[113] Exner has since determined by measurement and calculation the optical properties of the eye of Hydrophilus. He finds that the focus of a corneal lens is about 3mm. away, and altogether behind the eye.
[114] Zur vergl. Phys. des Gesichtsinnes.
[115] A critical history of the whole discussion is to be found in Grenacher’s “Sehorgan der Arthropoden” (1879), from which we take many historical and structural details.
[116] Flies, whose eyes are in several respects exceptional, have almost completely separated rods, notwithstanding their quick sight.
[117] Bull. de l’Acad. Roy. de Belgique, 1885.
[118] References to the literature of the question are given by Hauser in Zeits. f. wiss. Zool., Bd. XXXIV., and by Plateau in Bull. Soc. Zool. de France, Tom. X.
[119] Zeits. f. wiss. Zool., 1885.
[120] Will confirms, by his own experiments (p. 685), Plateau’s conclusion (Supra, p. [46]), that the maxillary and labial palps have nothing to do with the choice of food.
[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.
[122] Here, as generally in the digestive tube of the adult Cockroach, the peritoneal layer is inconspicuous or wanting. It occasionally becomes visible—e.g., in the outer wall of the Malpighian tubules, and in the tubular prolongation of the gizzard.