Velvet worms, otherwise known as Onychophora, are reclusive little animals that have changed very little in the last 500 million years.
Scientists have described some 180 modern species. They can be found in moist, dark places all around the tropics and Australia and New Zealand. Smaller species are less than an inch long, while the largest reach lengths of about 8 inches.
They come in a dazzling array of colors and exhibit some pretty weird and complex behaviors. I'm sure you'll be just as charmed by them as I am.
__1. Velvet worms have hydrostatic skeletons. __Velvet worms don't have hard exoskeletons like arthropods. Instead, their fluid-filled body cavities are covered in a thin skin and kept rigid by their pressurized internal liquids. They move by the alteration of fluid pressure in the limbs as they extend and contract along the body.
2. They have velvety, water-proof skin. Their entire bodies are covered with papillae, tiny protrusions with bristles sensitive to touch and smell. The papillae are made up of overlapping scales, which gives the velvet worm its velvety appearance. It also makes their skin water-repellant.
3. They have lots of little stubby feet. Their feet are described as conical, baggy appendages. Depending on the species, a velvet worm can have between 13 and 43 pairs of feet. The feet are hollow, fluid-filled, and have no joints.
4. Each little stubby foot has a claw. Each foot is outfitted with a hooked claw made of chitin (lending the group its scientific name, *Onychophora *('Claw-Bearers')). Velvet worms use their claws when walking on uneven terrain; on smoother surfaces, they retract their claws and walk on the foot cushions at the base of the claws.
5. Velvet worms are vulnerable to dehydration. Like insects, velvet worms breathe through holes along their bodies called tracheae. Unlike insects, velvet worms cannot close these holes to prevent water loss, so they easily dry out. For this reason, velvet worms spend most of their time hidden in moist areas in the soil, under rocks, and in rotting logs. They're most active at night and during rainy weather.
6. They use slime as a weapon. Velvet worms are ambush predators, hunting other small invertebrates by night. To subdue their prey, they squirt a sticky, quick-hardening slime from a pair of glands on their heads. After the prey is ensnared, the velvet worm bites into it, injecting digestive saliva that helps liquefy the insides for easier snacking. The slime is energetically costly to make, so velvet worms will often eat any excess slime they have produced to shore up their reserves. Check out a sliming in action here.
7. At least one species is highly social with a strict dominance hierarchy. *Euperipatoides rowelli *live in groups of up to 15 individuals, ruled over by a dominant female. The group hunts together, and after a kill the dominant female always feeds first, followed by the other females, then the males, and finally the young. The social hierarchy is established and maintained through aggression: higher-ranking individuals will chase, bite, kick, and crawl over subordinates.
8. Velvet worms are survivors. They belong to a clade that has been around for over 500 million years. Fossilized marine versions of velvet worms from the Cambrian period have been found in the Burgess Shale in Canada (505 million years old) and the Chengjiang formation in China (520 million years old). Velvet worms are now considered to be close relatives of arthropods and tardigrades. They're of interest to paleontologists because they might help provide an idea of what the ancestors of arthropods were like.
9. They have a number of bizarre reproductive strategies. All the velvet worms reproduce sexually, except for Epiperipatus imthurni (they reproduce by parthenogenesis and no males have ever been observed).
The other velvet worm species have evolved several creative ways to deliver the male's sperm to the female's egg. Some species deposit their spermatophores directly into the female's genital opening, though the means by which they do this varies. In some species the male uses special structures on his head; other species have spikes, spines,or pits to either hold their sperm or transfer it to the female.
The males in the genus Peripatopsis just deposit their spermatophore on seemingly random spots on the female's body. This stimulates a localized breakdown of her skin so the sperm can pass into her body and migrate to her ovaries, where fertilization takes place.
10. Most velvet worms give birth to live young. Female velvet worms can store sperm for many months before using them to fertilize their eggs. Their gestation period can last up to 15 months in some species. Most give birth to live young, although a few species lay eggs. Young velvet worms are born fully developed and looking like miniature versions of the adults.
References and Other Resources:
Campbell, L. I., Rota-Stabelli, O., Edgecombe, G. D., Marchioro, T., Longhorn, S. J., Telford, M., J., ... Pisani, D. (2011). MicroRNAs and phylogenomics resolve the relationship of Tardigrada and suggest that velvet worms are the sister group of Arthropoda. Proceedings of the National Academy of Sciences USA. 108: 15920-15924. doi: 10.1073/pnas.1105499108.
"Introduction to the Onychophora," UC Berkeley. Accessed 3/27/2014.
Monge-Najera, J. (1995). Phylogeny, Biogeography and Reproductive Trends in the Onychophora. Zoological Journal of the Linnean Society 114 (1): 21–60. doi:10.1111/j.1096-3642.1995.tb00111.x.
Oliveira I. de S., Read V. M. S. J. and Mayer G. (2012). A world checklist of Onychophora (velvet worms), with notes on nomenclature and status of names. ZooKeys 211 (211): 1–70. doi:10.3897/zookeys.211.3463.
Poinar, G. (1996). Fossil Velvet Worms in Baltic and Dominican Amber: Onychophoran Evolution and Biogeography. Science 273 (5280): 1370–1371. doi:10.1126/science.273.5280.1370.
Reinhard, J. and Rowell, D. M. (2005). Social behaviour in an Australian velvet worm, Euperipatoides rowelli (Onychophora: Peripatopsidae). Journal of Zoology, 267, pp 1-7. doi:10.1017/S0952836905007090.
Van Roy, P., Orr, P. J., Botting, J. P., Muir, L. A., Vinther, J., Lefebvre, B., Hariri, K. E. and Briggs, D. E. G. (2010). Ordovician faunas of Burgess Shale type. Nature 465 (7295): 215. doi:10.1038/nature09038.
"Velvet Worm," Australian Museum. Accessed 3/27/2014.
