196 , MB. A. 1). MICHAEL ON AN [Mar. 5, 



female would appear to be that in the OribatidsB, 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 Hydrachnidae, 

 and are greatly developed in the genus Hych-odrorna. They were 

 noticed by Neuman ' and Haller '\ 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 membi'anous 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, pro- 

 trusible ; in each case the plates are excavated to allow these 

 organs 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 mm. 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 



'^ " Om Sveriges Hydrachnider," Stockholm, 1880, Kongl. Svensk. Vetenskaps- 

 Akad. Handling., Band. 17, No. 3, p, 112. 



^ "Die Hydrachniden der Schweiz," Mittheil d. Sern. naturf. Gtesellsch. 

 1881, Heft ii. p. 48 (1882). 



