Look closely at this Narceus millipede and you’ll see two small hitch-hikers (one near the center of the coil and one at opposite end of the millipede). These mites have a very close relationship with their millipede host.
Narceus millipedes live on and in dead logs. At this time of year, they gather together in writhing mating tangles. Because the adult mites never wander away from their millipede, this is their only chance to meet other mites, so they also breed now, having synchronized their life-cycle to that of their host. Both millipede and mite will then overwinter buried in logs or the soil. In spring, both species lay eggs in millipede frass (dung) in logs; the eggs hatch out together.
The millipedes and mites live for several years. Millipedes feed on fungi and dead wood. Adult mites feed on exudations from the millipedes’ bodies. Young mites wait for millipedes to hide from the sun inside a log, then wander off the host to feed on small invertebrates.
This whole family of mites, the Heterozerconidae, live exclusively on millipedes.
For more info, see: Gerdeman and Garcia (2010). Heterozerconidae: A comparison between a temperate and a tropical species. Trends in Arachology.