Cephalothorax

Definition

The anterior body tagma of certain arthropods, formed by the fusion of the head and thorax into a single functional unit, distinct from the posterior abdomen. In arachnids and horseshoe crabs, the cephalothorax (also termed prosoma) is rigid, bears the eyes, mouthparts, and all walking legs, and houses the muscles controlling limb movement. In crustaceans, it is typically protected dorsally by a carapace. The term applies to chelicerates, crustaceans, and some highly modified insects (e.g., female Stylopidia), but not to Hexapoda, where the head and thorax remain separate tagmata.

Etymology

From Greek kephalē (head) + thōrax (chest/breastplate)

Example

In spiders, the cephalothorax bears eight legs, the chelicerae, and usually eight eyes, while the unsegmented abdomen behind contains the heart, digestive glands, and reproductive organs.

Synonyms

• prosoma

Related Terms

• Abdomen
• opisthosoma
• tagma
• Carapace
• chelicerate
• Thorax
• prosoma

Usage Notes

Prosoma is preferred in arachnology for spiders, scorpions, and allies; cephalothorax is more widely used across arthropod groups and in general zoological contexts. The two terms are often treated as synonyms for arachnids but prosoma technically refers only to the anterior tagma, while cephalothorax emphasizes the head-thorax fusion homology. Not applied to insects except in rare parasitic modifications. Contrast with the separate head and thorax of insects.