SUMMARY 1. The net-winged midges (Diptera: Blephariceridae), with highly specific habitat requirements and specialised morphological adaptations, exhibit high habitat fidelity and a limited potential for dispersal. Given the longitudinal and hierarchical nature of lotic systems, along with the geological structure of catchment units, we hypothesise that populations of net-winged midge should exhibit a high degree of population sub-structuring.
2. Sequence variation in the cytochrome c oxidase subunit I (COI) region of the mitochondrial DNA (mtDNA) was examined to determine patterns of genetic variation and infer historical and contemporary processes important in the genetic structuring of populations of Elporia barnardi. The DNA variation was examined at sites within streams, between streams in the same range, and between mountain ranges in the south-western Cape of South Africa.
3. Twenty-five haplotypes, 641 bp in length, were identified from the 93 individuals sampled. A neighbour-joining tree revealed two highly divergent clades (∼5%) corresponding to populations from the two mountain ranges. A number of monophyletic groups were identified within each clade, associated with individual catchment units.
4. The distribution of genetic variation was examined using analysis of molecular variance (amova). This showed most of the variation to be distributed among the two ranges (∼80%), with a small percentage (∼15%) distributed among streams within each range. Similarly, variation among streams on Table Mountain was primarily distributed among catchment units (86%). A Mantel's test revealed a significant relationship between genetic differentiation and geographical distance, suggesting isolation by distance (P < 0.001).
5. Levels of sequence divergence between the two major clades, representing the two mountain ranges, are comparable with those of some intra-generic species comparisons. Vicariant events, such as the isolation of the Peninsula mountain chain and Table Mountain, may have been important in the evolution of what is now a highly endemic fauna.
6. The monophyletic nature of the catchment units suggests that dispersal is confined to the stream environment and that mountain ridges provide effective physical barriers to dispersal of E. barnardi.