To date, the taxonomy of most Erythraeidae (Parasitengonae) lacks clear correlations of deutonymph and adult stages with their conspecific parasitic larval instar. Hitherto, the taxonomy in almost all cases is based upon descriptions of isolated ontogenetical stages only. In controlled rearing experiments, all instars of Leptus trimaculatus (Hermann, 1804) (Erythraeidae: Leptinae), being formerly known as adult only, were reared. A description of the larva as well as data on parasitism, times of development, phenology, nutrition and habitat requirements of all instars are given. L. trimaculatus is an univoltine erythraeid, reproducing during late spring and hibernating in the adult instar. Unlike other Erythraeidae, the larva parasitizes a broad spectrum of arthropod hosts. Both postlarval active instars prey on immobile or slow moving arthropods. All stages of L. trimaculatus show a preference for humid and temperate habitat-conditions, although they may not be restricted to them. The results are discussed in comparison with other Leptus species.