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Show 196 ME. A. D. MICHAEL ON AN [Mar. 5, female would appear to be that in the Oribatidae, which possess precisely similar organs, there is a long extensible ovipositor through which the egg passes ; so that it could not possibly be touched or guided by these " genital suckers." Organs which must be homologized with and which greatly resemble these so-called genital suckers exist in the Hydrachnidse, and are greatly developed in the genus Hydrodroma. They were noticed by Neuruan ' and Haller 2 The latter, although he calls the organs suckers (Haftnapfe), generally puts the word between quotation-marks, as if he did not wish to be responsible for the name. He pointed out that in this genus these suckers, although somewhat extensible, were externally convex chitinous knobs, and he gives a rough drawing of the chitinous parts. Schaub, in the work so often quoted (p. 46), describes two pairs of such organs, which he terms the " so-called suckers," and one pair of chitinous rings which he regards as true suckers. These last-named pair are on the membranous folds or labia, which are situated between the genital plates and border the genital opening; the two pairs of convex chitinous knobs are at the anterior and posterior ends of the genital plates themselves, not forming one piece with the plate, but each knob is described as capable of protrusion, and as being a hollow hemisphere with its convex side on the exterior of the body, i. e. downwards, and with a short thick bundle of striped muscles inserted into its concave or inner side, and practically filling the hemisphere. Schaub says that the position and arrangement of these muscles leaves no doubt that the organs subserve copulation. In Thyas petrophilus I find three pairs of these so-called suckers- two pairs in the labia between the genital plates, and one at the posterior angles of the plates, all slightly, but only slightly, protrusible ; in each case the plates are excavated to allow these orgaus to pass. The posterior pair are rather the largest, but in other respects all three pairs are alike ; there is not any ring-like sucker as in Schaub's species. Each so-called " sucker " (I use the expression for want of a better one) is provided exteriorly with a strong hemispherical cap of homogeneous chitin, with the convex side to the exterior, as in Schaub's species; the chitin covers the whole exterior of the organ, is about -004 m m . thick and of a light reddish colour (fig. 19, cm.). This chitinous cap is supported by an irregular-shaped band of thicker chitin of a clear yellow colour standing at right angles to the cap : this chitinous band is attached to the soft cuticle of the ventral surface of the body, and forms the margin of an opening in the cuticle which just allows the narrowest part of the organ to pass through it, both the cap and the inner part of the organ being larger than the opening. Thus in the chitinous cap and the external appearance and position the 1 " Om Sveriges Hydrachnider," Stockholm, 1880, Kongl. Svensk. Vetenskaps- Akad. Handling., Band. 17, No. 3, p. 112. 2 "Die Hydrachniden der Schweiz," Mittheil d. Bern, naturf. Gesellsch. 1881, Heft ii. p. 48 (1882). |
