This study undertakes a cytogenetic analysis of the New Zealand blackfly species Austrosimulium australense (Schiner). The principles of such an approach are outlined, and previous taxonomic studies of the genus Austrosimulium, in particular the taxonomic position of A. australense, are discussed. Populations from North Island localities covering a wide area were sampled and analysed for polymorphisms in the polytene chromosomes, taken from salivary glands of larvae. In all, 1018 larvae from 49 sites were analysed. A polytene chromosome map of A. australense is presented, with details of chromosomal inversions found in salivary gland cells. Three geographical zones are designated, according to the incidence of certain chromosomal polymorphisms.

We present population cytogenetic data from an analysis of polytene chromosomes of individuals of the common New Zealand blackfly Austrosimulium australense (Schiner). These data show that, in an area of the central North Island of New Zealand (Tarawera), there are, firstly, a number of abrupt changes in inversion polymorphisms between this area and adjacent collection sites. Secondly, there is non-random association of heterozygous inversion pairs in samples from the Tarawera area. Thirdly, we present evidence of a deficiency of heterozygotes for a number of inversions found in this area. We conclude that this may provide evidence for the existence of cryptic species within our samples. We discuss this possibility in terms of the morphological and genetical changes which are likely to occur at speciation, and briefly discuss the mounting evidence for the ubiquity of cryptic species in Diptera.