VELVET WORMS: Onychophora

PHYSICAL CHARACTERISTICS

Velvet worms have bilateral symmetry (bye-LAT-er-uhl SIH-muh-tree) and can only be divided into similar halves along one plane. They resemble caterpillars and have long, soft, and flexible bodies. Adults measure from 0.5 to 8 inches (13 to 203 millimeters) long. Most range in color from black to blue, red, brown, or gray. Some species are striped or have beautiful patterns. Their skin, or exoskeleton, is very elastic and covered with small bumps called papillae (pah-PIH-lee). The papillae are made up of small scales that give them a velvety appearance. Larger papillae have a single sensory bristle that helps velvet worms to feel their surroundings.

Along the sides of the bodies are breathing holes, or spiracles (SPIH-reh-kulz). The spiracles lead to a network of respiratory tubes inside the body, similar to insects and spiders. The head has a pair of soft antennae, clawlike jaws, and soft, fleshy papillae on either side of the mouth. Inside the mouth is a rough, tonguelike structure that helps to grind food. The cone-shaped legs are stumpy and unsegmented, but there are 13 to 43 pairs of legs, depending on the age, sex, and species of velvet worm. Each leg is tipped with 3 to 5 pads and a pair of claws.

GEOGRAPHIC RANGE

Velvet worms are found in Mexico, Central America, Chile, tropical West Africa, South Africa, southeast Asia, New Guinea, Australia, and New Zealand.

HABITAT

All velvet worms live in leaf litter, under stones or logs, or in soil in moist and humid habitats, such as tropical and subtropical forests.

DIET

Velvet worms are carnivores (KAR-nih-vorz), or meat eaters, and eat mainly insects, spiders, other arthropods, and snails.

BEHAVIOR AND REPRODUCTION

As with arthropods, which include insects, spiders, and their relatives, velvet worms must molt, or shed their exoskeleton, in order to grow. These secretive animals capture prey with threads of clear, sticky slime shot from the oral papillae. The slime is also used to discourage predators and can be squirted up to 1.6 feet (0.5 meters) in distance. During the dry season, or periods of low temperature, velvet worms crawl down into crevices in the soil and remain there until conditions on the surface improve.

Mating has been observed in very few species. Males produce special chemicals, or pheromones (FEH-re-moans), from glands located at the bases of their legs to attract females. In some species, males deposit sperm packets directly into the female's reproductive opening. In other species, the packets are placed on the female's body and are absorbed directly through the exoskeleton. The sperm is sometimes stored for several months before the eggs are fertilized.

Some velvet worms deposit their eggs in the soil, and the young develop and are nourished inside the egg until they hatch later. Others also produce eggs, but they hatch inside the female's body and young are born live. A few species give live birth to young that are nourished by the mother's body until they are born, headfirst. Whether born or hatched, all young velvet worms resemble small adults.

A MISSING LINK?

Velvet worms have changed very little since their marine ancestors first came on land about 400 million years ago. Discovered in 1826, velvet worms were first thought to be a kind of slug. Later scientists realized that they shared features of both segmented worms (Annelida) and arthropods, and they were placed in their own phylum. They have been called the "missing link" that connects these two groups, but recent studies show that they are more closely related to arthropods than annelids.

VELVET WORMS AND PEOPLE

Velvet worms are particularly valuable research animals. Their distributions are studied to help track the movements of continents over millions of years. Their sticky slime is also being studied as a possible glue for special kinds of surgery.

CONSERVATION STATUS

Eleven species of velvet worms are listed by the World Conservation Union (IUCN). Three species are listed as Critically Endangered, which means they face an extremely high risk of extinction in the wild. Two species are listed as Endangered or facing a very high risk of extinction in the wild. Four species are listed as Vulnerable or facing a high risk of extinction in the wild. One species is listed as Lower Risk, or at risk of becoming threatened with extinction in the future. Another is Data Deficient and lacks sufficient information to determine its vulnerability to extinction. The greatest threat to their existence is habitat loss.

NO COMMON NAME (Epiperipatus biolleyi): SPECIES ACCOUNT

Physical characteristics: Epiperipatus biolleyi adults measure up to 1.5 to 2 inches (38 to 52 millimeters) in length and are rusty brown or pinkish with dark papillae and a stripe along the back. Antennae and legs are gray. Females have 30 pairs of legs, while males have 26 to 28.

Geographic range: They are found in Costa Rica.

Habitat: Epiperipatus biolleyi (abbreviated as E. biolleyi) live in low mountain forests inside rotting logs or in natural cavities in the soil.

Diet: Nothing is known about their diet in the wild.

Behavior and reproduction: E. biolleyi avoid light and walk at speeds up to 0.4 inches (10 millimeters) per second. Individuals found in the wild often carry scars and missing legs.

Sperm packets are deposited directly into the reproductive opening. They are viviparous.

Epiperipatus biolleyi and people: E. biolleyi are not known to impact people or their activities.

Conservation status: This species is not considered endangered or threatened. ∎

FOR MORE INFORMATION

Books:

Tavolacci, J., ed. Insects and Spiders of the World. Volume 9, Stonefly-Velvet Worm. Velvet Worm. New York: Marshal Cavendish, 2003.

Periodicals:

Ghiselin, M. T. "A Moveable Feaster." Natural History 94, no. 9 (September 1985): 54-60.

Mendez, R. "Keeping a Missing Link—The Velvet Worm." 1997 Invertebrates in Captivity Proceedings, 72-74.

Monge-Nágera, J., and J. P. Alfaro. "Geographic Variation of Habitats in Costa Rican Velvet Worms (Onychophora: Peripatidae)." Biogeographica 71, no. 3 (1995): 97-108.

New, T. R. "Onychophora in Invertebrate Conservation: Priorities, Practice and Prospects." Zoological Journal of the Linnean Society 114, no. 1 (1995): 77-89.

New, T. R. "Velvet Worms: Charismatic Invertebrates for Conservation." Wings 27, no. 2 (Fall 2004): 12-15.

Sunnucks, P., and N. Tait. "Tales of the Unexpected." Nature Australia 27, no. 1 (2001): 60-69.

Web sites:

Introduction to the Onychophora.http://www.ucmp.berkeley.edu/onychoph/onychophora.html (accessed on January 18, 2005).

The Onychophora.http://www.mnhn.fr/assoc/myriapoda/ONYCHO.HTM (accessed on January 18, 2005).

Onychophora Homepage.http://www.sciref.org/onychophora/ (accessed on January 18, 2005).