The evolutionary transformation from muscular to hydraulic movements in spider genitalia: A study based on histological serial sections

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The male genitalia of many "higher" spiders (Entelegynae) are intricate and complex structures that perform various types of movements (rotation, tilt, twist) before and during intromission. One of the most extraordinary features is that these movements are not effected by muscles but by hydraulic pressure causing membranes (hematodochae) to unfold in a way determined by the folding pattern itself. In "primitive" spiders, however, movements are caused by a pair of muscles that are thought to be homologous to the muscles that move the claws in walking legs. A preliminary study has shown that neither the distribution of characters nor the character states are that simple: muscles occur also in certain entelegyne spiders, and instead of two character states (pair of muscles versus hydraulics), there are up to five, with substantial variation in the number and attachment sites of muscles. This study aims at a comprehensive treatment of this aspect of spider genital morphology as a basis for an unterstanding of how and why spiders have given up on musles and evolved hydraulic movement instead.