Velvet Worm

Despite their common name, velvet worms are not actually worms. They have been thought of as the missing link between arthropods and annelids(worms), but they are actually in their own phylum, Onychophora. Onychophorans are small terrestrial caterpillar-like animals with antenna, modified legs called ‘oral tubes’ that help catch prey, and 14-43 pairs of legs. They move by the alternating fluid pressure in their limbs, giving them a wandering, weaving movement. They can range from two to 10 centimeters. They are usually orange, red, brown, blue, gold, or white, and are covered in overlapping papillae scales, giving them a velvety look, tiny hairs that are sensitive to touch and smell, and making their skin hydrophobic, helping to keep them dry in their wet environments.

There are about 200 species of velvet worms that can be found across the world. They are categorized into two families, peripatidae and peripatopsidae. The first can be found in tropical America, western Africa, and south east Asia. Peripatopsids are found in countries such as Chile, South Africa, Australia, New Guinea and New Zealand that were previously connected in the supercontinent Gondwanaland. Australia has the most diversity, with almost 74 species. In these places they are found in warm, tropical areas in moist rotting logs and leaf litter. Velvet worms breath through holes, or trachea, across their body. Because they can’t close their trachea, water is easily lost. This is why they must stay in wet, humid areas. They are photonegative, meaning they try to stay away from light.

Velvet worms are carnivores, eating other small invertebrates like termites, woodlice, and spiders. They capture this prey by squirting out sticky slime from their oral tubes, entangling the prey. They then bite off parts of the prey, partially digest it outside of their body using their saliva, and then eat it. Velvet worms also use this slime as a defense against predators. Unfortunately, the slime takes a while to remake, so they often will eat the dried slime with their prey. In fact, 11% of the velvet worm’s body weight is it’s slime. Because of this limitation, one Australian species of velvet worm forms packs that hunt together and protect each other. The largest and most aggressive female is picked as the alpha female, who has the benefit of eating first.

Some velvet worms reproduce when males deposit sperm on the back or sides of a female. The female’s skin then collapses, and the sperm is brought to her ovaries. Some females can even store sperm for several months! Males release a pheromone to attract females. Females are mostly larger than males, and only reproduce once. Some species of velvet worms lay eggs, and some keep eggs in the female body until hatching. However, once the velvet worms hatch, all are fully developed, but smaller and without the adult’s pigmentation.

Evolutionarily, velvet worms are very interesting. They have stayed mostly the same for half a billion years. Although once they were thought to be many things, including slugs, the link between arthropods and worms, or the precursors to arthropods, in reality, onychophorans evolved along side the arthropods. They are closely related, but evolved separately. In fact, the closest relatives to velvet worms are tardigrades!