In the southwestern suburbs of Sapporo, northern Japan, the flightless leaf beetle Chrysolina angusticollis species complex is represented by three phenotypically distinct forms which are distributed in a strictly parapatric manner. Boundaries between the forms usually coincided
with narrow topographic barriers such as streams and cliffs, and areas with low density of host plants. In only four out of 237 sites was more than one form discovered in sympatry. In these exceptional sites, putative hybrid individuals were also found. This stricdy parapatric distribution
and coincidence of boundaries with topographic barriers suggest that the boundaries are tension zones, i.e. hybrid zones maintained by dispersal of native individuals from outside of the zones and selection against hybrids in the zones.