The Insect World. Figuier Louis

Чтение книги онлайн.

СКАЧАТЬ functions are most actively employed by them. In fact, if one compares the oxygen they imbibe with the heavy organic matter of which their body is composed, the amount is enormous.

      Before finishing this rapid examination of the body of an insect, we shall have to say a few words on the nervous system.

      This system is chiefly composed of a double series of ganglions, or collections of nerves, which are united together by longitudinal cords. The number of these ganglions corresponds with that of the segments. Sometimes they are at equal distances, and extend in a chain from one end of the body to the other; at others they are many of them close together, so as to form a single mass.

      The cephalic ganglions are two in number; they have been described by anatomists under the name of brain. "This expression," says M. Lacordaire, "would be apt to mislead the reader, as it would induce him to suppose the existence of a concentration of faculties to control the feelings and excite the movements, which is not the case." [4] The same naturalist observes, "All the ganglions of the ventral chain are endowed with nearly the same properties, and represent each other uniformly."

      The ganglion situated above the œsophagus gives rise to the optic nerves, which are the most considerable of all those of the body, and to the nerves of the antennæ. The ganglion beneath the œsophagus provides the nerves of the mandibles, of the jaws, and of the lower lip. The three pairs of ganglions which follow those placed immediately below the œsophagus, belong to the three segments of the thorax, and give rise to the nerves of the feet and wings. They are in general more voluminous than the following pairs, which occupy the abdomen.

      Fig. 15 represents the nervous system of the Carabus auratus: A is the cephalic ganglion; B, the sub-œsophagian ganglion; C, the prothoracic ganglion; D and E are the ganglions of the mesothorax and metathorax. The remainder, F F, are the abdominal ganglions.

      

Fig. 15.—Nervous system of Carabus auratus.

      Before finishing these preliminary observations, it is necessary to say that the preceding remarks only apply absolutely to insects arrived at the perfect state. It is important to make this remark, as insects, before arriving at that state, pass through various other stages. These stages are often so different from each other, that it would be difficult to imagine that they are only modifications of the same animal; one would suppose that they were as many different kinds of animals, if there was not abundant proof of the contrary.

      The successive stages through which an insect passes are four in number:—the egg; the larva; the pupa, nymph, or chrysalis; and the perfect insect, or imago.

      

      The egg state, which is common to them, as to all other articulate animals, it is unnecessary to explain. Nearly all insects lay eggs, though some few are viviparous. There often exists in the extremity of the abdomen of the female a peculiar organ, called the ovipositor, which is destined to make holes for the reception of the eggs. By a wonderful instinct the mother always lays her eggs in a place where her young, on being hatched, can find an abundance of nutritious substances. It will not be needless to observe that in most cases, these aliments are quite different to those which the mother seeks for herself.

      In the second stage, that is to say, on leaving the egg—the larva period—the insect presents itself in a soft state, without wings, and resembles a worm. In ordinary language, it is nearly always called a worm, or grub, and in certain cases, a caterpillar.

      Linnæus was the first to use the term "larva"—taken from the Latin word larva, "a mask"—as he considered that, in this form, the insect was as it were masked. During this period of its life the insect eats voraciously, and often changes its skin. At a certain period it ceases to eat, retires to some hidden spot, and, after changing its skin for the last time, enters the third stage of its existence, and becomes a chrysalis. In this state it resembles a mummy enveloped in bandages, or a child in its swaddling clothes. It is generally incapable of either moving or nourishing itself. It continues so for days, weeks, months, and sometimes even for years.

      While the insect is thus apparently dead, a slow but certain change is going on in the interior of its body. A marvellous work, though not visible outside, is being effected, for the different organs of the insect are developing by degrees under the covering which surrounds them. When their formation is complete, the insect disengages itself from the narrow prison in which it was enclosed, and makes its appearance, provided with wings, and capable of propagating its kind; in short, of enjoying all the faculties which Nature has accorded to its species. It has thrown off the mask; the larva and pupa has disappeared, and given place to the perfect insect.

      To show the reader the four states through which the insect passes in succession, in Fig. 16 is represented the insect known as the Hydrophilus, [5] firstly, in the egg state; secondly, as the larva, or caterpillar; thirdly in the pupa; and fourthly as the perfect insect or imago. The different degrees of transformation and evolution which we have just described, are those which take place either completely or incompletely in all insects. Their metamorphoses are then at an end. There are certain insects, however, that show no difference in their various stages, except by absence of wings in the larva; and in these the chrysalis is only characterised by the growth of the wings, which, at first folded back and hidden under the skin, afterwards become free, but are not wholly developed till the last skin is cast. These insects are said to undergo incomplete metamorphoses, the former complete metamorphoses. Some never possess wings; indeed, there are others which undergo no metamorphosis, and are born possessed of all the organs with which it is necessary they should be provided.

Fig. 16.—Hydrophilus in its four states. A, eggs; B, larva; C, pupa; D, imago, or perfect insect.

      Some curious researches have been lately made on the strength of insects. M. Felix Plateau, of Brussels, has published some observations on this point, which we think of sufficient interest to reproduce here.

      

      In order to measure the muscular strength of man, or of animals—as the horse, for instance—many different dynamometric apparatuses have been invented, composed of springs, or systems of unequal levers. The Turks' heads which are seen at fairs, or in the Champs Élysées, at Paris, and on which the person who wishes to try his strength gives a strong blow with his fist, represent a dynamometer of this kind. The one which Buffon had constructed by Régnier the mechanician, and which is known by the name of Régnier's Dynamometer, is much more precise. It consists of an oval spring, of which the two ends approach each other; when they are pulled in opposite directions, a needle, which works on a dial marked with figures, indicates the force exercised on the spring. It has been proved, with this instrument, that the muscular effort of a man pulling with both hands is about 124 lbs., and that of a woman only 74 lbs. The ordinary effort of strength of a man in lifting a weight is 292 lbs.; and a horse, in pulling, shows a strength of 675 lbs.; a man, under the same circumstances, exhibiting a strength of 90 lbs.

      Physiologists have not as yet given their attention to the strength of invertebrate animals. It is, relatively speaking, immense. Many people have observed how out of proportion a jump of a flea is to its size. A flea is not more than an eighth of an inch in length, and it jumps a yard; in proportion, a lion ought to jump two-thirds of a mile. Pliny shows, in his "Natural History," that the weights carried by ants appear exceedingly great when they are compared with the size of these indefatigable labourers. The strength of these insects is still more striking, when one considers the edifices they are able to construct, and the devastations СКАЧАТЬ