The more important of the special characters alluded to above {181}as being possessed by some but not by all members of the Order are (1) a remarkable leaping apparatus, consisting of two elongate processes at the under side of the termination of the body; (2) a peculiar ventral tube, usually seen in the condition of a papilla with invaginated summit, and placed on the first abdominal segment (see Fig. 100, p. 194); (3) the scales covering the body; (4) the existence of abdominal appendages in the form of long cerci or processes at the termination of the body, or of short processes on the sides of the under surface of the abdominal segments. Throughout the Order the general shape approximates to that of a larva; this is shown by the diagrammatic section of the body of _Machilis_ (Fig. 90). There is a succession of rings differing little from one another, except so far as the head is concerned; even the division of thorax from abdomen is but little evident, and although in some of the forms the three thoracic segments may differ considerably among themselves, yet they never assume the consolidated form that they do to a greater or less extent in the imago stage of the other Orders. Fig. 90 shows the larva-like structure of the body, and also exhibits the inequalities in size between some of the dorsal and the corresponding ventral plates. This phenomenon is here displayed only to a small extent, so that the true relations of the dorsal and ventral plates can be readily detected; but in the higher Insects want of correspondence of this kind may be much more extensive. [Illustration: FIG. 90.--Section of body of _Machilis_: _o_, ovipositor. (After Oudemans.)] The respiratory system is in many of these Insects very inferior in development, and may even be, so far as tracheae and spiracles are concerned, entirely absent, but in other members of the Aptera it is well developed. In the other internal organs there is also great variety, as there is in the external structure.