Thecephalothorax, also calledprosoma in some groups, is atagma of variousarthropods, comprising the head and thethorax fused together, as distinct from theabdomen behind.[1] (The termsprosoma andopisthosoma are equivalent tocephalothorax andabdomen in some groups. The termsprosoma andopisthosoma 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 theopisthosoma contains organs atypical of a trueabdomen, such as a heart and respiratory organs.[2]) The wordcephalothorax is derived from theGreek words for head (κεφαλή,kephalé) and thorax (θώραξ,thórax).[3] This fusion of the head and thorax is seen inchelicerates andcrustaceans; in other groups, such as theHexapoda (includinginsects), the head remains free of the thorax.[1] Inhorseshoe crabs and manycrustaceans, a hard shell called thecarapace covers the cephalothorax.[4]
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 sometarantulas (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 theocelli found in most arachnids.[8] Inharvestmen, 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]
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