The greenhouse millipedes: small, banded, pantropical travelers spread worldwide by the plant trade, and now resident in warm enclosures on every continent, often arriving unintentionally in substrate.
They are effective micro-detritivores, working soil the giants ignore, and function as cleanup infrastructure in humid setups: deep litter, calcium, moisture, done. Colonies establish from small numbers and persist indefinitely.
The economics match the biology: nearly every keeper's culture produces surplus, wild-type stock is effectively free, and intentional listings price at the floor that self-spreading species enforce. The genus trades as a tool rather than a display, when it trades at all, and its most common route into a collection remains hitchhiking, which is the most honest price on the site.