Care guide · Order Spirobolida
Black Round Millipede
Tylobolus uncigerus (Cook, 1904)
Tylobolus uncigerus (Cook, 1904)
Tylobolus uncigerus is the only member of the order Spirobolida found west of the Cascades — its cylindrical, glossy body sets it immediately apart from the flat-backed polydesmids also common in the Pacific Northwest. Adults reach up to four inches in length and are dark brown to black with smooth, gently arched segment rings. A diagnostic groove (suture) runs down the middle of the frons to the labrum, shared by all Spirobolida. Eyes appear as two widely separated patches of ocelli on either side of the head.
Juveniles are smaller and paler — they may be confused with members of Bollmaniulus or Litiulus, but those genera are notably narrower and often patterned. Introduced julid millipedes can also look superficially similar but have striations across the entire segment surface, which T. uncigerus lacks. If you found yours in moist forest under a log or in deep leaf litter west of the Cascades, this is almost certainly your species.
Tylobolus uncigerus occupies moist to mesic forests and woodlands west of the Cascades, ranging from southwest Washington through Oregon into northern California. It can be found in second-growth riparian forest, Douglas fir stands, and mixed hardwood slopes — sheltering beneath logs, stones, and deep accumulations of decaying leaf litter, emerging during and after rain to forage on the surface.
As a detritivore it plays a genuine ecological role: adults feed on decaying leaves, humus, and rotting wood; juveniles feed on fine humus particulate. Some evidence suggests they consume their own frass to replenish gut bacteria. Unlike the yellow-spotted millipede (Harpaphe haydeniana) which produces hydrogen cyanide, T. uncigerus relies on benzoquinones and hydroquinones — acrid and foul-smelling, irritating to skin, but significantly less acutely toxic.
When threatened, T. uncigerus curls into a tight coil and exudes secretions from repugnatorial glands on most body segments. The smell is unpleasant and acrid. This only happens when the animal feels genuinely stressed — calm, minimal handling usually won't trigger it.
A 4–6 quart container works for two juveniles; plan to upsize as they grow. Cross-ventilation is essential — mesh on both lid and at least one side panel prevents stagnant air. No supplemental lighting needed; keep away from direct sunlight entirely.
| Parameter | Recommendation |
|---|---|
| Size (juvenile) | 4–6 qt tub for two individuals minimum |
| Size (adult) | 10+ qt — adults reach 4 in and need room to move and moult undisturbed |
| Ventilation | Cross-ventilation essential — mesh lid + side panel. No sealed containers. |
| Lighting | None. No direct sunlight — overheats and desiccates rapidly. |
Target 60–68 °F (15–20 °C). Portland homes are mostly fine year-round, but summer heat events are genuinely dangerous — move the enclosure to a basement or the coolest room available when temperatures rise. A natural cool-down toward 55 °F in winter is likely beneficial and may support breeding behavior, though this is unconfirmed specifically for this species.
The second most critical factor after temperature, and arguably the most important for long-term health. The substrate is both the home and the primary food source — upwards of 80% of the diet comes from consuming it directly. An indigestible filler like coconut coir doesn't just add nothing; it physically displaces food and can cause impaction. Every component of the mix needs to be something the animal can eat.
For a PNW native like T. uncigerus, locally collected material is strongly preferred over commercial substrates. Ideally, source wood and leaf litter from the same habitat type the animals were collected from. Alternatively, flake soil (fermented hardwood) can serve as the digestible base — the same standard used for tropical species.
| Component | Notes |
|---|---|
| Rotting hardwood (primary) | Well-rotted alder, bigleaf maple, or Oregon white oak. Soft, crumbly, and dark — not just dried or surface-mouldy. 50–70% of mix. This is the main food. |
| Mixed leaf litter | Alder and maple preferred. Layer 1–2 in on surface; replenish continuously as consumed. Partially broken-down is better than bone dry. |
| Forest topsoil | Unfertilized woodland topsoil from a clean site. Adds microfauna and organic structure. Sterilize before use. |
| Flake soil (alternative) | Fermented hardwood can substitute or supplement locally collected wood if field material isn't available. Use oak or alder base. |
| Depth | Minimum 3–4 in, but deeper is always better. Substrate depth should match or exceed the animal's body length. Juveniles moult entirely buried — inadequate depth is a moult risk. |
| Avoid | Coconut coir (impaction risk). Pine, cedar, fruit tree wood — terpenes and compounds in these are harmful. Fertilized, treated, or potting soil. Peat moss. |
Bake collected wood and soil at 250 °F for 30 minutes before use to clear mites and parasites. Let cool fully before adding animals. Leaf litter can be frozen for 48 hours as a gentler alternative.
Target 70–80% relative humidity. The substrate should feel like a wrung-out sponge — damp throughout but no standing water. Lightly mist one side every 2–3 days, leaving the other slightly drier to create a moisture gradient. Wipe away heavy condensation. Good ventilation and maintained moisture work together, not against each other.
The substrate is the primary diet — 80%+ of intake comes from consuming it directly. Supplemental feeding supports this but doesn't replace it. For juveniles especially, a well-structured substrate is more important than any surface offering. Protein supplementation is recommended weekly; the millipede exoskeleton is chitin-based and requires protein for proper formation and moult recovery. Fruits and vegetables are treats, not dietary staples.
| Food | Frequency | Notes |
|---|---|---|
| Rotting hardwood & leaf litter | Continuous | Alder, maple, oak — the main diet. Replenish leaf litter layer as it disappears. |
| Protein — fish flakes | Weekly | Covers protein and mineral needs efficiently. A small pinch on the substrate surface. |
| Protein — freeze-dried minnows | Weekly | More substantial. One or two per feeding for juveniles. |
| Mushroom | Weekly or biweekly | Accepted and natural. Oyster mushrooms or wild hardwood fungi. |
| Vegetables (treat) | Every 1–2 weeks | Squash, cucumber. Nutritionally limited but accepted. Remove within 24–48 hrs. |
| Cuttlebone | Continuously | Leave a piece in the enclosure at all times. Important around moults. |
| Avoid | — | Citrus, onion, anything salty or processed. Fruit tree wood (apple, cherry). Pine, cedar. |
Juveniles add segments and legs with each moult, gradually darkening toward adult coloration. A moulting animal will burrow deep and go completely still — do not disturb the enclosure or probe the substrate. Disturbing a moulting millipede can be fatal. Leave freshly moulted animals alone for at least a week; they'll consume their shed exuviae for calcium, so don't remove it. Growth rate is undocumented for this species — expect it to be slow.
Keep it minimal, especially with juveniles. They'll likely curl tightly when picked up — let them uncurl on their own, never force it open. Scoop from below rather than grasping. These are better as display animals than handling animals; watching through the enclosure walls is the gentlest interaction. Minimize substrate disturbance too.
Both were found under a large rotting alder log in a shaded ravine. The substrate there was very deep — easily six inches of accumulated leaf litter and wood fragments above clay-heavy soil. They were found together, which felt like a good sign.
At time of collection they were clearly juvenile — much paler than adult photos I'd referenced, and quite small. They burrowed within minutes of being placed in the enclosure. One disappeared entirely for about a week, presumably moulting. It emerged noticeably darker.
Life history documentation for this species in captivity is essentially nonexistent. Everything here is provisional. I'll update as I learn more from watching them.