Allozyme diversity and population genetic structure of the caddisfly Orthopsyche fimbriata and the mayfly Acanthophlebia cruentata in New Zealand streams

1. Allozymes were used to measure genetic variation within and among region al populations of the caddisfly Orthopsyche fimbriata and the mayfly Acanth ophlebia cruentata in North Island New Zealand streams. 2. High levels of genetic differentiation were recorded in populations of O . fimbriata within and among catchments separated by more than 100 km, but little or no differentiation in populations separated by around 10 km. The Auckland isthmus appears to be a major barrier to north-south gene flow, wi th nearly fixed allelic differences at one locus. Genotype frequencies conf ormed to Hardy-Weinberg equilibrium. 3. Acanthophlebia cruentata had low levels of genetic variation; the result s are unexpected given that O. fimbriata apparently has greater potential f or dispersal. The limited genetic data for A. cruentata provided evidence f or genetic differentiation among populations separated by around 100 km, or more, within catchments and some differentiation between catchment populat ions separated by only 25 km.