Organisms often hitch rides on other organisms (see here and here for some recent examples). They do this for a variety of reasons. Some need to conserve energy. Others prefer to get to their destination in a more efficient manner (maybe it’s to get away from a predator). For the parasitic water mite, Partnuniella thermals, it’s because they need a host to survive.
These mites live in the hydrothermal areas of Yellowstone National Park, specifically on algal-bacterial mats. The mites use brine flies (Diptera: Ephydridae) as hosts. In Yellowstone, there are three large species of brine flies that all feed and lay eggs on the hydrothermal mats. Ephyrdra thermophila live in the acidic thermal areas, Ephydra bruesi thrive in low-productive alkaline areas, and Paracoenia turbida inhabit the highly productive alkaline pools (Collins 1977). All of them seem to be vehicles for mites.
It’s actually the mats that drive the relationship between parasite (mite) and host (fly). When the mats are young and thin, they are not accessible to the flies because they are covered by hot water. As time passes and the mats grow and exceed the the water level and divert water flow. The mats then cool to below 40˚C as they are exposed to the air. This cooling gives the flies just enough time to land, feed and lay eggs. As the larvae hatch they immediately begin feeding on the mats, which causes the mats to loose their thickness and dip back below the water. Adults or un-emerged larvae that over stay their welcome on the mats are washed out. This provides a continuous cycle of growth and regrowth for the mats, but also requires a window of perfect timing for the flies.