Opisthosoma

The opisthosoma is the posterior part of the body in some arthropods, behind the prosoma (cephalothorax).

[1] It is a distinctive feature of the subphylum Chelicerata (arachnids, horseshoe crabs and others).

Although it is similar in most respects to an abdomen (and is often referred to as such), the opisthosoma is differentiated by its inclusion of the respiratory organs (book lungs or book gills) and the heart.

[2] In general, appendages are absent or reduced, although in horseshoe crabs they persist as large plate-like limbs, called opercula or branchiophores, bearing the book gills, and that function in locomotion and gas exchange.

In most chelicerates the opisthosomal limbs are greatly reduced and persist only as specialized structures, such as the silk-producing spinnerets of spiders or the pectines of scorpions.