Invertebrates
Velvet Worm
Onychophora · also called velvet worm, onychophoran, peripatus
The velvet worm looks like a caterpillar crossed with a slug, creeping through the damp dark of a rotting log. It is neither — it is the last of an ancient line, a soft-bodied predator that has barely changed in half a billion years and that hunts by shooting glue from a pair of cannons beside its mouth.
Overview
Velvet worms (phylum Onychophora, about 200 species) live in consistently moist, dark places — rotting logs, leaf litter, soil, caves — in the tropics and the southern-temperate zone (Central and South America, Africa, Southeast Asia, Australia, New Zealand). They are nocturnal and slow, with a soft, velvety, segmented body carried on 13–43 pairs of stubby, clawed legs, and they navigate by touch and air movement with a pair of antennae.
The slime cannons
Beside its mouth, a velvet worm has two nozzle-tipped appendages. Detecting prey, it fires jets of sticky protein glue from both at once, whipping them side to side to lay down a net. The slime sets in seconds into rubbery threads that entangle the prey; the worm then bites in, injects digestive saliva, and sucks out the contents — and eats the used slime to recycle it.
A living bridge
Onychophorans appear in Cambrian fossils ~500 million years old and have changed little. The phylum sits between the segmented worms and the arthropods, sharing features with both, and it helps show how the arthropod body plan evolved. Its closest living relatives are the tardigrades.
Surprising lives
Many velvet worms give live birth, some with a placenta-like connection to the embryos. And some Australian species hunt in groups — up to 15 in one log, led by a dominant female, subduing large prey together and feeding in rank order — cooperative behaviour almost unheard of in a soft-bodied invertebrate.
Status
Velvet worms have not been assessed as a group, but many species have tiny ranges in threatened forests and cannot disperse or tolerate drying, so several are considered Endangered or Critically Endangered.
Velvet Worm: common questions
Is a velvet worm a worm, an insect, or something else?
Something else — it has its own phylum, Onychophora, and sits between the annelid worms and the arthropods on the tree of life. It shares features with both — a soft, segmented, worm-like body and simple eyes on the one hand; a chitinous cuticle it must moult, an open circulatory system, and clawed limbs on the other. For a long time it was considered a "missing link," and while modern biology avoids that phrase, the velvet worm genuinely does help illuminate how the enormously successful arthropod body plan (insects, crustaceans, spiders) evolved from soft-bodied ancestors. Its closest living relatives are the tardigrades and the arthropods.
How does the slime attack work?
On each side of its mouth a velvet worm has a small nozzle-tipped appendage. When it detects prey by touch and air movement with its antennae, it approaches slowly, then fires jets of a sticky, protein-rich fluid from both nozzles at once, whipping them from side to side several times a second so the streams paint a zig-zag net over the target. The slime is liquid as it leaves the body but sets into strong, rubbery threads within seconds, entangling the prey's legs and wings. The velvet worm then walks up, bites a hole in the immobilised animal, injects digestive saliva, and sucks out the liquefied contents — and it eats the spent slime as well, because the glue is metabolically expensive to make and worth recovering.
Why do velvet worms live only in damp, hidden places?
Their skin is thin and highly permeable, with no waxy waterproofing layer, and unlike insects they lack an efficient system for controlling water loss. In dry air a velvet worm desiccates and dies within hours. They are therefore confined to consistently humid microhabitats — the inside of rotting logs, deep leaf litter, moss, soil crevices, and caves — and they are mostly active at night and after rain. This dependence on stable moisture, combined with very limited ability to disperse (they cannot fly, swim, or travel far), means many species are restricted to single forests or even single hillsides, and are highly vulnerable to habitat drying and clearance.
Do velvet worms really hunt in packs?
Some do. Studies of Australian velvet worms in the genus Euperipatoides found that individuals live in groups of up to about 15 in a single rotting log, that the groups are led by a dominant female, and that they hunt together — the group piles onto large prey such as a cricket, subduing it with combined slime, and then feeds in a strict order, with the dominant female eating first and subordinates and juveniles last. Aggression is used to establish and maintain the hierarchy. Cooperative, hierarchical group hunting like this is extremely rare outside the social insects and a few vertebrates, and its discovery in a "primitive" soft-bodied invertebrate was a genuine surprise.