Chalcidoidea are less resistant to rhythmical floodings with sea water than other arthropods from the intertidal region of the North Sea coast. The reactions of specimens from the intertidal region and from inland are alike. The resistance of small species (up to 3.5 mm) inereases in proportion to their body length, whereas the resistance of larger species decreases with length.The spiracles in the propodeum of all species are easily reached by water. It is likely that those Chalcidoidea that have spirales with a diameter of more than 50 μ are damaged by penetrating water. In the Cynipidae, however, which measure more than 3.5 mm in length and have spiracles of more than 50 μ in diameter, the propodeum is covered with hairs which retain a thin film of air under water and thus protect the spiracles.Morphology and behaviour of the adult Chalcidoidea from the intertidal region are not adapted to the floodings. The species only survive because of the predispositions of their juvenile stages. The most important condition of their existence are their hosts.