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Temporal range: 40–0Ma
Euperipatoides kanangrensis crop.jpg
Scientific classification e
Kingdom: Animalia
Superphylum: Ecdysozoa
(unranked): Panarthropoda
Phylum: Onychophora
Grube, 1853

Sole class: Udeonychophora[1]

Onychophora dis.png
Global range of Onychophora: Peripatidae in green, Peripatopsidae in blue

The velvet worms (Onychophora — literally "nail bearers", also known as Protracheata) are a minor ecdysozoan phylum with ~180 species. These obscurely segmented[2] organisms have tiny eyes, antennae, multiple pairs of legs and slime glands.[3] They have variously been compared to worms with legs, caterpillars and slugs.[4] Most common in tropical regions of the Southern Hemisphere, they prey on smaller animals such as insects, which they catch by squirting an adhesive mucus. In modern zoology, they are particularly renowned for their curious mating behaviour and for bearing live young.







Body appendages[edit]

Euperipatoides kanangrensis, overturned to show lobopod legs









  • On the belly side of the second head segment is the labrum, a mouth opening surrounded by sensitive "lips". In the velvet worms, this structure is a muscular outgrowth of the throat, so, despite its name, it is probably not homologous to the labrum of the Arthropoda and is used for feeding. Deep within the oral cavity lie the sharp, crescent-shaped "jaws", or mandibles, which are strongly hardened and resemble the claws of the feet, with which they are probably homologous;[8]:146 early in development, the jaw appendages have a similar position and shape to the subsequent legs.[9] The jaws are divided into internal and external mandibles and their concave surface bears fine denticles. They move backward and forward in a longitudinal direction, tearing apart the prey.

The surface of the mandibles is smooth, with no ornamentation.[10] The cuticle in the mandibles (and claws) is distinct from the rest of the body. It has an inner and outer component; the outer component has just two layers (whereas body cuticle has four), and these outer layers (in particular the inner epicuticle) are dehydrated and strongly tanned, affording toughness.[10]

Slime glands[edit]



Skin and musculature[edit]

A member of the Peripatidae family.








Haemocoel and circulation[edit]








Unlike the arthropods, the velvet worms are unable to control the openings of their tracheae; the tracheae are always open, entailing considerable water loss in arid conditions. Water is lost twice as fast as in earthworms and forty times faster than in caterpillars.[14] For this reason, velvet worms are dependent upon habitats with high air humidity.

Digestive system[edit]



Excretory organs[edit]



Sensory organs[edit]

Euperipatoides kanangrensis, showing antennae


The rhabdomeric eyes of the Onychophora are thought to be homologous with the median ocelli of arthropods; this would imply that the last common ancestor of arthropods bore only median ocelli.[15] However, the innervation shows that the homology is limited: the eyes of Onychophora form behind the antenna, whereas the opposite is true in arthropods.[16]

Reproductive organs[edit]





Distribution and habitat[edit]


Velvet worms live in all tropical habitats and in the temperate zone of the Southern Hemisphere, showing a circumtropical and circumaustral distribution. Individual species are found in Central and South America; the Caribbean islands; equatorial West Africa and South Africa; northeastern India;[17][18] Indonesia and parts of Malaysia; New Guinea; Australia; and New Zealand. Fossils have been found in Baltic amber, indicating that they were formerly more widespread in the Northern Hemisphere when conditions were more suitable.[19]




The largest measured population density is very low, at approximately ten individuals per square meter; velvet worms are often difficult to find in their natural habitat.


The slime of the Onychophora is forcefully squirted from a pair of slime glands (oral papillae) in defence against predators and to capture prey.[20] The slime glands, positioned on the sides of the head below the antennae, are a pair of highly modified limbs and typically propel the slime around a centimetre.[21] The slime can be propelled up to four centimetres, although accuracy drops with range, which is usually much shorter than this.[22] One squirt usually suffices to snare a prey item, although larger prey may be further immobilised by smaller squirts targeted at the limbs; additionally, the fangs of spiders are sometimes targeted.[22]

The slime, which can account for up to 11% of the organism's dry weight,[22] is 90% water; its dry residue consists mainly of proteins—primarily a collagen-type protein.[21] 1.3% of the slime's dry weight consists of sugars, mainly glactosamine.[21] The slime also contains lipids and the surfactant nonylphenol. Onychophora are the only organisms known to produce this latter substance.[21]

The proteinaceous composition accounts for the slime's high tensile strength and stretchiness.[21] Upon ejection, it forms a net of threads about 20 µm in diameter, with evenly spaced droplets of viscous adhesive fluid along their length.[21] It subsequently dries, shrinking, losing its stickiness, and becoming brittle.[21] Onychophora will eat and "reuse" any dried slime.


