The courtship and copulation behaviour of the inquiline gall wasp Synergus japonicus complex, which consists of sp. A and sp. B, was studied under laboratory conditions. Copulation occurred between sp. A adults emerging from two different types of galls (AS and AL). Once copulated, females of AS and AL never copulated again. During courtship, males of both AS and AL did not discriminate between virgin (potentially receptive) and once-copulated (non-receptive) females. The patterns of courtship and copulation behaviour of the two sibling species were the same, but the duration of acceptance posture after copulation in sp. B females was significantly longer than that in sp. A females. No copulation occurred between the two species, but females of each species possessed some degree of attractiveness to non-conspecific males. The results of the experiments suggest that the overwintering generation of sp. A depends not only on normal unisexual galls of Andricus mukaigawai but also on unisexual galls of A. mukaigawai which have already been inhabited by sp. B. The term "multi-inquilinism" is proposed to denote that a host gall fosters more than one inquiline species.
