Opisthosoma

Opisthosoma, from Greek opistho meaning behind and soma meaning body, is the posterior body tagma of many arachnids, especially spiders and scorpions. It is the rear portion of the body, opposite the prosoma (cephalothorax). In mites and ticks, usage varies; the posterior region is often fused with the prosoma to form the idiosoma.

In spiders, the opisthosoma is the abdomen. It contains parts of the digestive and reproductive systems, and hosts silk glands whose ducts lead to spinnerets at the posterior end. It connects to the prosoma by a narrow pedicel, allowing some movement and growth without disturbing the prosoma. The abdomen is segmentally structured and can expand during feeding.

In scorpions, the opisthosoma is subdivided into the mesosoma (preabdomen) and metasoma (postabdomen or tail); the metasomal segments carry the stinger at the tip. The opisthosoma houses internal organs and the reproductive tract; the metasoma is a flexible, elongated segment used for locomotion and stinging.

In acarology, many mites and ticks have an idiosoma—an opisthosoma-like posterior section whose dorsal shield covers both the abdomen and much of the body when fused. In these groups, the boundary between prosoma and opisthosoma is not always distinct.

The term opisthosoma is chiefly used in arachnology to describe the posterior tagma and is less common in other arthropod groups. Understanding its structure aids in studying anatomy, physiology, and taxonomy of spiders, scorpions, and related orders.