It takes an onychophoran around 24 days to replenish an exhausted slime repository.[22]



Peripatoides sp., clearly showing the stub feet

Velvet worms move in a slow and gradual motion that makes them difficult for prey to notice.[22] Their trunk is raised relatively high above the ground, and they walk with non-overlapping steps.[23]:21 To move from place to place, the velvet worm crawls forward using its legs; unlike in arthropods, both legs of a pair are moved simultaneously. The claws of the feet are only used on hard, rough terrain where a firm grip is needed; on soft substrates, such as moss, the velvet worm walks on the foot cushions at the base of the claws.


Social interaction[edit]

The brains of Onychophora, though small, are very complex; consequently, the organisms are capable of rather sophisticated social interactions.[24] Behaviour may vary from genus to genus, so this article reflects the most studied genus, Euperipatoides.

They form social groups of up to fifteen individuals, usually closely related, which will typically live and hunt together. Groups usually live together; an example in drier regions would be in a region of the moist interior of a rotting log. Group members are extremely aggressive towards individuals from other logs.[24] Dominance is achieved through aggression and maintained through submissive behaviour.[24] After a kill, the dominant female always feeds first, followed in turn by the other females, then males, then the young.[24] Social hierarchy is established by a number of interactions: higher-ranking individuals will chase, bite and crawl on top of their subordinates.[24] Juveniles never engage in aggressive behaviour, but climb on top of adults, which tolerate their presence on their backs.[24] When assessing other individuals, individuals often measure one another up by running their antennae down the length of the other individual.[24] Once hierarchy has been established, paired individuals will often cluster together to form an aggregate; this is fastest in male-female pairings, followed by pairs of females, then pairs of males.[24] Hierarchy is quickly established between individuals from a single group, but not between organisms from different groups; these are substantially more aggressive and very rarely climb one another or form aggregates.[24]




Velvet worms are ambush predators, hunting only by night,[22] and are able to capture animals at least their own size, although it may take almost all of their slime-secreting capacity to capture a large prey item.[25] They feed on almost any small invertebrates, including woodlice (Isopoda), termites (Isoptera), crickets (Gryllidae), book/bark lice (Psocoptera), cockroaches (Blattidae), millipedes and centipedes (Myriapoda), spiders (Araneae),[25] various worms and even large snails (Gastropoda). Depending on their size, they eat on average every one to four weeks.[22] They are considered to be ecologically equivalent to centipedes (Chilopoda).

Potential victims can be detected from up to four centimetres away and are investigated by the gentle application of the antennae. If they are judged to be a suitable size, slime is ejected to immobilise the prey item.[22] The most energetically favourable prey are two-fifths the size of the hunting onychophoran.[22] The onychophoran bites into the prey and injects saliva, which further reduces motion and may initiate digestion of the prey item's innards.[22] Ninety percent of the time involved in eating a specimen is spent ingesting it; re-ingestion of the slime used to trap the insect is performed while the onychophoran locates a suitable place to puncture the prey, and this phase accounts for around 8% of the feeding time, with the remaining time evenly split between examining, squirting and injecting the prey.[22]

Onychophora probably do not primarily use vision to detect their prey; although their tiny eyes do have a good image-forming capacity, their forward vision is obscured by their antennae;[22] their nocturnal habit also limits the utility of eyesight. Air currents, formed by prey motion, are thought to be the primary mode of locating prey; the role of scent, if any, is unclear.[22] The animals literally creep up on their prey, with their smooth, gradual and fluid movement escaping detection by predators.[22] Once they reach their prey, they touch it very softly with their antennae to assess its size and nutritional value. After each poke, the antenna is hastily retracted to avoid detection by the prey individual.[22] This investigation may last anywhere upwards of ten seconds, until the velvet worm makes a decision as to whether to attack it—or until it disturbs the prey and flees.[clarification needed][22] Hungry Onychophora spend less time investigating their prey and are quicker to apply their slime.[22] Once slime has been squirted, Onychophora will stop at nothing to hound down and devour their prey, in order to recoup the energetic investment. They have been observed to spend up to ten minutes searching for removed prey, after which they return to their slime to eat it.[22] In the case of smaller prey, they may opt not to slime it at all.[22]


In social groups, the dominant female is the first to feed, not permitting competitors access to the prey item for the first hour of feeding. Subsequently, subordinate individuals begin to feed. The number of males reaches a peak after females start to leave the prey item.[24] After feeding, individuals clean their antennae and mouth parts before re-joining the rest of their group.[24] Because it takes so long to ingest a prey item, hunting mainly happens around dusk; the onychophorans will abandon their prey at sunrise.[22]


