Onychophorans

Onychophora, commonly known as velvet worms, are a small phylum of soft-bodied, predatory invertebrates found primarily in moist tropical and subtropical regions. Described species are grouped into two living lineages: Peripatidae, which are mostly tropical, and Peripatopsidae, which occur in temperate regions of the Southern Hemisphere and parts of Asia. With roughly 180 described species, onychophorans are a key group for understanding early arthropod evolution.

Physiology and morphology: Velvet worms resemble small, worm-like caterpillars and possess numerous pairs of short, stubby legs. The number of leg pairs varies by species, typically ranging from about 13 to more than 20. They have antennae, a pair of jaws, and oral papillae that eject adhesive slime. The slime glands are located in the head region and are used to entangle prey. Gas exchange occurs through a system of tracheal tubules connected to surface spiracles, and the body accommodates a hydrostatic skeleton that aids movement.

Ecology and behavior: Velvet worms inhabit leaf litter, soil, and decaying wood in humid environments and are usually nocturnal and slow-moving. They are carnivorous, feeding on small invertebrates. Prey capture involves a rapid discharge of sticky slime to immobilize targets, followed by biting with their jaws.

Reproduction and development: Reproduction is sexual and involves internal fertilization. Reproductive modes vary among species, with some laying eggs (oviparous) and others giving birth to live young (viviparous). Development is direct, with juveniles resembling miniature adults.

Evolution and conservation: Onychophorans are often cited as a close living relative of arthropods, providing insight into the evolution of segmented bodies and jointed limbs. The fossil record is sparse due to soft bodies, but related lobopod fossils illuminate early terrestrial adaptation. Many species have restricted ranges and are threatened by habitat loss and climate change.