What are Onychophorans

Velvet worms. Nice name, but what are they? Why are they in their own Phylum, the Onychophora? Where are they found and why have most people never heard of them, much less seen them? Velvet worms only occur in remnants of the ancient ecosystems such as those that once formed part of the supercontinent of Gondwana, so you must look in Australia, New Guinea and New Zealand plus the southern tip of Africa and parts of Central and South America to find them. They are small and usually live cryptic lives in the leaf litter of forests. However they are becoming better known as they have been introduced to the ‘exotic’ pet trade.

There are two living groups of Onychophora, the Peripatidae in Africa, South America and parts of Asia, and the Paripatopsidae in the Australian region. They have been considered living fossils because they are similar to the ancestral segmented worms that developed legs and became the first arthropods. They are literally worms with legs.

I first discovered the group in my Invertebrate Zoology course in Canada many years ago. There were no onychophorans in Canada so when I moved to New Zealand, one of my goals was to see a living Velvet worm. I found my beast in the southern beech forests of Arthur’s Pass, inland of Christchurch on the South Island. It was about 5 cm long, velvety black in colour with irridescent blue spiracles on the sides of its segmented body. It was rummaging around under a rotten log and it was an exciting find. I didn’t see another one for twenty odd years but one day as I was walking in my forest in Australia, there was one on the ground, crawling along. That was exciting too. It was mottled brown in colour and its spiracles were grey.

This is an animal that gives us an idea of what must have happened long ago in the Cambrian Period. Worms evolved as the first predators of those ancient ecosystems. Some were able to come ashore and begin the colonisation of the land. Legs became a useful adaptation on the land and some of the early worms developed legs and then a hard exoskeleton. From there, these animals radiated out to become the vast, diverse and successful Phylum Arthropoda. But some of the original worms with legs survived. They survived against the competition with all the arthropods. They survived the great extinction events that wiped out the trilobites and then later the dinosaurs. They survived to the present time when we can look at them and marvel at the adaptations that led both to the modern Arthropods and the surviving Onychophora.

Velvet worms live in moist, dark habitats in the leaf litter of tropical and temperate forests and some grasslands. Two species are cave dwellers. The distinctive respiratory spiracles on their bodies cannot be closed so they must live in moist or humid conditions to avoid dessication. They are feisty little predators that attack insects, worms and spiders, millipedes and centipedes. They have a unique prey capture method: they hit them with glue secreted from slime glands. Then when the prey is immobilized, they hit it again with digestive juices and suck up the resultant liquids. In their turn, velvet worms are preyed upon by larger spiders, small mammals and birds.

Velvet worms have separate sexes and the females are generally larger than the males. Fertilisation is internal, with the male placing a spermatophore in the female. Young can be born live or laid as eggs. The young have to moult in order to grow and mature in less than a year. Life spans can be several years long.

Velvet worms, like so many other animals, are threatened by human activities such as land clearing, swamp drainage and other habitat altering activities. Three species are already extinct and at least four more are endangered. It is to be hoped that with greater awareness of these unique animals, efforts will be made to save them. It would be a shame if, after surviving for up to five hundred million years, they became extinct because of humans.