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Show 444 ARTICULATA. which gives off arterial branches, all(l receives venous ones from them~ but their mode of respiration varies, some of them still having true pulmonary organs which open on the sides of the abdomen, while others receive air by trachere, like Insects. In both of them, however, we observe lateral openings or true stigmata. The INSECT A constitute the fourth class of the Articulata, and the most numerous of all the animal kingdom. With the exception of some genera, the Myriapoda, in which the body is divided into numerous and nearly equal parts, it is always divided into three portions: the head, furnished with the antennre, eyes and mouth; the thorax, to which are appended the feet and wings, when they exist; and the abdomen, which is suspended behind the thorax and contains the principal viscera. Those which have wings, only receive them at a certain age, and frequently pass through two more or less difl,'erent forms before they assume that of the winged insect. In all their states they respire by trachere ; that is, by elastic vessels which receive air through stigmata pierced on their sides, and distribute it by infinite ramifications to every part of the body. A vestige of a heart only is perceptible, consisting of a dorsal vessel which experiences an alternate contraction and dilatation, but to which no branch has ever been discovered, so that we are forced to believe that nutrition is effected in this class of animals by im bihition. It is, probably, this sort of nutrition which necessitated the kind of respiration proper to insects; for as the nutritive fluid is not contained in vessels(l ), and could not be directed towards pulmonary organs in search of air, it was requisite that this air should be diffused throughout the body to reach the fluid. This is also the reason why insects have no secretory glands, but are provided with mere spongy vessels, which, by the extent of their surface, appear (1) M. Carus has observed regular movements in the fluid which fills the bodies of certain larvre of Insects; but this movement does not take place in a. system of closed vessels, as in the superior animals. See his treatise entitled "Discovery of a simple circulation oftlte blood, &c." in German, Leipzig, 1827, 4to. ARTICULATA. 445 to absorb the peculiar juices they are to produce, from the mass of the nutritive fluid( I). Insects vary infini~ely as to the form of the organs of the mouth, and those of digestion, as well as in their industry and mode of life ; the sexes are always separated. The Crustacea and Arachnides were long united with the In~ecta unde~ one common name, and resemble them in many pomts of their external form, in the disposition of their organs of motion, and of the sensations, and ev.en in those of manducation. {1) On this subject see my Memoir on the nutrition of Insects, printed 1799, Mem.. de la. Soc. d'Hist. Nat. de Paris. Baudouin, an vii, 4to, p. 32. |