The Insect World. Figuier Louis

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СКАЧАТЬ 6), the second primary division of the body of insects, plays almost as important a part as the head. It consists of three segments or rings, which are in general joined together—the prothorax, the mesothorax, and the metathorax, each of which bears a pair of legs. The wings are attached to the two posterior segments.

      All insects have six true legs. There is no exception whatever to this rule, though some may not be developed.

      From the segments to which they are attached, the legs are called anterior, posterior, and intermediate. The legs are composed of four parts: the trochanter, a short joint which unites the thigh to the body; the thigh or femur; the tibia, answering to the shank in animals; and the tarsus, or foot, composed of a variable number of pieces placed end to end, and called the phalanges.

      We take as examples the hind leg of a Heterocerus (Fig. 7), and the front leg of a Zophosis (Fig. 8) (genera of beetles).

      

      We shall not dwell on the different parts, as they perform functions which will occupy us later, when speaking of the various species of the great class of insects.

Fig. 7. Hind leg of a Heterocerus.		Fig. 8. Front leg of a Zophosis.
Fig. 9.—Posterior leg of a jumping insect.

      The functions which the legs of insects have to perform consist in walking, swimming, or jumping.

      In walking, says M. Lacordaire, insects move their legs in different ways. Some move their six legs successively, or only two or three at a time without distinction, but never both legs of the same pair together, consequently one step is not the same as another. The walk of insects is sometimes very irregular, especially when the legs are long; and they often hop rather than walk. Others have one kind of step, and walk very regularly. They commence by moving the posterior and anterior legs on the same side and the intermediate ones on the opposite side. The first step made, these legs are put down, and the others raised in their turn to make a second.

      Running does not change the order of the movements, it only makes them quicker—very rapid in some species, and surpassing in proportion that of all other animals; but in others the pace is slow. Some insects rather crawl than walk.

      In swimming, the posterior legs play the principal part. The other legs striking the water upwards or downwards, produce an upward or downward motion. The animal changes its course at will by using the legs on one side only, in the same way as one turns a rowing boat with one oar without the aid of a rudder. Swimming differs essentially from walking, for the foot being surrounded by a resisting medium, the legs on both sides are moved at the same time.

      The act of jumping is principally performed by the hind legs. Insects which jump have these legs very largely developed, as in Fig. 9. When about to jump they bring the tibia into contact with the thigh, which is often furnished with a groove to receive it, having on each side a row of spines. The leg then suddenly straightens like a spring, and the foot being placed firmly on the ground, sends the insect into the air, and at the same time propels forward. The jump is greater in proportion as the leg is longer.

      To treat here in a general manner of the wings of insects would be useless. We shall refer to them at length in their proper place, when treating of the various types of winged insects.

      In the perfect insect the abdomen does not carry either the wings or the legs. It is formed of nine segments, which are without appendages, with the exception of the posterior ones, which often carry small organs differing much in form and function. These are saws, probes, forceps, stings, augers, &c. We shall consider these different organs in their proper places.

      With vertebrate animals, which have an interior skeleton suited to furnish points of resistance for their various movements, the skin is a more or less soft covering, uniformly diffused over the exterior of the body, and intended only to protect it against external injury. In insects the points of resistance are changed from the interior to the exterior. The skin is altered by Nature to fit it to this purpose. It is hard, and presents between the segments only membranous intervals, which allow the hard parts to move in all directions.

      We are examining a perfect insect; we have glanced at its skeleton, and the different appendages which spring from it. The principal organs which are contained in the body remain to be examined.

      We will first study the digestive apparatus. This apparatus consists of a lengthened tubular organ, swollen at certain points, forming more or less numerous convolutions, and provided with two distinct orifices. This alimentary canal is always situated in the median line of the body, traverses its whole length, and is at first surrounded by, and then passes above, the nervous ganglia. [1]

Fig. 10.—Digestive apparatus of Carabus auratus.

      In its most complicated form the alimentary canal is composed of an œsophagus, or gullet, of a crop, of a gizzard, of a chylific ventricle or stomach, a small intestine, a large intestine, divers appendages, salivary, biliary, and urinary glands. The œsophagus is often not wider than a hair, and part of it in many species is enlarged into a pouch, which is called the crop, because it occupies the same position, and performs analogous functions with that organ in birds. It is enough to say that the food remains there some time before passing on to the other parts of the intestinal canal, and undergoes a certain amount of preparation. It is in the gizzard, when one exists, that the food, separated by the masticatory organs of the mouth, undergoes another and more complete grinding. Its structure is suited to its office. It is, in fact, very muscular, often half cartilaginous, and strongly contractile. Its interior walls are provided with a grinding apparatus, which varies according to the species, and consists of teeth, plates, spines, and notches, which convert the food into pulp. It only exists among insects which live on solid matters, hard vegetables, small animals, tough skin, &c. This apparatus is absent in sucking insects and those which live on soft substances, such as the pollen of flowers, &c.

      The chylific ventricle or stomach is never absent; it is the organ which performs the principal part in the act of digestion.

      Two kinds of appendages belong to the chylific ventricle, but only in certain families. The first are papillæ, in the form of the fingers of a glove, which bristle over the exterior of this organ, and in which it is believed that the food begins to be converted into chyle. The second are cæca, and larger and less numerous.

      They have been considered as secretory organs, answering to the pancreas in vertebrate animals.

      

      Fig. 10, which СКАЧАТЬ