Velvet worms (Phylum Onychophora)
Despite comprising the only exclusively terrestrial (land-dwelling) animal phylum, velvet worms require moist air and tend to live in humid forests in the southern hemisphere and in the tropics and subtropics. Some 80 species are known from Australia, particularly in temperate forests in the country's southeast.
Velvet worms resemble large caterpillars and range from 0.5 to 15 cm long. They have soft, velvety, often finely patterned skin, 14-43 pairs of stubby legs, fleshy antennae, large clawed jaws, and slime glands. Velvet worms are extremely retiring: they avoid light and hide during the day in moist, humid environments like rotting logs, leaf litter, and soil. Their legs, while appearing somewhat similar to those of a caterpillar, work very differently. They lack joints and are essentially baggy appendages of the body that are kept rigid by hydraulic pressure.
Famously, velvet worms are slow-motion ambush predators. Hunting exclusively at night, and very slowly (less than one metre per hour), a velvet worm will silently explore the forest floor until it bumps into a potential prey item, such as a cricket or spider. Then, using its antennae to pinpoint its target's location, it slowly prepares for a lightning-fast attack: a clear fluid shoots in a stream from special slime glands flanking the mouth, covering the target with overlapping strands.
The fluid quickly hardens, trapping the target. If the prey is large, other slime ejections may be used to immobilise the legs; if it's a spider, an ejection may also be used to render the fangs immobile and harmless. The velvet worm then begins feeding, using its sharp jaws to break open its prey and injecting saliva that softens tissues and begins the digestive process. Then the claws help pull out the flesh, walking it stepwise into the velvet worm's mouth. A velvet worm may spend an entire night slowly devouring a single prey item; in the morning, only indigestible shell elements remain - even the velvet worm's own slimy glue is consumed for recycling.
The Australian Euperipatoides rowelli hunting. Photo: Andras Keszai, CC-BY-NC-SA 2.0. Source: https://www.flickr.com/photos/nascenthought/5517509102
A well-studied Australian velvet worm, Euperipatoides rowelli, has remarkably complex behaviour. Individuals form social networks, usually of related individuals occupying the same habitat such as a rotted log. Well-developed dominance hierarchies are established and maintained using dominant and submissive behaviours. The group hunts cooperatively and shares prey items. Members of one group will aggressively defend their territory from non-group members. All these behaviours are more reminiscent of higher mammals such as chimpanzees than they are of invertebrates.
Most velvet worms lay eggs, either inside or outside the mother's body. Some Australian species, however, do not lay eggs; instead, developing young receive nutrition directly from the mother across a tissue bridge between mother and young - similar to human pregnancy. Such velvet worms, like humans and other mammals, give birth to live young.
The sharing of nutrients between mother and offspring (developmental placentation) was long assumed to be exclusive to mammals; however, recent research, including on Australian velvet worms, has helped show that placentation is in fact widespread throughout the animal kingdom, even among invertebrates.
These velvet worms form a social network and exhibit remarkably elaborate behaviours. Their brains, though small, are surprisingly complex. Photo: M. Trenerry; source: https://www.wettropics.gov.au/the-velvet-worm
Over one third of the world's 220 velvet worm species are Australian, and every one of Australia's 80 species is found nowhere else in the world. This makes velvet worms something of an invertebrate icon for Australia.
Despite their diversity, velvet worms have very precise environmental requirements, sometimes limited to tiny microclimatic areas, and are probably very susceptible to habitat loss and fragmentation. Professor Dave Rowell at ANU is currently spearheading projects to improve our understanding of Australia's velvet worms.
Thanks to Robin Hare from the University of Western Australia for help preparing this profile
Read more about velvet worms:
Velvet worms at the Australian Museum
St J. Read, V. M. and Hughes, R. N. (1987) Feeding behaviour and prey choice in Macroperipatus torquatus (Onychophora). Proceedings of the Royal Society of London B: Biological Sciences, 230(1261), pp. 483-506.
J. Reinhard & D.M. Rowell (2005). Social behaviour in an Australian velvet worm, Euperipatoides rowelli (Onychophora : Peripatopsidae). Journal of Zoology 267(01). DOI: 10.1017/S0952836905007090