Tasmania is home to many relict species of animals and plants that are among the most ancient on Earth. One such group of species is the velvet worms (or Peripatus, as they are also known). These remarkable creatures are the sole members of the ancient phylum Onychophora - the only animal phylum to include no marine species. Fossils of the Onychophora have been found dating back to the Cambrian period, 500 million years ago.
Currently there are around 10 genera and 110 species recognized within two families distributed across the southern continents which once made up Gondwana. The family Peripatidae is known from the circumtropical regions of Mexico, Central and northern South America, equatorial West Africa, and South East Asia and the family Peripatopsidae is found in Chile, South Africa, Australia including Tasmania, and New Zealand.
Tasmania is the world centre of diversity for the Onychophora, with at least 20 species.
The densest populations of velvet worms seem to be in the open eucalypt forests in the eastern and southern portions of the Central Plateau. Three species, the Giant velvet worm (Tasmanipatus barretti), Northwest velvet worm (Ooperipatellus cryptus) and the Blind velvet worm (T. anophthalmus) are listed under the Tasmanian Threatened Species Protection Act 1995.
Velvet worms breathe through little holes (trachea) over the body that are permanently open, so water from the body can easily be lost. Consequently, they are restricted to moist places, such as rotting logs, or under stones and leaf litter.
Worms or Arthropods?
The velvet worm is so named because of the soft velvety appearance of the skin which is covered with fine hair-like papillae, but it is not a worm.
They have soft, segmented bodies that give them a worm-like appearance like members of the Annelid phylum, but they also have walking legs (which gives rise to them being known in some parts of the world as "walking worms"), antennae and a tracheal breathing system — all characteristics of insects, spiders and other members of the arthropod phylum. Unlike arthropods, velvet worms do not have a stiff exoskeleton.
Australian velvet worms have between 14 and 16 pairs of legs, although species from other parts of the world may have up to 43 pairs of legs. Their characteristic flowing movement is caused by the alteration of fluid pressure in the limbs as they extend and contract along the body. This movement led to the common name, peripatus, from 'peripatetic', which means 'wandering'.
Most species grow to about 10-20 mm, although the giant velvet worm, the largest known in Tasmania, measures 35 to 40 mm when resting and extends up to 75 mm in length when walking.
Interesting dining habits
The velvet worm is a voracious, nocturnal predator. The small appendages on either side of the mouth (oral papillae, and not to be confused with the two anntenae) can squirt a white, sticky fluid for up to 30 cm which ensnares its prey (mostly small insects). Such behaviour has led to the term "spitting worms" being used in some parts of the world as the common name for the onychophores. The fluid is also squirted in self-defence.
It then bites a hole in its prey's exoskeleton with its mandibles, through which it injects enzymes which breakdown the internal tissues and begins sucking out its prey's pre-digested innards.
An even more interesting sex life
Reproduction among the velvet worms is one of the many remarkable aspects of the biology of these creatures.
The male's head has a pit adorned with hooks or spikes. The male scoops a packet of sperm from his genital opening and stores it in the pit on his head. When he finds a mate he attaches himself, head first, to the female’s genital opening.
Some velvet worms produce live young which have been nourished by an attachment similar to a placenta.