The biology and parasitism of Centrotrombidium schneideri Kramer, 1896 (Johnstonianidae) and Valgothrombium major (Halbert) 1920 (Microthrombidiidae) were investigated in the field and in the laboratory. For the larval instars of both species taxonomical descriptions are given. From laboratory experiments, a confinement of both species to hygric biotopes was established. It appears that both species have univoltine life cycles. The postlarval instars of both species prey on larvae of Nematocera. The larvae of both species parasitize Culicoides sp. (Diptera Ceratopogonidae) and display an unique host recognition behaviour (hitherto unknown for terrestrial Parasitengonae), in that they recognize the pupae of the host. The larvae then wait for the imaginal host instar to emerge. The parasitic phase is restricted to the imaginal host instar. This type of host recognition is evidently a good tactic when short-lived instars of Insecta serve as hosts. The relatively high host specificity for hygrophilic Ceratopogonidae decreases the risk of the mite larvae being displaced into unsuitable biotopes.