[143] Newport, in Todd’s Cyclopædia of Anatomy and Physiology, Art. Insecta, pp. 981–2.
[144] Beitr. zur näheren Kenntniss von Periplaneta orientalis, p. 19.
[145] The termination of the aorta has been described by Newport, in Sphinx (Phil. Trans., 1832, Pt. I., p. 385) Vanessa, Meloe, Blaps and Timarcha. (Todd’s Cycl., Art. “Insecta,” p. 978.)
[146] Moseley, Q. J. Micr. Sci. (1871).
[147] The oldest Tracheate actually known to bear spiracles is the Silurian Scorpion of Gothland and Scotland (Scudder, in Zittel’s Palæontologie, p. 738). We need not say that this is very far removed from the primitive Tracheate which morphological theory requires. The existing Peripatus makes a nearer approach to the ideal ancestor of all Tracheates, if we suppose that all Tracheates had a common ancestor of any kind, which is not as yet beyond doubt.
[148] The longitudinal air-tubes are characteristic of the more specialised Tracheata. In Araneidæ, many Julidæ, and Peripatus each spiracle has a separate tracheal system of its own.
[149] Investigators are not yet agreed as to the minute structure of the tracheal thread. Chun (Abh. d. Senkenberg. Naturf. Gesells., Bd. X., 1876) considers it an independent chitinous formation, not a mere thickening of the intima. He describes the thread as solid. The intima itself is, he believes, divisible in the larger tubes into an inner and an outer layer, into both of which the thread is sunk. Macloskie (Amer. Nat., June, 1884) describes the spiral as a fine tubule, opening by a fissure along its length. He regards it as a hollow crenulation of the intima, and continuous therewith. Packard (Amer. Nat. Mag., May, 1886) endeavours to show that the thread is not spiral, but consists of parallel thickenings of the intima. He is unable to find proof of the tubular structure, or of the external fissure. We have specially examined the trachea of the Cockroach, and find that the thread can readily be unwound for several turns. It is truly spiral.
[150] It has been supposed that these irregular cells of the tracheal endings pass into those of the fat-body, but the latter can always be distinguished by their larger and more spherical nuclei.
[151] In the first abdominal spiracle the setæ are developed only on that lip which carries the bow.
[152] This subject is treated at greater length in Prof. Plateau’s contribution on Respiratory Movements of Insects. (Infra, p. [159].)