The opisthosoma is the posterior part of the body in some arthropods, behind the prosoma. It is a distinctive feature of the subphylum Chelicerata. It is similar in most respects to an abdomen.

Arthrolycosidae is an extinct family of arachnids, possibly spiders. Fossils placed in the family were found in the period spanning Carboniferous to Permian. They were considered by Alexander Petrunke