We provide the first complete description of the water requirements for the hissing-cockroach mite, Gromphadorholaelaps schaeferi, focusing on characteristics that result from the restriction of all stages to the Madagascar hissing-cockroach (Gromphadorhina portentosa). Particularly, we determine how G. schaeferi spends its entire life on the same individual cockroach. This mite is not parasitic, rather they feed on cockroach saliva and other moist organic debris that accumulates between the cockroach's legs. Water balance characteristics of this mite show that it is extremely hydrophilic and that it must maintain a high percentage body water content to function properly despite being very porous (high net transpiration rate) and sensitive to water loss, tolerating only 20% loss of their water content before death. This hydrophilic trend starts with the larva and is retained into adulthood. Developmentally, a shift occurs during postlarval development from an emphasis on water gain (low critical equilibrium activity for water vapor absorption from drier air) in the protonymph to an emphasis on water retention (reduced net transpiration rate for water conservation) in the adult. The minute-sized larva is prevented from replenishing water stores by water vapor absorption or feeding because it lacks functional mouthparts, thus dries up rapidly. To avoid dehydration and survive, the larval stage utilizes a quick shoot-through molt to the protonymph that can feed and grow. Our conclusion is that the hissing-cockroach creates an ideal, stable moisture-rich microhabitat that satisfies the high water demand for G. schaeferi during all stages, fixing this mite to a single cockroach as an ecological niche.