Onychophora (Velvet Worms)

Abstract

Abstract The phylum Onychophora, also known as ‘peripatus’, ‘velvet worms’ or ‘walking worms’, comprises over 180 described species. Onychophorans are exclusively terrestrial, but are susceptible to desiccation and are restricted to humid microsites. The majority of species inhabit tropical forest litter. Their flexible trunk, retractile limbs and ability to squeeze through small interstices all make them excellently adapted for life in decomposing wood and leaf litter. The head appendages are modified for form sensory antennae, slicing mandibles and slime papillae. The last are a unique trait of onychophorans and eject a rapidly polymerising glue, which is used to entangle their animal prey. Both morphological and molecular phylogenetic studies place the Onychophora at the base of the phylum Arthropoda. Gas exchange via an invaginated tracheal system and water‐conserving uricotely represent show convergent evolution with similar terrestrial adaptations in insects. Key Concepts: Onychophorans possess typical arthropod traits: a chitinous cuticle, metameric segmentation with paired segmental limbs and periodic moulting regulated by steroid ecdysones. Gas exchange in onychophorans is accomplished by spiracles scattered over the body surface and which open into fan‐like clusters of tracheal tubules. The flexible trunk functions as a hydrostatic skeleton, deformed by antagonising circular and longitudinal muscle layers beneath the integument. Onychophorans possess little resistance to desiccation and are mostly confined to humid habitats in the tropics and sub‐tropics. Some species are able to take up water by eversible coxal vesicles at the leg‐bases. The anterior limbs are specialised as sensory antennae, slicing mandibles and slime papillae that eject a fast‐polymerising glue to entangle prey. Most onychophorans possess separate sexes (gonochoristic) and fertilisation involves various mechanisms of spermatophore transfer, often during an elaborate courtship.