OCR Text |
Show 204 MB. A. D. MICHAEL ON AN [Mar. 5, is set with the point upward; they therefore form the levator muscles of the mandible. The sigmoid piece is attached, by a strong ligament (fig. 21, L.) which rises from a projection near the middle of its anterior edge, to the chitinous bridge or shelf (fig. 21, B.) on which the mandibles rest; thus the action of the fan-shaped muscles cannot pull the sigmoid piece out of position. Forming a fulcrum for the mandibles and a point of attachment for their muscles is not the only office of the sigmoid pieces; each is hollow and its interior is an air-chamber (ac.); it is lined by a thin loose membrane, which is in fact a continuation of the main tracheal trunk from the stigma ; thus the air-chamber apparently varies in shape according to the amount of air which it contains for the moment: it, however, really occupies almost the whole interior of the sigmoid piece, and is broadest about the middle; it does not, however, extend to the upper (anterior) end and it diminishes to a very fine tube in the lowTer (posterior) curve of the S and ends blindly some distance before the point of this portion of the chitin. There are two passages out of this air-chamber, one is situated in the posterior edge of the sigmoid piece near to but not at its upper (anterior) end; through this opening passes the principal tracheal trunk (fig. 21, tra.) which runs to the stigma (S.). This trunk is the only trachea in the body in which I have been able to detect any ringing, but here it is very distinct; it at first rises between the mandibles, but when it has reached their upper edge it turns forward, and runs above and parallel to the mandible on its own side for about one-fourth of the length of that organ; it then enlarges to a small bulb, pointed anteriorly, which contains the stigma (S.). The second passage (tre.) out of the air-chamber is smaller and is placed in the upper edge of the projection from the middle of the anterior edge of the sigmoid piece to which, as before stated, the ligament (L.) is attached ; a small branch of the air-chamber leads to this opening; out of which passes the efferent tracheal trunk, which shortly divides dichotomously, sending one trunk forward and one backward, which almost immediately breaks up into a multitude of extremely fine and delicate tracheae which supply the body. In addition to an arrangement practically almost similar to this Schaub describes and figures (see his Taf. iii. fig. 8, tr.) a number of very fine tracheae passing direct into the air-chamber through the chitin of the sigmoid piece, and not springing from any tracheal trunk. I cannot say what there may be in Hydrodroma, but I can say with some confidence that nothing of the kind exists in Thyas petrophilus. I have several times obtained precisely the appearance figured by Schaub; but this, in the species I am describing, has certainly arisen from the fan of muscles being torn or cut away, leaving the numerous tendons by which they were attached behind them ; these tendons part from the muscle much more readily than from the sigmoid piece. |