prosoma

Prosoma, from the Greek pro- 'forward' and soma 'body', is the anterior tagma of most chelicerate arthropods. In common usage it is synonymous with the cephalothorax in spiders, scorpions, harvestmen, and their relatives, marking the front part of the body that bears the mouthparts and limbs while the hind part is called the opisthosoma or abdomen.

Anatomy: The prosoma carries the mouthparts, including the chelicerae, and in most species the pedipalps. It bears the four pairs of walking legs in spiders and many other chelicerates, and often a dorsal carapace or shield that protects internal organs. Sensory structures, including eyes, are located on the prosomal shield in many groups.

Development and variation: The prosoma forms from tagmosis during embryonic development, resulting in a fused head–thorax region in arachnids. In spiders, the male's pedipalps frequently become specialized for reproduction. In some mites and related groups the boundaries between prosoma and opisthosoma are less distinct, and body organization can appear more compact.

Function and evolution: By housing primary sensory organs, feeding structures, and locomotor appendages, the prosoma supports predation, defense, and interaction with the environment. The consolidation of the prosoma with the anterior segments reflects the chelicerate body plan's evolutionary tendency toward fusion of the head and thorax.

See also: Cephalothorax; Opisthosoma; Tagma; Arachnid anatomy.