opisthosome

Opisthosoma is the posterior body region in many arachnids, comprising the part behind the prosoma (the cephalothorax). The term comes from Greek, with opistēs meaning behind and sōma meaning body. In most arachnids, the body is divided into a prosoma and an opisthosoma, which are connected by a narrow pedicel.

In spiders, the opisthosoma corresponds to the abdomen and contains most of the digestive, reproductive, and excretory organs. The posterior end bears the spinnerets, which produce silk. The structure and segmentation of the opisthosoma can vary among spider groups, but its overall function centers on housing internal organs and silk production.

In scorpions, the opisthosoma is divided into the mesosoma (preabdomen) and the metasoma (postabdomen or tail), with the telson at the end forming the venomous stinger. This division reflects differences in sensory and reproductive anatomy and is a common basis for taxonomic distinctions within the group.

Among harvestmen (Opiliones) and many mites and ticks (Acari), the term is used for the posterior body region as well. In some Acari, the opisthosoma is fused with the main body into a single idiosoma that contains most visceral organs, although the term opisthosoma may still be encountered in descriptive texts.

Overall, the opisthosoma denotes the rear portion of the body in arachnids, contrasting with the prosoma, and its specific morphology and contents vary across different orders. See also prosoma, pedicel, mesosoma, metasoma, and idiosoma.