Onychophora (velvet worms, spitting worms)

Onychophora, velvet worm fast facts

  • Worm-like and bilaterally symmetrical.
  • 14 - 43 pairs of fleshy, short, unjointed legs ending in a pad and a pair of claws
  • Tubular tracheae (tubes carrying air to inside of body) from numerous irregularly placed spiracles (tracheal opening on body surface).
  • A mouth with a pair of claw-like mandibles, a straight gut and anus
  • A hydrostatic skeleton with a tubular heart
  • A thin, chitinous cuticle-covered epidermis
  • Layers of circular, oblique and longitudinal smooth muscles
  • Fertilisation is internal via spermatophores (sperm enclosed in a protective packet).
  • Development is direct
  • Free-living and terrestrial
  • Mainly nocturnal
  • Greek: onychos = claws; phoros = bearer

Onychophora overview

Onychophorans are commonly known as the velvet worms. They got this name because their skin is covered in small bumps many of which have a small filament sticking out of the top.

There are about 120 species, and they range in length from 1.5 - 15 cm. They are found in tropical and sub-tropical forests.

Velvet worm evolutionary relationships

For many years they were thought to be the missing link between the worms and the Arthropods, because they share characteristics with both. The fossil record shows that they have changed little in the last 500 million years.

However it is now believed that their Arthropod-like characteristics are examples of parallel evolution, and that they are not an Arthropod ancestor. The main reason for this is that their tracheae form numerous branches near the spiracle opening, but rarely ever branch again. Also the spiracles appear to be randomly located on the body surface and have no closing mechanism. This lack of closing mechanism confines them to the most humid of environments such as the forest floor, leaf litter and under stones. In fact in an ordinary room Peripatus (below) can lose as much as one third of its body weight in moisture in just 4 hours.

It is now believed that they evolved from the nematodes some time during the Cambrian.

Velvet worm body

Velvet worm cuticle never hardens like Arthropod cuticle, and is not moulted all at one time, but in patches. On the head they have a pair of antennae each with an eye at the base, the eye is similar to Annelid eyes (see Peripatopsis capensis below right).

Their legs are unjointed, operated hydraulically, and end in a pair of claws. The colour of the upper body varies from dark grey, green, brown or red; with the underneath whitish or light coloured.

Their mouth has a pair of claw-like mandibles and papillae. The papillae are the exits for a pair of slime glands.

Onycophora, velvet worm

Onychophoran hunting and defensive behaviour

Onychphorans are predators and they disable their prey by squirting out a glue-like slime from the slime glands, usually from a distance of 0.5 cm, but for spiders and other dangerous prey they can aim from as far as 4 cm away. They can squirt up to 30 times aiming at the legs and jaws of dangerous prey, but for less dangerous prey they may squirt only once. Their prey consists of small insects such as termites, crustaceans such as woodlice, and they will also eat larger insects such as crickets when they find them already dead.

The slime rapidly hardens on contact. When the prey is immobilised the onychophora bites through the exoskeleton and injects saliva to digest the body contents. While waiting for the saliva to work the slime is eaten as it is a protein and so too nutritious to waste. The slime is also used in defence. Their usual prey is insects, snails and worms.

Onychophora, Peripatopsis capensis
Peripatus, Onychophora

Velvet worm sex life

Onychophora have, to us, a rather lacklustre sex life. A male packages his sperm into a bundle called a spermatophore. Having succeeded in doing this, it appears he feels he has has done all that can reasonably be expected of him. So then deposits his bundle on the next onychophoran he meets, whether it is a male, female or juvenile. Then wanders off. If the spermatophore lands on a mature female the skin beneath the spermatophore dissolves, and the sperm enters the body cavity, where some eventually migrate to the ovaries where fertilization occurs.

Males are usually a little smaller than females, and often have fewer legs.

 
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