By means of serial sections and dissection, 537 individual muscles were found in the adult female Latrodectus mactans, and 533 in the male. The absence of muscles in the male palpal pretarsus represents the only substantial difference found between the muscular systems of the two sexes. The musculature corresponds generally to that described for other species of spiders. Among muscles found in the black widow spider, but not described in other spiders, are those of the coxal gland, the colulus, the ampullate silk duct, and the mid-gut, and some of the muscles of the pedicel, abdomen, and abdominal appendages. The muscles designated "abdominal sac" by various authors are considered to be vestiges of a laterally placed dorsoventral musculature originally joining tergites and sternites. The nature of the intrinsic musculature of the palpi supports the belief that these appendages represent modified legs. Homologies are proposed between the muscles of the pedicel and those of the abdomen, between the extrinsic muscles of the spinnerets and those of the coxae of the legs, and between the intrinsic muscles of the lateral spinnerets and those acting on the trochanter of the legs.
Southern black widow Latrodectus mactans (Fabricius) (Arachnida: Araneae: Theridiidae)
