Auditory organs are best developed in such Insects as produce sounds as a call to each other. The Cockroach is dumb, and it is, therefore, not a matter of surprise that no structure which can be considered auditory should have ever been detected in this Insect.[121] [121] For a popular account of auditory organs in Insects, see Graber’s Insekten, Vol. I., page 287; also J. Müller, Vergl. Phys. d. Gesichssinn, p. 439; Siebold, Arch. f. Naturg., 1844; Leydig, Müller’s Arch. 1855 and 1860; Hensen, Zeits. f. wiss. Zool., 1866; Graber, Denkschr. der Akad. der wiss. Wien, 1875; and Schmidt, Arch. f. mikr. Anat., 1875. The sensory hairs of the skin have been already noticed (p. 31). CHAPTER VII. THE ALIMENTARY CANAL AND ITS APPENDAGES. _SPECIAL REFERENCES._ CHOLODKOWSKY. Zur Frage über den Bau und über die Innervation der Speicheldrüsen der Blattiden. Horæ Soc. Entom. Rossicæ, Tom. XVI. (1881). [Salivary Glands of Cockroaches.] SCHINDLER. Beiträge zur Kenntniss der Malpighi’schen Gefässe der Insekten. Zeits. f. wiss. Zool., Bd. XXX. (1878). [Malpighian Tubules of Insects.] CHUN. Ueber den Bau, die Entwickelung, und physiologische Bedeutung der Rectaldrüsen bei den Insekten. Abh. der Senkenbergischen Naturforschers Gesellschaft, Bd. X. (1876). [Rectal Glands of Insects.] LEYDIG. Lehrbuch der Histologie, &c., and VIALLANES. (_Loc. cit._ _supra_, chap. iv.) [Histology of Alimentary Canal.] BASCH. Untersuchungen über das Chylopoëtische und Uropoëtische System der Blatta orientalis. Kais. Akad. der Wissenschaften. (Math-Nat. Classe.), Bd. XXXIII. (1858). [Digestive and Excretory Organs of Blatta.] SIRODOT. Recherches sur les Sécrétions chez les Insectes. Ann. Sci. Nat., 4^e Série, Zool., Tom. X. (1859). [Digestive and Excretory Organs of Oryctes, &c.] JOUSSET DE BELLESME. Recherches expérimentales sur la digestion des Insectes et en particulier de la Blatte (1875). PLATEAU. Recherches sur les Phénomènes de la Digestion chez les Insectes. Mem. de l’Acad. Roy. de Belgique, Tom. XLI. (1874). [Now the principal authority on the Digestion of Insects. The other physiological memoirs cited (Nos. 5, 6, 7) are chiefly of historical interest.] PLATEAU. Note additionelle. Bull. Acad. Roy. de Belgique, 2^e Sér., Tom. XLIV. (1877). [Contains some corrections of importance.] _The Alimentary Canal._ The alimentary canal of the Cockroach measures about 2-3/4 inches in length, and is therefore about 2-3/4 times the length of the body. In herbivorous Insects the relative length of the alimentary canal may be much greater than this; it is five times the length of the body in Hydrophilus. Parts of the canal are specialised for different digestive offices, and their order and relative size are given in the following table:--