[[File:Spider-characteristics.png|thumb|right|180px|Arachnid anatomy:
(1) four pairs of
legs
(2) prosoma (cephalothorax)
(3) opisthosoma (abdomen)]]
The
opisthosoma {3} is the posterior part of the body in some
, behind the
prosoma (
cephalothorax).
It is a distinctive feature of the subphylum
Chelicerata (
,
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.
Segments
The number of segments and appendages on the opisthosoma vary.
have 13, but the first is only seen during its embryological development. Other arachnids have fewer;
Opiliones, for instance, have only ten.
In general, appendages are absent or reduced, although in
they persist as large plate-like limbs, called opercula or branchiophores, bearing the
, and that function in locomotion and gas exchange. In most
Chelicerata the opisthosomal limbs are greatly reduced and persist only as specialized structures, such as the silk-producing spinnerets of
or the pectines of scorpions. In animals like
and
the first two 'sternites' bearing the
may actually be highly modified opisthosomal limbs.
Segmentation of the opisthosoma in adult spiders is not visible, but embryo spiders typically have 13 segments, the posterior segments being called the presegmental zone.
