Peripatoides and their relatives are among the most ancient predators still alive, members of a lineage that has remained essentially unchanged for over 500 million years. They predate the colonization of land by vertebrates. They predate trees. They were hunting in the leaf litter of Cambrian forests before forests existed in any form we would recognize, and they are still doing it today in the humid forests of the Southern Hemisphere, using exactly the same method.
The hunting method is the reason they have not needed to change. Velvet worms hunt by spraying slime.
Two slime glands run the length of the body and open through nozzles on either side of the head. When a velvet worm detects prey, typically a small arthropod, it fires jets of slime from these nozzles in a rapid oscillating pattern that covers the prey in a sticky net before it can escape. The slime hardens on contact with air, immobilizing the prey within seconds. The velvet worm then bites through the prey’s cuticle, injects digestive saliva, and drinks the liquefied contents.
The slime is produced from a protein that is liquid inside the gland and solidifies on contact with air through a process that materials scientists have studied with considerable interest. It is strong, elastic, and produced at ambient temperature from water-soluble proteins, properties that make it a candidate model for biodegradable adhesives and fibers. A 500-million-year-old hunting tool is being investigated as a template for sustainable materials.
Velvet worms are social in a way that is unusual for invertebrate predators. Groups of up to 15 individuals, typically related females and their offspring, share a territory and cooperate in hunting, with a dominant female feeding first on shared kills. The social hierarchy is maintained through aggression, with the dominant female biting subordinates that attempt to feed out of turn.
The term living fossil implies stasis, and velvet worms are often described that way. But the body plan that has persisted for 500 million years contains considerable diversity. There are approximately 180 described species across the Southern Hemisphere, occupying habitats from tropical rainforest to temperate forest, varying in size, coloration, and social behavior. The ancient blueprint has been adapted and refined across an enormous range of environments without being fundamentally altered.
What the persistence of the velvet worm body plan actually demonstrates is not evolutionary stagnation but evolutionary sufficiency. The slime-spraying, fluid-drinking, socially hierarchical predator of the leaf litter found a solution that works across an extraordinary range of conditions, and 500 million years of changing environments have not produced a situation where a fundamentally different approach would work better.