The larvae of the water mitesHydrachna conjecta andH. cruenta are parasitic on corixids.H. conjecta larvae are attached to the underside of the hemelytra of the host and those ofH. cruenta to the legs. The growth ofH. cruenta larvae is less than that of the more shelteredH. conjecta larvae. The infection influences the egg production of the host: infectedSigara striata females have fewer eggs in the ovarioles than uninfected ones. The number of eggs in the ovarioles of the host is related to the size of the parasite. It may be assumed that the presence ofH. conjecta larvae causes a lower fecundity in the host than those ofH. cruenta. InCymatia coleoptrata, a much smaller host thanS. striata, the fecundity is even more impaired by both parasite species. For in the ovarioles of infectedC. coleoptrata females no developed eggs were found at all. So, it is clear that it is anyhow the relation between body size of host and parasite that matters.