Onychophora

Onychophora (from the Greek onyx, "claw," and pherein, "to bear," commonly known as velvet worms or peripatus) is a phylum of terrestrial, nocturnal, ambush-predatory invertebrates belonging to the clade Panarthropoda (alongside Tardigrada and Arthropoda) within the molting superphylum Ecdysozoa. Comprising approximately 220 described extant species inhabiting damp rotting logs, leaf litter, and caves across tropical and southern temperate Gondwanan forests, velvet worms range from 1.5 to 20 cm in length and bear 13 to 43 pairs of unjointed, walking legs (lobopods) tipped with paired sclerotized claws.

In evolutionary biology and paleontology, Onychophora is the only surviving lineage of the Cambrian lobopodian radiation (which included marine armored lobopodians such as Hallucigenia sparsa, Aysheaia pedunculata, and Microdictyon) and represents the closest living sister phylum to Euarthropoda (Chelicerata, Myriapoda, and Crustacea / Hexapoda). Because velvet worms combine a soft, flexible, non-calcified α-chitin** cuticle** (periodically molted under ecdysteroid control) and an annelid-like muscular-hydrostat body wall with arthropod-like hemocoelic circulation, dorsal ostiate heart, tracheal respiration, and appendage-derived jaws, they provide critical insight into how the segmented arthropod body plan was assembled during the Cambrian explosion.

1. Hydrostatic Velvet Cuticle and Unjointed Lobopods: The body wall is covered by a thin (~1–2 μm), flexible α-chitin and protein cuticle studded with microscopic, overlapping hydrophobic scales and sensory bristles (sensilla) that give the animal a water-repellent, velvety sheen. Beneath the epidermis lie three continuous smooth-muscle layers (outer circular, middle diagonal, and inner longitudinal) surrounding a blood-filled hemocoel partitioned into a median and two lateral sinuses. Rather than relying on jointed exoskeletal levers, each lobopod is a hollow fluid-filled cone extended by local hemocoelic blood pressure and swung forward and backward by intrinsic leg muscles, terminating in an eversible distal foot pad (planta) bearing two curved chitinous claws.
2. Unspecialized Spiracles and Obligate Humid Microhabitats: Velvet worms breathe air via hundreds of tiny, unbranched tracheal tufts that pipe gaseous O₂ directly to internal tissues. However, unlike insects, myriapods, and arachnids—which evolved muscular valved spiracles and waxy epicuticular hydrocarbons to prevent respiratory water loss—onychophoran spiracles (stigmata) are permanently open pits ( up to 75 per body segment) that cannot be closed and their cuticle lacks a waterproof lipid barrier. Consequently, velvet worms lose water by evaporation ~40 times faster than caterpillars of comparable size, restricting all living species to saturated microhabitats (relative humidity >90%) inside rotting wood and soil crevices and driving them to periodic ecdysis every 2–3 weeks (after which they immediately eat their shed exuvia to recycle nitrogen and chitin).
3. Cephalic Appendage Homology (Six3, Otd, and Hox Alignment): Developmental gene expression studies by Georg Mayer, Maximilian J. Telford, Graham E. Budd, and Ralf J. Sommer resolved a century-long debate over how the three anterior head segments of Onychophora align with the head segments of Euarthropoda:
   • Segment 1 (Protocerebral / Ocular Segment): Bears the paired annulated antennae (which are therefore protocerebral appendages homologous to the frontal filaments of crustaceans and the raptorial "great appendages" of Cambrian radiodonts like Anomalocaris, and not homologous to the deutocerebral first antennae of insects, myriapods, and crustaceans!).
   • Segment 2 (Deutocerebral Segment): Bears the paired internal sickle-bladed jaws (mandibles), which move anteroposteriorly inside the mouth cavity to slice prey (homologous to the chelicerae of Chelicerata and the first antennae of Mandibulata).
   • Segment 3 (Tritocerebral Segment): Bears the paired slime papillae flanking the head (homologous to the pedipalps of chelicerates and the second antennae of crustaceans).

To capture fast-moving crickets, cockroaches, and woodlice despite walking at only ~1–2 cm/s, velvet worms deploy a biomechanical weapon unique in the animal kingdom—oscillating liquid-crystalline slime jets:

