What does cephalothorax mean?
The cephalothorax is a single body region found in many arthropods, formed by the complete fusion of the head and the thorax. It is a defining feature of spiders and other arachnids, of most crustaceans such as crabs, lobsters, and shrimp, and of horseshoe crabs. In these animals, structures that would otherwise occupy separate segments — eyes, mouthparts, brain, walking legs, and in spiders the bases of the limbs — are concentrated in this one unified section, which in many species is protected by a hard plate called the carapace. Behind the cephalothorax lies a distinct rear portion: the abdomen in spiders or the pleon in crustaceans. The word derives from Greek roots meaning 'head' (kephalē) and 'chest' (thōrax). Though chiefly a term of zoology and anatomy, it appears in general science writing whenever the body plan of spiders or shellfish is described, making it useful vocabulary for readers interested in biology.
nounThe fused head and thorax found in many arthropods, such as spiders, crustaceans, and horseshoe crabs, functioning as a single body region.
- The anterior body division of certain arthropods (spiders, crustaceans, horseshoe crabs) formed by the fusion of the head and thorax.
"A spider's legs all attach to the cephalothorax, not to the head alone."
"The crab's cephalothorax is shielded by a hard carapace."
"Under the microscope, the students could see eight tiny eyes arranged across the spider's cephalothorax."
Both plurals are accepted; 'cephalothoraces' follows the Latin-style pluralization of words ending in -x, while 'cephalothoraxes' is the anglicized form common in modern scientific writing.
"Comparing the cephalothoraces of the two specimens revealed differences in eye arrangement."
In a spider, there's no separate head at all — eyes, brain, stomach, and leg muscles are all packed into one armored fusion called the cephalothorax.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of cephalothorax
Cephalothorax comes from New Latin, combining two Ancient Greek roots: kephalē, meaning 'head', and thōrax, meaning 'chest' or 'breastplate'. The compound was coined by naturalists to name the fused head-thorax region observed in crustaceans and arachnids, and English use of the term dates from the early nineteenth century as comparative anatomy flourished. The same Greek elements give us numerous cognates, including 'cephalic' (relating to the head), 'cephalopod' (the class containing octopuses and squids), 'encephalitis' (inflammation of the brain), and 'thoracic' (relating to the chest).
Related word forms
How cephalothorax is actually used
A technical term used almost exclusively in zoology, entomology, and arachnology; it appears rarely outside scientific or educational writing. In crustacean anatomy, some specialists prefer the term 'cephalothorax' for the fused region, while in arachnology it is sometimes called the prosoma.
Easily confused with cephalothorax
A cephalopod is a mollusc such as an octopus or squid, whereas a cephalothorax is a fused body segment found in arthropods like spiders and crabs.
The thorax is the middle body region of an insect lying behind the head, while the cephalothorax is a single structure formed by the head and thorax being fused together.