Peripatus or velvet worms are little-known nocturnal creatures living in damp forest in New Zealand and other southern lands. Varying in length from a few millimetres to 22 centimetres, they look like skinny caterpillars with stumpy legs. Some species have striking colours and textures.
New Zealand has five named species, but others are yet to be named.
Peripatus are called velvet worms because their skin has a velvety appearance, caused by thousands of tiny papillae (bumps) with fine bristles. They have short legs tipped with spiny pads and a hooked claw. The number of legs varies depending on the species. New Zealand velvet worms have 13–16 pairs of legs; species elsewhere have from 13 to 43 pairs.
Their distinctive legs earned them the name peripatus, from the same root as the word peripatetic – to walk or wander about. Peripatus is just one genus of velvet worm, but the name is loosely used to include the entire velvet worm phylum or subphylum, Onychophora ('claw-bearers'). Their Māori name is ngaokeoke, from ngaoki, which means to crawl.
Onychophorans were once seen as the missing link between annelids (segmented worms) and arthropods (insects, spiders, crustacea). Like the annelids, they have a hydrostatic ‘skeleton’ – a fluid-filled core surrounded by muscle – instead of a hard exoskeleton like arthropods or a bony skeleton like vertebrates. But like the arthropods, they have chitin (a tough compound, from which their claws are formed), and a tracheal system – a series of tubes that carry oxygen into the body.
Today, the Onychophora are usually placed in the arthropod line of evolution, either as a phylum in their own right or a subphylum in the Arthropoda.
Although most velvet worms have a pair of simple eyes, it is doubtful they can do more than detect light. Their eyes help them avoid daylight, when they would be at greater risk of drying out. Two overseas species lack eyes entirely.
Velvet worms are ancient survivors from the distant past. Fossil remains date from the early Cambrian period, around 520 million years ago. These early fossils are nearly all from the sea and include the strange Hallucigenia – a small worm with pincer-tipped tentacles along one side of its body and spines along the other. Some researchers think that marine peripatus may have been out-competed by marine polychaete worms (bristle worms). This may explain why only those worms that were able to leave the sea and live on land have survived.
Velvet worms alive today are grouped into two families:
- Peripatidae are mainly equatorial and are found in Antilles, Mexico, Central America, northern South America, equatorial West Africa and South-East Asia.
- Peripatopsidae live further south, in Chile, South Africa, New Guinea, Australia and New Zealand.
The two families may have diverged before the break up of the Gondwana supercontinent, about 85 million years ago. But this has been questioned since the discovery of 20–40-million-year-old fossils in amber from the Baltic region of Europe, and the Dominican Republic. The fossils in amber show important differences to modern species, and may represent two new Onychophoran families. Dominican fossils also appear to be ancestors of the Peripatidae family. The presence of peripatus ancestors in the Baltic is also evidence for a wider past distribution. This means it may be misleading to use the distribution of the two living velvet worm families to understand their evolution.
Currently about 140 species are recognised in 10 genera, two of which occur in New Zealand (Peripatoides and Ooperipatellus). With more DNA analysis this figure will probably increase.