part as a tent, to increase the size of its exit from its habitation. It thrusts it into the hole and draws it out again many times in the course of two or three days, and the oftener this is repeated, the longer it is able to retain its posterior end in the opening, as the hole becomes larger. On the day preceding that on which the worm is to come out, the posterior part is to be found almost continually in the hole. At last, it comes out backwards, and falls to the ground, when it gets under a stone, or buries itself in the turf; remaining quiet and preparing for its last transformation. Its skin hardens, the rings disappear, and it becomes black. Thenceforth the insect is detached from the outer skin, which forms a cocoon, or box. At the front СКАЧАТЬ