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Bauplan of a malacostracan; the cephalothorax is the region including cephalon and thorax, marked in yellow.

The cephalothorax, also called prosoma in some groups, is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind.[1] (The terms prosoma and opisthosoma are equivalent to cephalothorax and abdomen in some groups. The terms prosoma and opisthosoma may be preferred by some researchers in cases such as arachnids, where there is neither fossil nor embryonic evidence animals in this class have ever had separate heads and thoraxes, and where the opisthosoma contains organs atypical of a true abdomen, such as a heart and respiratory organs[2]). The word cephalothorax is derived from the Greek words for head (κεφαλή, kephalé) and thorax (θώραξ, thórax).[3] This fusion of the head and thorax is seen in chelicerates and crustaceans; in other groups, such as the Hexapoda (including insects), the head remains free of the thorax.[1] In horseshoe crabs and many crustaceans, a hard shell called the carapace covers the cephalothorax.[4]

Arachnid anatomy[edit]


The fovea is the centre of the cephalothorax and is located behind the head (only in spiders).[5] It is often important in identification. It can be transverse or procurved [6] and can, in some tarantulas (e.g. Ceratogyrus darlingi) have a "horn".[7]


The clypeus is the space between the anterior of the cephalothorax and the ocularium. It is found in most arachnids.[6] It is connected to the labrum of the invertebrate, between the labrum and the face.


The ocularium is a "turret" for the ocelli found in most arachnids.[8] In harvestmen, it may have the ornament of spines.[9]


The trident is a small group of (usually three) spines found in harvestmen exclusively. It is located in front of the ocularium. It varies in size amongst species; in some it is completely absent, and in others it is enlarged considerably.[9]


  1. ^ a b Eldra Pearl Solomon, Linda R. Berg & Diana W. Martin (2004). "The animal kingdom: an introduction to animal diversity". Biology (7th ed.). Cengage Learning. pp. 534–549. ISBN 978-0-534-49276-2.
  2. ^ Shultz, Stanley; Shultz, Marguerite (2009). The Tarantula Keeper's Guide. Barron's. p. 23. ISBN 978-0-7641-3885-0.{{cite book}}: CS1 maint: multiple names: authors list (link)
  3. ^ Timothy J. Gibb & C. Y. Oseto (2006). "Glossary". Arthropod Collection and Identification: Field and Laboratory Techniques. Academic Press. ISBN 978-0-12-369545-1.
  4. ^ Andrew J. Martinez (2003). "Arthropoda (crabs, shrimps, lobsters)". Marine Life of the North Atlantic: Canada to New England (3rd ed.). Aqua Quest Publications. pp. 144–175. ISBN 978-1-881652-32-8.
  5. ^ Dalton, Steve (2008). Spiders; The Ultimate Predators. A & C Black, London. P.p. 19. ISBN 9781408106976.
  6. ^ a b Smith, A. M. (1990c). Baboon spiders: Tarantulas of Africa and the Middle East. Fitzgerald Publishing, London, pp. 138. Retrieved February 13, 2016.
  7. ^ Gallon, R.C. (2008). "On some poorly known African Harpactirinae, with notes on Avicuscodra arabica Strand, 1908 and Scodra pachypoda Strand, 1908 (Araneae, Theraphosidae)". Bulletin of the British Arachnological Society. 14: 238.
  8. ^ Spiders... Archived 2021-11-28 at the Wayback Machine Yorkshire Naturalists' Union. Retrieved February 13, 2016.
  9. ^ a b Sankey, John & Savory, Theodore. British Harvestmen. Academic Press. P.p. 1-75. ISBN 012619050X.