Reproduction and life-cycle[edit]








Predators and parasites[edit]



Conservation status[edit]




The primary threat comes from destruction and fragmentation of velvet worm habitat due to industrialisation, draining of wetlands and "slash and burn" for agriculture. Many species also have naturally low population densities and closely restricted geographic ranges; as a result, relatively small localised disturbances of important ecosystems can lead to the extinction of entire populations or species. Collection of specimens for universities or research institutes also plays a role on a local scale.[citation needed]



In their present forms, the velvet worms are probably very closely related to the arthropods, a very extensive taxon that incorporates, for instance, the crustaceans, insects and arachnids. They share, among other things, an exoskeleton consisting of α-chitin and non-collagenous proteins; gonads and waste-elimination organs enclosed in true coelom tissue; an open blood system with a tubular heart situated at the rear; an abdominal cavity divided into pericardial and perivisceral cavities; respiration via tracheae; and similar embryonic development. Segmentation, with two body appendages per segment, is also shared. However, antennae, mandibles and oral papillae are probably not homologous to the corresponding features in arthropods, i.e., they probably developed independently.




Velvet worms (Onychophora)


Water bears (Tardigrada)

Arthropods (Arthropoda)









Panarthropoda (arthropods, velvet worms, water bears)

Cycloneuralia (threadworms, horsehair worms and others)

Lophotrochozoa (annelids, molluscs and others)




Evolutionary history[edit]

See also: Lobopodia

Fossils from the early Cambrian bear a striking resemblance to the velvet worms. These fossils, known collectively as the lobopodians, were marine and may represent a stem group to the onychophorans[31] or a grade from which arthropods and onychophora arose.[32] They are found in the Cambrian,[33] Ordovician (possibly),[34] Silurian[35] and Pennsylvanian[note 1][36] periods.


Importantly, none of the Cambrian fossils bear features that distinctively unite them with the Onychophora; none can be confidently assigned to the onychophoran crown or even stem group.[37]



The low preservation potential of the non-mineralised Onychophora means that they have a sparse fossil record. Stem-group members include Ilyodes (=Helenodora, Carboniferous), Tertiapatus dominicanus and Succinipatopsis balticus (Tertiary);[1] crown group representatives are known only from amber—there is a single, partial specimen from the Cretaceous,[note 2][31] and a more comprehensive record in Eocene deposits from 40 million years ago.[38] But, in fact, some of these amber-borne specimens lack slime papillae and separate feet, thus may belong in the stem group.[37]




A Peripatid onychophoran


By 2011, some 180 modern species, comprising 49 genera, had been described;[40] the actual number of species is probably about twice this. According to more recent study, 82 species of Peripatidae and 115 species of Peripatopsidae have been described thus far. However, among these 197 species, 20 are nomina dubia due to major taxonomic inconsistencies.[41] The best-known is the type genus Peripatus, which was described as early as 1825 and which, in English-speaking countries, stands representative for all velvet worms.




See also[edit]


  1. ^ the Mazon Creek Helenodora


  1. ^ a b Jr, P.; George, (2000). "Fossil onychophorans from Dominican and Baltic amber: Tertiapatus dominicanus n.g., n.sp. (Tertiapatidae n.fam.) and Succinipatopsis balticus n.g., n.sp. (Succinipatopsidae n.fam.) with a proposed classification of the subphylum Onychophora". Invertebrate Biology 119: 104. doi:10.1111/j.1744-7410.2000.tb00178.x.  edit
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  4. ^ Ruppert, E. E.; Fox, R. S.; Barnes, R. D. (2004). Invertebrate Zoology: A Functional Evolutionary Approach (7th ed.). Belmont, CA: Thomson-Brooks / Cole. ISBN 978-0-03-025982-1 , p. 505
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  19. ^ Poinar, G. (1996). "Fossil Velvet Worms in Baltic and Dominican Amber: Onychophoran Evolution and Biogeography". Science 273 (5280): 1370–1371. Bibcode:1996Sci...273.1370P. doi:10.1126/science.273.5280.1370. 
  21. ^ a b c d e f g h Benkendorff, K.; Beardmore, K.; Gooley, A. A.; Packer, N. H.; Tait, N. N. (1999). "Characterisation of the slime gland secretion from the peripatus, Euperipatoides kanangrensis (Onychophora: Peripatopsidae)". Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology 124 (4): 457–465. doi:10.1016/S0305-0491(99)00145-5.  edit
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  28. ^ 2007 IUCN Red List – Search
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External links[edit]