Velvet worm

Introduction

While the body structure of onychophorans is a very simple one, it works well. It has enabled them to live very successfully on land, largely unchanged in structure, for about 500 million years. Although first mistakenly described in 1826 as a type of slug, the evolutionary history of onychophorans has long fascinated scientists. They have been described as a missing link between the arthropods (a group that includes insects and spiders) and the annelids, or segmented worms (which includes earthworms and beach worms). It is now known that onychophorans are much more closely related to arthropods than to annelids.

Identification

Velvet worms belong to a phylum of their own, the Onychophora, meaning 'claw-bearers'. They are small, terrestrial (land-dwelling) worms that look rather like caterpillars, with antennae and clawed legs down the whole length of their bodies.

Velvet worms range up to about 10 cm in length, but those most often encountered in Australia are between two and four centimetres long. Apart from a few white cave-dwelling species, they are generally blue-grey or brownish in colour, often intricately and beautifully patterned, with stripes, diamonds, spots or chevrons. The body is covered by a very thin layer of chitin and the skin is covered with numerous papillae and close-set ring-like markings. The papillae, comprised of delicate rows of overlapping scales, give onychophorans their velvety appearance (explaining their common name). They make the skin hydrophobic, or water repelling. This is a useful trait to keep the skin dry when inhabiting moist environments. The papillae also have at their tips tiny hairs that are sensitive to touch and smell.

Australian velvet worms have between 14 and 16 pairs of lobe-like, stumpy legs, although species from other parts of the world may have up to 43 pairs of legs. Their characteristic flowing movement is caused by the alteration of fluid pressure in the limbs as they extend and contract along the body. This movement led to a second common name, peripatus, from 'peripatetic', which means 'wandering'.

Velvet worms also have two conspicuous antennae on the head, and, behind these, two simple-lensed eyes and a pair of modified legs, called 'oral tubes'. These are used to assist in prey capture. The paired, toothed, sickle-shaped jaws sit in the middle of a rounded, fleshy pad on the underside of the head.

Habitat

In Australia, velvet worms can be found in regions as diverse as sub-alpine south-eastern Australia, the wet tropics, and the forests of south-western Western Australia. Most species are found in relatively moist coastal areas, including tropical and temperate rainforest as well as eucalypt forests and woodlands with pockets of remnant wet forest. These habitats have become fragmented by past climatic changes and, more recently, by human disturbance.

They live in moist places, such as rotting logs and leaf litter.

Distribution

To date, only a couple of hundred species of onychophorans are known, but it is very likely that many more remain to be discovered and described. They fall into two family groups, the Peripatidae and the Peripatopsidae. The peripatids inhabit tropical America, and have also been found in tropical western Africa and south-eastern Asia. Peripatopsids are found in countries associated with the previously connected southern super-continent, Gondwanaland, such as Chile, South Africa, Australia, New Guinea and New Zealand. Of these, Australia is richly endowed with the greatest number and diversity of onychophorans, with 74 described species (compared with three species from South America, nine from South Africa, and eight each from New Zealand and New Guinea).

Feeding and diet

Despite their apparently gentle appearance, velvet worms are voracious and active carnivores, feasting on other small invertebrates (for example, termites, woodlice and small spiders) that they encounter during their travels.

Velvet worms capture their prey by squirting sticky slime from their oral tubes. The slime effectively entangles the prey so it can't escape. The velvet worm bites off parts of the prey then sucks them up after they have been softened by digestive saliva extruded from the velvet worm's mouth. Any undigested portions are excreted by the anus at the rear end of the body. Segmental excretory organs (the nephridia) also remove metabolic wastes.

The slime is also squirted in self-defence. An enemy with a face full of slime gives the velvet worm time to escape.

Other behaviours and adaptations

Velvet worms are quite secretive and display 'photonegative' behaviour, meaning they hide away from light.

Velvet worms breathe through little holes called 'trachea' that are scattered over the body. These pores are permanently open, so water from the body can easily be lost. The porous nature of their cuticle means that velvet worms can easily dry out, so they are restricted to areas of high humidity, such as in logs, under stones, in the soil, or among leaf litter.

Life history cycle

Reproduction takes place in an extremely curious manner. The male deposits packets of sperm (spermatophores) on the body of the female, for example on her back or sides. Some species deposit their spermatophores directly into the genital opening of the female. This method has been observed in a few Australian species. In those species that deposit sperm on the skin of a female, the skin tissue collapses where the sperm are deposited and the sperm migrate into the female's body, where they penetrate the ovaries to fertilise the eggs. Many species have a pair of sperm receptacles close to the ovaries, and these can be used to store sperm for many months to fertilise eggs as needed. This is a useful adaptation when encounters between males and females may occur only rarely. The males of most species have glands on the underside of the legs that secrete a pheromone to attract females.

Embryonic development is unusually diverse in onychophorans. Some species lay large yolk-filled eggs, while others retain yolky eggs within the female until they are ready to hatch. Some other species (though none of the Australian species) have small eggs without a yolky food source, and the developing young obtain nourishment from their mother's body in a manner similar to placental mammals. In all cases, the young are fully developed when born, and, apart from lacking complete pigmentation, look like miniature adults.

Breeding behaviours

In a number of Australian species, the males place their spermatophores on their heads like tiny trophies in readiness to present them to a female. Some species have developed elaborate structures, including spikes, spines, hollow stylets, pits, and depressions to either hold the sperm, or assist its transfer to the female. These structures may also have a role in species recognition, in a fashion similar to the often elaborate genital structures of insects (but this is not known for certain). Two species, Florelliceps stutchburyae and Planipapillus annae, have been observed mating. In both species, the head of the male holding a package of spermatophores was placed against the genital opening of the female at the rear end of her body and the spermatophores were transferred directly into her genital opening.

References

• Reid, A. L. 1996. A review of the Peripatopsidae (Onychophora) in Australia, with descriptions of new genera and species, and comments on Peripatopsid relationships. Invertebrate Taxonomy 10(4): 663-936.
• Storch, V. and Ruhberg, H. 1993. Onychophora. In Microscopic Anatomy of Invertebrates: Treatise Vol. 12, Onychophora, Chilopoda and Lesser Protostomata. (Eds F. W. Harrison and M. E. Rice.) Pp. 11-56. (Wiley-Liss, Inc., New York.).
• Sunnucks, P. and Tait, N. 2001. Tales of the unexpected. Nature Australia Winter 2001, 27(1): 60-69.