Onychophora (Velvet Worms)

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.

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.

close

References

Alexander AJ (1957) Notes on onychophoran behaviour. Annals of the Natal Museum 14: 35–43.

Alexander AJ and Ewer DW (1955) A note on the function of the eversible sacs of the onychophoran Opisthopatus cinctipes Purcell. Annals of the Natal Museum 13: 217–222.

Ballard JWO, Olsen GJ, Faith DP et al. (1992) Evidence from 12S ribosomal sequences that onychophorans are modified arthropods. Science 258: 1345–1348.

Clusella‐Trullas S and Chown SL (2009) Investigating onychophoran gas exchange and water balance as a means to inform current controversies in arthropod physiology. Journal of Experimental Biology 211: 3139–3148.

Giribet G (2008) Assembling the lophotrochozoan (=spiralian) tree of life. Philosophical Transactions of the Royal Society B 363: 1513–1522.

Giribet G, Carranza S, Baguna J, Ruitort M and Ribera C (1996) First molecular evidence for the existence of Tardigrada+Arthropoda clade. Molecular Biology and Evolution 13: 76–84.

Giribet G, Edgecombe GD and Wheeler WC (2001) Arthropod phylogeny based on eight morphological loci and morphology. Nature 413: 157–161.

Hackman RH and Goldberg M (1975) Peripatus: its affinities and its cuticle. Science 190: 582–583.

Haritos VS, Niranjane A, Weisman S et al. (2010) Harnessing disorder: onychophorans use highly unstructured proteins, not silks, for prey capture. Proceedings of the Royal Society B 277: 3255–3263.

Kusche K, Ruhberg H and Burmester T (2002) A hemocyanin from the Onychophora and the emergence of respiratory proteins. Proceedings of the National Academy of Sciences of the USA 99(16): 10545–10548.

Lavallard R and Campiglia SS (1981) Fine‐structure of the coxal vesicles ion Peripatus acacioi Marcus and Marcus (Onychophora). Brazilian Journal of Medical and Biological Research 14(2‐3): 212.

Manton SM (1977) The Arthropoda. Cambridge: Cambridge University Press.

Manton SM and Heatley NG (1937) Studies on the Onychophora II. The feeding, digestion, excretion and food storage of Peripatopsis, with biochemical estimations and analyses. Philosophical Transactions of the Royal Society B 227: 411–464.

Mesibov R and Ruhberg H (1991) Ecology and conservation of Tasmanipatus barretti and T. anophthalmus, parapatric onychophorans (Onychophora: Peripatopsidae) from Northeastern Tasmania. Papers and Proceedings of the Royal Society of Tasmania 125: 11–16.

Meyer E and Eisenbeis G (1985) Water relations in millipedes from some Alpine habitat types (Central Alps, Tyrol) (Diplopoda). Bijdragen tot de Dierkunde 55: 131–142.

Monge‐Najera J (1994) Reproductive trends, habitat type and body characteristics in velvet worms (Onychophora). Revista de Biologia Tropical 42(3): 611–622.

Moseleyi HN (1874) See Cuenot L, 1949. Les Onychophores. In: Grassé P (ed.) Traité de Zoologie VI, pp. 6–37. Paris: Masson et Cie.

Oliveira IS, Lacorte GA, Fonseca CG et al. (2011) Cryptic Speciation in Brazilian Epiperipatus (Onychophora: Peripatidae) Reveals an underestimated diversity among the peripatid velvet Worms. PloS One 6(6): e19973.

Ou Q, Liu J, Shu D et al. (2011) A rare onychophoran‐like lobopodian from the lower Cambrian ChengJiang Lagerstatte, Southwestern China, and its phylogenetic implications. Journal of Paleontology 85(3): 587–594.

Peterson KJ and Eernisse D (2001) Animal phylogeny and the ancestry of bilaterians: inferences from morphology and 18S rDNA gene sequences. Evolution and Development 3(3): 170–205.

Regier JC, Schultz JW, Zwick A et al. (2010) Arthropod relationships revealed by phylogenomic analysis of nuclear protein‐coding sequences. Nature 463: 1079–1084.

Robson EA (1964) The cuticle of Peripatopsis moseleyi. Journal of Microscopy Science 25: 449–468.

Storch V (1984) Onychophora. In: Bereiter‐Hahn J, Matolsky AG and Richards KS (eds) Biology of the Integument, Chap. 36, pp. 703–708. Berlin: Springer‐Verlag.

Strausfeld NJ, Strausfeld CM, Loesel R, Rowell D and Stowe S (2006) Arthropod phylogeny: onychophoran brain organization suggests an archaic relationship with a chelicerate stem lineage. Proceedings of the Royal Society B 273: 1857–1866.

Tait NN and Norman JM (2001) Novel mating behaviour in Florelliceps stutchburyae gen. nov., sp. nov. (Onychophora: Peripatopsidae) from Australia. Journal of Zoology (London) 253: 301–308.

Thompson I and Jones D (1980) A possible onychophoran from the middle Pennsylvanian Mazon Creek beds of northern Illinois. Journal of Paleontology 54: 588–596.

Wright JC and Luke BM (1989) Ultrastructural and histochemical investigations of Peripatus integument. Tissue and Cell 21: 605–625.

Further Reading

Anderson DT (ed.) (2001) Invertebrate Zoology, 2nd edn. Oxford University Press. 476 pp.

Camatini M (ed.) (1980) Myriapod Biology. London: Academic Press. 428 pp.

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.

Contact Editor close
Submit a note to the editor about this article by filling in the form below.

* Required Field

How to Cite close
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]