OCR Text |
Show 230 lNSECTA. the term thorax, or corselet by the French. It is composed of three segments, not well distinguished at first, the relative proportions of which vary considerably· Sometimes, as in the Coleoptera, the anterior, much the largest, separated from the following one by an articulation, movable, and alone exposed, appears at the first glance to constitute the entit·e trunk, and is called the thorax or corselet; sometimes, as in the Hymenopte. ra, Lepidoptera, &c., it is much shorter than the ensuing one, has the appearance of a coilar, and, with the two others, forms a common body, attached to the abdomen by a pedicle, or adhering closely to it across its whole posterior width, and is also called thorax. These distinctions were insufficient, and freqnently ambiguous, inasmuch as they were not based on a ternary division, distinctly announced by me in the first edition of this work, as a character proper to the Hexapoda. M. Kirby having already employed the denomination of me. tathm·ax, to designate the after-thorax(!), that of prothorax and mcsotlwrax, the ternary division once established, natu· raily presented itself to the mind, and the celebrated professor Nitzsch was the first to employ it. Some naturalists have since designated the prothorax or anterior segment, that which bears the two first feet, by the term collar, collare. Wishing to retain the denomination of corselet, but to restrain -~----~--- (1) This segment should not be restricted, in the Hymenoptera, to this superior, very short, and tr·ansverse division of the thorax, on the sides of which the second wings are inserted. It is also formed of that portion of the thorax which extends backwards to the origin of the abdomen, a circumstance which evidently demon· strates the position of the two b.st stigmata of the trunk, they being placed on the sides of this extremity, behind the wings, and above the last pair of legs. I am even of the opinion that this observation will apply to all winged Insects. Their meta thorax should be divided, at least above, into two parts or semi-segments, one, in the Tetraptcra, bearing the second wings and destitute of stigmata, and the other furnished with them; sometimes this b.tter portion, as in nearly all Insects, the Hymenoptera with a pediculated abdomen, the Uhipiptera and Diptera ex· cepted, appears to belong to the abdomen; sometimes it is incorporated with the trunk or thorax nnd closes it posteriorly, as in those last mentioned. In the Or· thoptera, Hymenoptera, Lepidoptera and I>iptera, the two anterior or thoracic segments are placed between the prothorax and the mesothorax. The abdomen will then consist of nine complete segments, the three last of which compose the or·gans of generation. I:XSECTA. 231 its application within pro~er limits, w.e will employ that term in all those cases where this segment IS much larger than the others, and where these latter are joined to the abdomen, and seem to constitute an integral part of it-a disposition proper to the Co1eoptera, Orthoptera, and several of the Hemiptera. When the prothorax is short, and forms with the succeeding segments a common and exposed mass, the trunk composed of the three wil1 retain the name of thorax. We will also con~ tinueto style pectus the inferior surface of the trunk, dividing it according to the segments, into three arere, the ante-pectus, medio·jJectus, and post-jJectus. The median line will c.lso constitute the sternum, which we divide into three parts: the ante-sternum, media-sternum, and post-sternum. The teguments of the thoracic segments, as welJ as of those of the abdomen, are usually divided into two annuli or semi~ annu1i, the one dorsal or superior, the other inferior, laterally united by a soft and flexible membrane, which, however, is but a portion of the same tegument that in many Insects, the Coleoptera particularly, is less firm. At the point of junction between these annuli we observe a little space of a more solid texture, or of the consistence of the annulus itself, which bears a stigma, so that the sides of the abdomen present a Ion~ ~tudinal series of small pieces, or each segment seems to be quadripartite. Other equally corneous pieces occupy the inferior sides of the meso thorax and meta thorax and immediate]y under the origin of the elytra and wings, which are supported by another longitudinal piece. T'he relations of these parts, the size and form of the first joint of the coxre, the manner in which they are articulated with the semi-annulus to which they belong, the extent and direction of that semi-annulus varying, furnish the thorax, thus considered, with a com hination of characters, which in a systematic point of view are of great importance. Some naturalists, Knoch in particular, had already employed them, but on no fixed principle, and under arbitrary denominations. A necessary preliminary step was the careful and comparative study of the thorax, as it exists in all the orders of the class of Insects. This was |