The Insect World. Figuier Louis

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СКАЧАТЬ alt=""/> Fig. 39.—A species of Helophilus.

      The Helophili (Fig. 39) deserve to be mentioned here on account of the singular form of many of their larvæ. The head is thick, fleshy, and varying a little in form. But the point by which they are easily to be distinguished from most other larvæ is, that they have always very long tails, sometimes, indeed, out of proportion to the length of the body. Réaumur called these larvæ "vers à queue de rat;" they are known in England as rat-tailed maggots, and their habits are aquatic. Having placed some of them in a bason of water, Réaumur saw that they kept in a perpendicular position at the bottom of the bason and parallel to one another, the extremities of their tails being on the surface of the water. He then increased the depth of the water by degrees; and, as it got deeper, observed that the tail of each worm became longer. These tails, which at first were only two inches long, at last attained to five.

      It will be remarked that the body of each worm does not exceed five lines in length. The tail is a peculiar organ, by the aid of which the worm breathes, although its body may be covered by water to the depth of several inches. It is composed of two tubes, one of which shuts into the other, like a telescope. Réaumur calls it the breathing tube. It terminates in a little brown knob, in which, according to Réaumur, are two holes for the purpose of receiving the air, and which have five little tufts of hair, which float on the surface of the water. When the time comes for the metamorphosis of these worms, they come out of the water and bury themselves in the earth; the skin then hardens and becomes a sort of cocoon. In this cocoon the insect loses the form of a worm, and takes by degrees that of the pupa, which it keeps until circumstances cause it to throw off its last coverings, and to appear in the winged state.

       Fig. 40.—Larvæ of a Helophilus.

      What an eventful life! what a life full of changes and turns of fortune is that of these insects, which pass the first and longest period of their existence under water, another part of their life under the ground, and, finally, after having existed in these two elements, enjoy, high in the air, the pleasures of flight!

      The third group of Brachycera is that of the Dichæta; that is, those flies having two-fibred suckers. Among these are classed the Œstri, the Conopes, and the flies properly so called.

      The genus Œstrus, the Gad, Bot-fly, or Breeze, comprises those formidable insects which attack the horse, the sheep, and the ox. [15] The labours of Réaumur, in his admirable Memoirs, and those of M. Joly, Professor of Zoology to the Faculté des Sciences de Toulouse, who published some most valuable researches on this subject, in 1846, will guide us in the following brief explanation.

      The following is the description given by M. Joly of the Gad-fly (Œstrus equi) represented in Figs. 41, 42, which are taken from a drawing which accompanies that naturalist's Memoirs.

      

I.—A Herd of Horses attacked by Gad-flies (Œstrus equi).

      The head of this insect is large and obtuse; the face light yellow, with whitish silky fur; the eyes blackish; the antennæ ferruginous; the thorax grey; and the abdomen of a reddish yellow, with black spots. The wings are whitish, not diaphanous, with a golden tint, and divided by a winding band of blackish colour. The feet are palish yellow.

Fig. 41.—Horse-fly, male (Œstrus [gasterophilus] equi).		Fig. 42.—Horse-fly, female (OEstrus [gasterophilus] equi).

      This species is found in France, in Italy, and also in the East, especially in Persia, and rarely in England. During the months of July and August the Œstrus frequents pastures, and deposits its eggs chiefly on the shoulders and knees of horses (Plate I.). In order to do this, the female suspends herself in the air for some seconds over the place she has chosen, falls upon it, and with her abdomen bent, sticks her eggs to the horse's hairs by means of a glutinous liquid with which they are provided, and which soon dries. This is repeated at very short intervals. It often happens that from four to five hundred eggs are thus deposited upon the same horse. Guided by a marvellous instinct, the female Œstrus generally places her eggs on those parts of the horse's body which can be most easily touched with the tongue, that is, at the inner part of the knees, on the shoulders, and rarely on the outer part of the mane.

Fig. 43.—Eggs of the Gad-fly (Œstrus [gasterophilus] equi) deposited on the hairs of a horse.

      The eggs of the Œstrus, which are white and of conical form, adhere to the horse's hair, as shown in Fig. 43. They are furnished with a lid, which at the time of hatching opens, to allow the exit of the young larva, which takes place, according to M. Joly, about twenty days after they are deposited. In fact, it is not in the egg state, but really in that of the larva, that the horse, as we shall explain, takes into his stomach these parasitical guests, to which Nature has allotted so singular an abode. When licking itself, the horse carries them into its mouth, and afterwards swallows them with his food, by which means they enter the stomach. It is a remarkable fact that it is sometimes other insects, as the Tabania for instance, that by their repeated stinging cause the horse to lick himself, and thus to receive his most cruel enemy. In the perilous journey they have to perform from the skin of the horse to his stomach, many of the larvæ of the Œstrus, as may be supposed, are destroyed, ground by the teeth of the animal, or crushed by the alimentary substances. There is hardly one Œstrus in fifty that arrives safely in the stomach of the horse; and yet if one were to open a horse which had been attacked by the Œstri, the stomach would be nearly always found to have many of the larvæ sticking to its inside. Fig. 44, taken from a drawing which accompanies M. Joly's Memoirs, represents the state of a horse's stomach attacked by the Gad-fly larvæ.

Fig. 44.—Portion of the stomach of the horse, and larvæ of Œstrus (gasterophilus) equi.

      The larvæ are of a reddish yellow, and each of their segments is armed at the posterior edge with a double row of triangular spines, large and small alternately, yellow at the base, and black at the point, which is always turned backwards. The head is furnished with two hooks, which serve to fasten the larva to the internal coats of the stomach. The spines with which the whole surface of the body is furnished contribute to fix it more perfectly, preventing the creatures, by the manner in which they are placed, from being carried away by the food which has gone through the first process of digestion.

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