Evolutionary transformation from muscular to hydraulic movements in spider (Arachnida, Araneae) genitalia: A study based on histological serial sections

The male genitalia of 107 spider species representing 73 families were serially sectioned and studied with an emphasis on muscles moving the genital bulb. As a rule, most non-Entelegynae have two bulbal muscles, most Entelegyne have none, but many exceptions occur. Variation also occurs with regard to origin and attachment of bulbal muscles. There appears to be a trend towards a shift of the origin from proximal (Liphistius, Atypus) to more distal palpal segments (Haplogynae). In most Entelegynae the muscular movement is replaced by hydraulic movement caused by expanding membranes (hematodochae). Hematodochae probably permit increased bulbal rotation and movements of higher complexity. New evidence is presented arguing against Palpimanidae being representatives of Entelegynae. Bulbal glands other than those discharging into the sperm duct (previously known in Amaurobiidae and Dictynidae only) are described in several entelegyne families.