—— Zelle und Gewebe, neue Beiträge zur Histologie des Tier-Körpers. Bonn, 1885, pp. 219, 6 Taf. =Walter.= Mikroscopische Studien über das Centralnervensystem wirbelloser Thiere. 1863. =Dietl, M. J.= Die Gewebselemente des Centralnervensystems bei wirbellosen Thieren. (Aus den Berichten des naturw.-medic. Vereins in Innsbruck.) Innsbruck, 1878. =Berger.= Untersuchungen über den Bau des Gehirns und der Retina der Arthropoden. (Arbeiten des zool. Instituts zu Wien, Heft 2, p. 173, 1878.) —— Nachtrag zu den Untersuchungen über den Bau des Gehirns und der Retina der Arthropoden. (Ibid., Heft 3.) =Viallanes, H.= Recherches sur l’histologie des insectes, etc. Paris, 1882. (Annales des Sciences nat., pp. 1–348, Pls. 1–18.) —— Sur la structure de la substance ponctuée des insectes. Paris, 1885. =Haller, B.= Ueber die sogenannte Leydig’sche Punktsubstantz im Centralnervensystem. (Morp. Jahrb., xi, 1886.) =Nansen, F.= The structure and combination of the histological elements of the central nervous system. (Bergen’s Museum Aarsberetning for 1886. Bergen, 1887.) Also the writings of Benedicenti, Holmgren. THE SENSORY ORGANS _a._ The eyes and insect vision [Illustration: FIG. 259.—Different forms of compound eyes. _A_, a bug (Pyrrhocoris). _B_, worker bee. _C_, drone. _D_, male Bibio, a holoptic insect.—From Judeich and Nitsche. ] Of the eyes of insects there are two kinds, the simple and the compound. Of the former there are usually three, arranged in a triangle near the top of the head, between the compound eyes (Fig. 259, _B_). The compound or facetted eyes, which are usually round and prominent, differ much in size and in the number of facets. The number of facets varies from 12 in Lepisma,—though in a Brazilian beetle (Lathridius) there are only seven unequal facets,—to 50 in the ant, and up to 4000 in the house-fly, 12,000 in _Acherontia atropos_, 17,000 in Papilio, 20,000 in the dragon-fly (Æschna), 25,000 in a beetle (Mordella), while in _Sphinx convolvuli_, the number reaches 27,000. The size of the facets seems to bear some relation to that of the insect, but even in the smallest species none have been observed less than 1⁄2000 of an inch in diameter. Day-flying Lepidoptera have smaller facets than moths (Lubbock). [Illustration: FIG. 260.—Section through the ocellus of a young Dyticus larva: _ct_, cuticula; _l_, corneal lens; _gh_, cells of the vitreous body, being modified hypodermal cells (_hy_); _st_, rods; _re_, retinal cells; _no_, optic nerve.—After Grenacher, from Lang. ]