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The Arrhenuri comprise the largest genus of the Hydrachnidz, and one of the most highly differentiated and widely distributed. The genus is easily recognized by the following characters. The thick chitinous covering of the body is pierced by large pores. A circular furrow on the dorsal side marks off an area which is entirely enclosed in the female, in the male usually open onto the posterior extension of the body. The females are oval in form and much alike ; the males are smaller and vary greatly from each other and from the females. They are characterized by a posterior prolongation of the body of varying length and complexity called the appendix, at the end of which there is often developed an accessory sex organ, the petiole. The epimera are in three groups. The capitulum has the form of a shield with a wedge-shaped notch closed by a membrane. The genital cleft in the female is flanked by two large semicircular discs, from each of which extends a wing-shaped area covered with small acetabula. The male genital area, at the base of the appendix, has narrow plates forming an elliptical plate for the cleft, from which extend narrow wing-shaped areas. The palpi are short and stout, ending in a pincer formed by the fifth segment and a prolongation from the distal end of the fourth. The legs are relatively short and much alike throughout the genus ; the last three pairs have swimming-hairs, and the fourth segment of the fourth leg in the male usually has a spur bearing a bunch of hairs.