1. Microfluidic Syringe Ejection and Aero-Elastic Whipping Oscillation: Paired slime glands running up to two-thirds the length of the body store a viscous aqueous solution (~90% water, ~5% high-molecular-weight intrinsically disordered proline-rich slime proteins, plus lipids, lectins, and surfactant nanoglobules) inside muscular reservoirs. When prey touches the antennae or stimulates air-current mechanoreceptors, the body wall contracts rapidly (~0.1–0.2 bar), forcing the slime from a wide reservoir through the narrow nozzle (~200 μm diameter) of the slime papillae at velocities of 3 to 5 m/s. As shown by Andrés Concha et al. (2015) using high-speed videography, fluid inertia flowing through the flexible, accordion-hinged papilla nozzle drives an aero-elastic fluid–structure instability (analogous to an untied garden hose whipping back and forth), causing the two papillae to oscillate at ~30–60 Hz and spray a crisscrossing net of sticky threads across a 3–5 cm target area in <60 milliseconds without requiring fast muscles or neural control!
2. Shear-Induced Nanoglobule Unfolding and Reversible Glassy Fiber Assembly: Inside the gland reservoir, the negatively and positively charged slime proteins are folded and sequestered with lipid–protein nanoglobules (~100–200 nm diameter) so the stored liquid does not clog the duct. As demonstrated by Matthew J. Harrington, Alexander Baer, and colleagues (2017), the high extensional and shear strain rates (>10⁴ s⁻¹) inside the constricting nozzle rupture the nanoglobules and stretch the disordered proteins into β-sheet-rich fibrils cross-linked electrostatically by divalent cations (Ca²⁺ and Mg²⁺) and phosphorylated residues. Within seconds of striking the prey and air-drying as the struggled insect pulls against the threads (draw-spinning), the slime solidifies into stiff, transparent polymeric glass fibers (Young's modulus E ≈ 4–5 GPa, comparable to nylon) that immobilize the prey.
3. Complete Water-Triggered Dissolubility and Recycling: Unlike spider silk or insect silk (which cross-link irreversibly into insoluble crystalline β-sheets), solidified velvet-worm slime fibers dissolve completely when placed in plain water and re-assemble into ~100 nm nanoglobules that can be drawn into stiff 4 GPa fibers again! In nature, after injecting digestive saliva into the immobilized prey and sucking out its liquefied tissues, the velvet worm eats its own dried slime threads to recover the metabolically expensive proteins.

All living Onychophora belong to the order Euonychophora, split since the breakup of the supercontinent Gondwana (~180–140 Ma) into two strictly allopatric families that exhibit virtually every reproductive mode known in animals:

How did the 2015 re-interpretation of Hallucigenia sparsa clarify the evolution of Onychophora and Panarthropoda?

Originally described from the Middle Cambrian Burgess Shale (~508 Ma) by Charles Doolittle Walcott in 1911 and famously reconstructed upside-down (walking on its rigid dorsal spines) by Simon Conway Morris in 1977 before Lars Ramsköld (1992) flipped it right-side-up onto its paired clawed lobopods, Hallucigenia sparsa was re-examined using electron-microscope elemental mapping by Martin R. Smith and Jean-Bernard Caron (2015). They discovered a smiling ring of radial circumoral plates and pharyngeal teeth in the anterior head (which had previously been mistaken for a tail decay stain!) along with cone-in-cone stacked jaw and claw elements identical in microstructure to the stacked chitinous claws and jaws of modern Onychophora—confirming Hallucigenia as a stem-group onychophoran and proving that the circumoral pharyngeal armature of Scalidophora and Nematoda was present in the ancestral panarthropod before being lost in modern velvet worms.

Why do some male Australian velvet worms carry spermatophores on their heads?

In scores of Australian Peripatopsidae genera (such as Florelliceps, Planipapillus, and Tasmanipatus), males possess elaborate species-specific head structures—including eversible spiked pits, rosetted crowns, or grasping cuticular hooks between the antennae—into which the male transfers a packet of sperm (spermatophore) from his posterior gonopore and presents it directly to the female's genital opening during courtship, while in South African Peripatopsis, males deposit spermatophores anywhere on the female's dorsal skin, where hemocytes lyse the underlying cuticle and allow spermatozoa to swim straight through the body wall and hemocoel into the ovaries (dermal insemination).

1. Mayer, G., Kato, C., Quast, B., Chisholm, R. H., Landman, K. A., & Quinn, L. M. (2010). Growth patterns in Onychophora (velvet worms): Lack of a posterior addition zone and relevance for evolutionary studies onayan segmentation. BMC Evolutionary Biology, 10, 339. doi:10.1186/1471-2148-10-339
2. Concha, A., Mellado, P., Morera-Brenes, B., Sampaio Costa, C., Mahadevan, L., & Monge-Nájera, J. (2015). Oscillation of the velvet worm slime jets by passive fluid–structure interactions. Nature Communications, 6, 6292. doi:10.1038/ncomms7292
3. Smith, M. R., & Caron, J.-B. (2015). *Hallucigenia*'s head and the pharyngeal armature of early ecdysozoans. Nature, 523(7558), 75–78. doi:10.1038/nature14573
4. Baer, A., Schmidt, S., Haensch, S., Eder, M., Mayer, G., & Harrington, M. J. (2017). Mechanoresponsive lipid-protein nanoglobules facilitate reversible control of velvet worm slime stiffness. Nature Communications, 8, 974. doi:10.1038/s41467-017-01142-x
5. Oliveira, I. S., Bai, M., Jahn, H., Gross, V., Martin, C., Hammel, J. U., ... & Mayer, G. (2016). Earliest onychophoran in amber reveals Gondwanan migration patterns. Current Biology, 26(19), 2594–2601. doi:10.1016/j.cub.2016.07.023