Onychophora (Velvet Worms)


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.

Keywords: Arthropoda; cuticle; lobopodia; peripatus; phylogeny; slime papillae; velvet worms

Figure 1.

A living onychophoran, Peripatopsis, from New Zealand. Photograph courtesy of Dr. Noel Tait.

Figure 2.

Generalised internal anatomy of an onychophoran. (a) Transverse section. (b) Dorsal dissection. Brain, B; cuticle, C; crural gland, CG; circular muscle, CM; gut, G; heart, H; longitudinal muscle, LM; nephridium, N; paired ventral nerve cords (with connecting commissures), NC; ovary, O; oblique muscle, OM; slime gland, SG; salivary gland, SaG. Reproduced with permission from Wright and Luke . Copyright © 1989 Harcourt Brace & Co.

Figure 3.

Transmission electron micrographs of the integument of Euperipatoides leuckarti. (a) General integument showing cuticle, epithelium and basal lamina. This animal is moulting and the old cuticle has just separated from the underlying new cuticle (bar, 5 μm). (b) Close‐up of the collagen basal lamina showing the shifting fibre angles (bar, 10 μm). Basement membrane, BM; cell membrane (epithelial junction), CM; epicuticle, EC; nucleus, N; procuticle, PC; epithelial pigment granules, PG; epithelial tonofibrils, T. Reproduced with permission from Wright and Luke . Copyright © 1989 Harcourt Brace & Co.

Figure 4.

Scanning electron micrographs of the integument of Euperipatoides leuckarti. (a) Ventral view of the anterior region (bar, 1 mm). (b) Close‐up of the integument (bar, 50 μm). antennae, A; buccal papillae, BP; crural gland, CG; jaws, J; primary papilla, PP; possible stretch sensillum, SS; slime papilla, SP. Reproduced with permission from Wright and Luke . Copyright © 1989 Harcourt Brace & Co.



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Little C (1990) The Terrestrial Invasion: An Ecophysiological Approach to the Origins of Land Animals. Cambridge: Cambridge University Press.

Ruppert EE, Fox RS and Barnes RD (2003) Invertebrate Zoology: A Functional Evolutionary Approach, 7th edn. Belmont, CA: Brooks/Cole.

Walker MH and Norman (eds) (1995) Onychophora: past and present. Zoological Journal of the Linnaean Society 114: 1–153.

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Wright, Jonathan C(Oct 2012) Onychophora (Velvet Worms). In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001610.pub3]