The ingenious hypothesis propounded by Gegenbaur, that the tracheal tubes of Insects were originally adapted to aquatic respiration, and that the stigmata arose as the scars of disused tracheal gills, has been discussed in chap. iv. Semper has suggested[162] that tracheæ may be modified segmental organs, but the most probable view of their origin is that put forth by Moseley,[163] that they arose as ramified cutaneous glands. In _Peripatus_ the openings are distributed irregularly over the body; the external orifices lead to pits, from which simple tubes, with but slight spiral markings, extend into the deeper tissues. [162] Arbeiten a. d. Zool. Zoot. Inst. Würzburg. Bd. II., 1874. [163] Phil. Trans., 1874, p. 757. CHAPTER IX. REPRODUCTION. _SPECIAL REFERENCES._ BRANDT, A. Ueber die Eiröhren der Blatta (Periplaneta) orientalis. Mem. Acad. St. Petersb. Ser. 7, Vol. XXI. (1874). [Ovarian Tubes of Cockroach.] LACAZE-DUTHIERS. Rech. sur l’armure génitale femelle des Insectes Orthoptères. Ann. Sci. Nat., Zool., 3^e Sér., Tom. XVII. (1852). [External reproductive organs of female Orthoptera.] BERLESE. Ricerde sugli organi genitali degli Ortotteri. Atti della R. Acad. dei Lincei. Ser. 3, Vol. XI. (1882). [Genital Organs of European Orthoptera.] KADYI. Beitr. zur Vorgänge beim. Eierlegen der Blatta Orientalis. Vorläufige Mittheilung. Zool. Anz., 1879, p. 632. [Formation of egg-capsules of Cockroach.] BREHM. Comparative structure of the reproductive organs in Blatta germanica and Periplaneta orientalis. Mem. Soc. Ent. St. Petersb., Tom. VIII. (1880). In Russian. [Male organs only.] RAJEWSKY. Ueber die Geschlechtsorgane von Blatta orientalis, &c. Nachr. d. kais. Gesellsch. d. Moskauer Universität., Bd. XVI. (1875). [Testes of Cockroach. The original paper is in Russian; an abstract is given in Hofmann and Schwalbe’s Jahresbericht, 1875, p. 425.] BÜTSCHLI. Bau u. Entwickelung d. Samenfäden bei Insekten u. Crustaceen. Zeits. f. wiss. Zool., Bd. XXI., pp. 402–414; 526–534. Pl. xl. xli. (1871). [Spermatozoa and spermatogenesis in the Cockroach.] LA VALETTE ST. GEORGE. Spermatologische Beiträge, II. _Blatta germanica._ Arch. f. mikr. Anat., Bd. XXVII. (1886). [Spermatogenesis in _B. germanica_.] MORAVITZ. Quædam ad anat. Blattæ germanicæ pertinentia. Dissertatio inauguralis. Dorpat. (1853). [An excellent early account of the anatomy of _B. germanica_, including a description of the male and female organs. The figures are not trustworthy.] _Female Reproductive Organs._