The Insect World. Figuier Louis

Чтение книги онлайн.

СКАЧАТЬ the digestive apparatus of Carabus auratus, a common beetle, presents to the eyes of the reader the different organs of which we are speaking.

Fig. 11.—Posterior extremity of the chylific ventricle, surrounded by the Malpighian vessels.

      A is the mouth of the insect, B the œsophagus, C the crop, D the gizzard, E the chylific ventricle, F and G the small and large intestines, and H the anus.

      It is not necessary to consider the other parts of the alimentary canal in insects, but only to refer to some of the appendages of this apparatus.

      The salivary glands pour into the digestive tube a liquid, generally colourless, which, from the place where it is secreted, and its alkaline nature, corresponds to the saliva in vertebrate animals. It is this liquid which comes from the tongue of sucking insects in the form of drops.

      These glands are always two in number. Their form is as variable as complicated. The most simple is that of a closed flexible tube, generally rolled into a ball, and opening on the sides of the œsophagus.

      At the posterior extremity of the chylific ventricle are inserted a variable number of fine tubes, usually elongated and flexible, and terminating in culs-de-sac at one end. Their colour, which depends on the liquid they may contain, is sometimes white, but more frequently brown, blackish, or green. They appear to be composed of a very slight and delicate membrane, as they are very easily torn, and nothing is more difficult than to unroll and to disengage them from the fatty or other tissues by which they are enveloped.

      The function of these vessels is uncertain. Cuvier and Léon Dufour supposed them to be analogous to the liver, and on that account they have been called biliary vessels; and they are often termed the Malpighian vessels, after the name of their discoverer.

      According to M. Lacordaire, their functions vary with their position. When they enter the chylific ventricle, they furnish only bile; bile and a urinary liquid when they enter the posterior part of the ventricle and the intestine; and urine alone when they are placed near the posterior extremity of the alimentary canal.

      Fig. 11 represents part of the preceding figure more highly magnified, showing the manner in which these tubes enter the chylific ventricle.

      In our rapid description of the digestive apparatus of insects, it only remains for us to mention certain purifying organs which secrete those fluids, generally blackish, caustic, or of peculiar smell, which some insects emit when they are irritated, and which cause a smarting when they get into one's eyes.

      Less well developed than the salivary organs, they are often of a very complicated structure. In Fig. 12 is represented the secretory apparatus of the Carabus auratus, which will serve for an example: A represents the secretory sacs aggregated together like a bunch of grapes, B the canal, C the pouch which receives the secretion, D the excretory duct.

Fig. 12. Secretory apparatus of Carabus auratus.

      Sometimes the secretion is liquid, and has a fœtid or ammoniacal odour; sometimes, as in the Bombardier beetle (Brachinus crepitans), it is gaseous, and is emitted, with an explosion, in the form of a whitish vapour, having a strong pungent odour analogous to that of nitric acid, and the same properties. It reddens litmus paper, and burns and reddens the skin, which after a time becomes brown, and continues so for a considerable time.

      About the middle of the seventeenth century Malpighi at Bologna, and Swammerdam at Utrecht, discovered a pulsatory organ occupying a median line of the back, which appeared to them to be a heart, in different insects. Nevertheless, Cuvier, having declared some time afterwards that there was no circula tion, properly so called, among insects, his opinion was universally adopted.

      But in 1827 a German naturalist named Carus discovered that there were real currents of blood circulating throughout the body, and returning to their point of departure. The observations of Carus were repeated and confirmed by many other naturalists, and we are thus enabled to form a sufficiently exact idea of the manner in which the blood circulates.

      The following summary of the phenomena of circulation among insects is borrowed from "Leçons sur la Physiologie et l'Anatomie comparée," by M. Milne-Edwards:—

      The tube which passes under the skin of the back of the head, and front part of the body, above the alimentary canal, has been known for a long time as the dorsal vessel. It is composed of two very distinct portions: the anterior, which is tubular and not contractile; and the posterior, which is larger, of more complicated structure, and which contracts and dilates at regular intervals.

      This latter part constitutes, then, more particularly the heart of the insect. Generally it occupies the whole length of the abdomen, and is fixed to the vault of the tegumentary skeleton by membranous expansions, in such a manner as to leave a free space around it, but shut above and below, so as to form a reservoir into which the blood pours before penetrating to the heart. This reservoir is often called the auricle, for it seems to act as an instrument of impulsion, and to drive the blood into the ventricle or heart, properly so called.

      The heart is fusiform, and is divided by numerous constrictions into chambers. These chambers have exits placed in pairs, and membranous folds which divide the cavity in the manner of a portcullis. The lips of the orifices, instead of terminating in a clean edge, penetrate into the interior of the heart in the form of the mouth-piece of a flute. The double membranous folds thus formed on each side of the dorsal vessel are in the shape of a half moon, and separate from each other when this organ dilates; but the contrary movement taking place, the passage is closed.

      By the aid of this valvular apparatus, the blood can penetrate into the heart from the pericardic chamber, the empty space surrounding the heart, but cannot flow back from the heart into that reservoir.

      The anterior or aortic portion of the dorsal vessels shows neither fan-shaped lateral expansions, nor orifices, and consists of a single membranous tube. The whole of the blood set in motion by the contractions of the cardial portion of the dorsal vessel runs into the cavity of the head, and circulates afterwards in irregular channels formed by the empty spaces left between the different organs. It is the unoccupied portions of the great visceral cavity which serve as channels for the blood, and through them run the main currents to the lateral and lower parts of the body. These currents regain the back part of the abdomen, and enter the heart after having passed over the internal organs. These principal channels are in continuity with other gaps between the muscles, or between the bundles of fibres of which these muscles are composed.

      The principal currents send into the network thus formed, minor branches, which having ramified in their turn among the principal parts of the organism, re-enter some main current to regain the dorsal vessel.

      In the transparent parts of the body the blood may be seen circulating in this way to a number of inter-organic channels, penetrating the limbs and the wings, when these appendages are not horny, and, in short, diffusing itself everywhere. "If, by means of coloured injections," says M. Milne-Edwards, "one studies the connections which exist between the cavities in which sanguineous currents have been found to exist and the rest of the economy, it is easy to see that the irrigatory system thus formed penetrates to the full depth of every organ, and should cause the rapid renewal of the nourishing fluid in all the parts where the process of vitality renders the passage of this fluid necessary."

      We shall see presently, in speaking of respiration, that the relations between СКАЧАТЬ