Opisthosoma

Segments

The number of segments and appendages on the opisthosoma vary. Scorpions have 13 segments, but the first is only seen during its embryological development. Other arachnids have fewer; harvestmen, for instance, have only ten.

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. In animals like whip scorpions and whip spiders the first two 'sternites' bearing the book lungs may actually be highly modified opisthosomal limbs.

Segmentation of the opisthosoma in adult spiders is generally not visible, but embryo spiders typically have 13 segments, the posterior segments being called the presegmental zone. The exception are the Mesothelae, a small group of spiders retaining ancestral characteristics. They split from other spiders around 300-445 million years ago, and they have a clearly segmented opisthosoma even in adult stages.

References

pl:Odwłok

References

1. Peter Ax. (28 August 2000). "Multicellular Animals". [[Springer Science+Business Media.
2. (2007). "Harvestmen: the Biology of Opiliones". [[Harvard University Press]].
3. (November 2010). "Patterning mechanisms and morphological diversity of spider appendages and their importance for spider evolution". Arthropod Structure & Development.
4. "TimeTree: The Timescale of Life".