The phylum Onychophora includes around 110 described species (and likely a similar number undescribed) of caterpillar-like relatives of arthropods. The first description of a living onychophoran was in 1826 (misinterpreted as a leg-bearing "slug", a mollusc). All living species are terrestrial, but a number of marine species are known from the fossil record going back to the Early Cambrian over 500 million years ago. Living Onychophora are found in humid habitats, stashing themselves away in burrows or other retreats and becoming inactive during dry periods. During wet periods, they can be found sifting through leaf litter. One particularly unusual aspect of onychophoran behavior is their method of prey capture, which involves shooting twin streams of a rapidly hardening adhesive slime up to 30 cm to entangle their prey, as can be seen here. (Brusca and Brusca 2003) With the exception of just a few species, onychophorans have not been well studied and the New World fauna is especially poorly known (Read 1988).
Conservation and Management
New (1995) reviewed the overall conservation status, threats, and conservation strategies for the phylum Onychophora.
Nearly all onychophorans are nocturnal carnivores that prey on small invertebrates such as snails, worms, termites, and other insects, which they pursue into cracks and crevices. Onychophora hunt by shooting twin streams of a rapidly hardening adhesive slime up to 30 cm to entangle their prey, as can be seen here. The jaws are used to grasp and cut up prey, which are partially digested by salivary secretions before the remaining semiliquid tissues are sucked into the mouth. (Brusca and Brusca 2003)
Barclay et al. (2000) showed that the males of the onychophoran Euperipatoides rowelli secrete a pheromone which acts as an attractant to both males and females of the species. Based on their studies of patterns of colonization of decomposing logs and differences in sex ratio between incipient versus established populations, the authors concluded that males are the initial dispersers and colonizers, finding suitable log habitats in an exploratory fashion, while females subsequently colonize logs. This results in a disproportionately high frequency of males in newly colonized logs, followed by a gradual increase in female proportion.
Modern onychophorans range from 5 mm to 15 cm in length and bear a superficial resemblance to caterpillars. Within a species, males are always smaller than females and have fewer legs. The head bears a pair of fleshy antennae, a pair of jaws, and a pair of fleshy oral papillae (or "slime papillae") adjacent to the jaws. Circular lips surround the jaws and a pair of eyes are located at the bases of the antennae. Onychophora have 13 to 43 pairs of simple saclike (not jointed or segmented) walking legs, or lobopods, each bearing a claw ("Onychophora" is derived from the Greek for "claw bearer"). Most onychophorans are distinctly colored blue, green, orange, or black, often with a velvety sheen (hence the common name "velvet worm"). Onychophorans move by extending and contracting the body using hydrostatic forces, which cause waves of contraction to pass from anterior (front) to posterior (rear). (Brusca and Brusca 2003)
Most onychophorans inhabit moist forests, but some species also occur in drier woodlands and mesic grasslands and in caves (New 1995; Barclay et al. 2000 and references therein).
The phylum Onychophora comprises two families, Peripatidae, which has a circumtropical distribution, and Peripatopsidae, which has a circumaustral distribution (i.e., confined to the temperate Southern Hemisphere) (Brusca and Brusca 2003). Few species appear to be widespread, and some cave-dwelling species are known from just a single cave (New 1995).
Local population densities of onychophorans may be quite high. For example, Sunnucks et al. (2000) report that as many as 150 Euperipatoides rowelli can be found in a single log.
Onychophorans probably live for several years, molting periodically (as often as every two weeks in some species) (Brusca and Brusca 2003).
With the exception of a single known parthenogenetic (all-female) species, all known oncychophorans have separate male and female individuals. Copulation has been observed in only a few onychophorans. In the southern African Peripatopsis the male deposits spermatophores apparently randomly over the female's body surface. The presence of the spermatophores stimulates a localized breakdown of the female's integument beneath the spermatophore, allowing sperm to pass into her body and eventually reach her ovaries, where fertilization takes place. Females may store sperm for many months (>9.5 months in one study) before using them to fertilize eggs. Most living Onychophora bear live young, although some species lay eggs. All onychophorans are direct developers (i.e., have no larval stage). (Sunnucks et al. 2000; Brusca and Brusca 2003)