What is a Velvet Worm?

The Velvet Worm, which belongs to a genus better known to scientists as Onychophora, is a tropical land animal which frequents dark, humid, moisture-rich environments. Large specimens of these carnivorous invertebrates can measure up to eight inches long and are typically encountered amongst decaying wood on the rainforest floor. There are around 180 recorded species of velvet worm, many of which have distinctive colouring and markings, and though their behaviour has attracted much attention, scientists are particularly interested in onychophora because the population has experienced no significant evolution in 500 million years.

Appearance

Onychophora ('claw bearers') have a smooth-looking outer skin and characteristically short stumpy feet equipped with fibrous hooked claws. Unlike arthropods (centipedes and spiders), to whom they are closely related, onychophora do not possess a hard skeleton. Instead they have a pressurised, fluid-filled body enclosed in a waterproof skin. This slug-like skin layer consists entirely of papillae: very small fleshy dimples or overlapping plates which form a sensory mechanism conveying touch and smell, and also give these worms their distinguishing velvety appearance.

Characteristics

Having anything from 13 to 43 feet, onychophora wriggle along by altering the fluid pressure inside their hollow feet. They have two distinct modes of walking, using their robust claws to gain traction on rough surfaces, and then switching to fluid-filled 'cushions' for less-demanding conditions. This primary dependence on body fluids makes the velvet worm, in common with most worm species, very vulnerable to the drying effects of sunshine and means the animal is generally reclusive by day, hunting overnight or otherwise during spells of damp weather.

Perhaps the most spectacular feature of onychophora is their ability to emit quantities of slime when hunting prey. Having first located and ambushed a smaller invertebrate, the velvet worm uses two powerful glands positioned on its head to quickly spray a layer of slime over its victim. This adhesive mucus substance hardens rapidly, allowing the worm to administer a fatal bite and then inject enough saliva into the resultant wound to break down the victim's internal organs into an easily digestible 'soup'. Field observations of these attacks show that, during the aftermath, worms are always keen to ingest any slime remnants, which would suggest sliming is an energy-intensive strategy for the hunter.

Evolutionary history

It has been shown that 500-million-year-old fossils from the early Cambrian period look remarkably similar to living onychophora. However, the evidence also reveals these fossil creatures lived a marine existence, and it is believed the major evolutionary change to a terrestrial lifestyle must have occurred between 10 and 70 million years later. Though accurate comparison of fossil records with living species is notoriously difficult, palaeontologists nevertheless believe the close similarities between arthropods and onychophora will inform the task of reconstructing a common arthropod ancestor.

Behaviour

Studies have revealed evidence of strong social hierarchies. For instance, one species forms matriarchal colonies controlled by one dominant female. For these groups, hunting is a communal activity in which the subsequent spoils are always enjoyed according to rank, and queue-jumping inferiors are routinely punished for code transgressions.

Sexual reproduction is the norm for onychophora - though parthenogenic reproduction occurs in one female-only species. Following fertilisation, and a gestation period which can extend to 15 months, almost all onychophora bear live young. There is no visible developmental phase, so at birth, each tiny worm arrives looking exactly like a small adult replica.