The Insect World. Figuier Louis

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СКАЧАТЬ Figs. 25, 26, 27.—Lancets of the Gnat.

      According to Réaumur, the sting of the gnat is composed of five parts. He acknowledges, however, that it is very difficult to be certain of the exact number of these parts, on account of the way in which they are united, and of their form. At the present day we know that there are six. Réaumur, as also Leuwenhoek, thought he saw two in the form of a sword blade with three edges. These have the points reversed, and are serrated on the convex side of the bend (Fig. 25). To form an idea of the shape of the other points, the reader should look at Figs. 26 and 27. He will then see that the gnat's sting is a sword in miniature.

      The prick made by so fine a point as that of the sting of the gnat ought not to cause any pain. "The point of the finest needle," says Réaumur, "compared to the sting of the gnat, is the same as the point of a sword compared to that of the needle." How is it then that so small a wound does not heal at once? How is it that small bumps arise on the part that is stung? The fact is, that it is not only a wound, but it has been imbued with an irritating liquid.

      This liquid may be seen to exude, under different circumstances, from the trunk of the gnat, like a drop of very clear water.

      Réaumur sometimes saw this liquid even in the trunk itself. "There is nothing better," he observes, "to prevent the bad effects of gnat bites than at once to dilute the liquid they have left in the wound with water. However small this wound may be, it will not be difficult for water to be introduced. By rubbing, it will be at once enlarged, and there is nothing to do but to wash it. I have sometimes found this remedy answer very well."

Fig. 28. Larva of the Gnat.

      The gnat is not always in the form of a winged insect, greedy for our blood. There is a period during which they leave us in repose. This is the larva period. It is in water, and in stagnant water in particular, that the larva of the insect which occupies our attention is to be found. It resembles a worm, and may be found in ponds from the month of May until the commencement of winter.

      If we desire to follow the larva of the gnat from the beginning, we have only to keep a bucket of water in the open air. After a few days this water will be observed to be full of the larvæ of the gnat (Fig. 28). They are very small, and come to the surface of the water to breathe; for which purpose they extend the opening of a pipe, A, which is attached to the last segment of the body, a little above the surface. They are, consequently, obliged to hold their heads down. By the side of the breathing-tube is another tube, B, shorter and thicker than the former, nearly perpendicular to the body, its orifice being the exterior termination of the digestive tube. At the anus it is fringed with long hairs, having the appearance, when in the water, of a funnel. At the end of the same tube, and inside the hair funnel, are four thin, oval, transparent, scaly blades, having the appearance of fins. They are placed in pairs, of which one emanates from the right side, the other from the left.

      

      These four blades or fins have the power of separating from each other. Each segment of the abdomen has on both sides a tuft of hair, and the thorax has three. The head is round and flat, and is provided with two simple brown eyes. Round the mouth are several wattles, furnished with hair, of which two of crescent-like form are the most conspicuous. These tufts move with great quickness, causing small currents of liquid to flow into the mouth, by means of which the necessary food, microscopic insects and particles of vegetable and earthy matter, is brought to the larva.

      They change their skin several times during their continuance in this state. This latter fact has been remarked by Dom Allou, a learned Carthusian, "whose pleasure," says Réaumur, "consisted in admiring the works of the Almighty, when not occupied in singing his praises." We think it will be interesting to repeat the few lines which accompany the mention made by Réaumur of this worthy Carthusian. They appear to us to be well worth reading, even at the present day.

      "If the pious monks who composed so many societies, possessed, like Dom Allou, the love of observing insects, we might hope that the most essential facts in the history of those little creatures would soon be made known to us. What enjoyment more worthy of the calling they have chosen could these pious men pursue than that which would place before their eyes the marvellous creations of an Almighty Power? Even their leisure would then incline them to adore that Power, and would furnish them the means to make others do so who are occupied by too serious or too frivolous employments."

Fig. 29. Pupa of the Gnat.

      After having changed its skin three times in a fortnight or three weeks, the larva of the gnat throws off its covering for a fourth time, and is no longer in the larva state. It is changed both in shape and condition. Instead of being oblong, its body is shortened, rounded, and bent in such a way that the tail is applied to the under part of the head. This is the case when the animal is in repose; but it is able to move and swim, and then, by bending its body and straightening it again, propels itself through the water.

      In this new condition, that is to say, in the pupa state (Fig. 29), it does not eat. It no longer possesses digestive organs, but it is necessary, even more than before its metamorphosis, that it should breathe atmospheric air. Besides, the organs of respiration are greatly changed. During the time the insect was in the larva state, it was through the long tube fixed to the posterior part that it received or expelled the air; but in casting its skin it loses the tube, two appendages resembling an ass's ears being for the pupa what the tube was for the larva, the opening of these ears being held above the surface of the water. From this pupa the perfect insect will emerge; it is developed little by little, and the principal members may be distinguished under the transparent membranous skin which envelopes it.

      When the insect is about to change from the pupa state, it lies on the surface of the water, straightening the hind part of its body, and extending itself on the surface of the water, above which the thorax is raised. Before it has been a moment in this position, its skin splits between the two breathing trumpets, the split increasing very rapidly in length and breadth.

      "It leaves uncovered," says Réaumur, "a portion of the thorax of the gnat, easily to be recognised by the freshness of its colour, which is green, and different from the skin in which it was before enveloped.

      "As soon is the split is enlarged—and to do so sufficiently is the work of a moment—the fore part of the perfect insect is not long in showing itself; and soon afterwards the head appears, rising above the edges of the opening. But this moment, and those which follow, until the gnat has entirely left its covering, are most critical, and when it is exposed to fearful danger. This insect, which lately lived in the water, is suddenly in a position in which it has nothing to fear so much as water. If it were upset on the water, and the water were to touch its thorax or body, it would be fatal. This is the way in which it acts in this critical position—As soon as it has got out its head and thorax, it lifts them as high as it is able above the opening through which they had emerged, and then draws the posterior part of its body through the same opening; or rather that part pushes itself forward by contracting a little and then lengthening again, the roughness of the covering from which it desires to extricate itself serving as an assistance.

       Fig. 30.—Gnats emerging.

      "A larger portion of the gnat is СКАЧАТЬ