O. Merkl szerk.: Folia Entomologica Hungarica 66. (Budapest, 2005)

less than 5% (Corixa punctata, M. pusilla, S. nigrolineata, S. limitata, Cymatia rogenhoferi). There is about one order of magnitude difference in occurrence frequency as compared to other species. The frequency distribution of the semiaquatic species is more balanced. Highest frequency was found in the case of Microvelia reticulata (34%). It is to be noted that 50% of the species have a frequency less than 5% (Hebrus pusillus, Hebrus rufipes, Gerris lacustris, G. thoracicus, Hydrometra gracilentum, Limnoporus rufoscutellatus). The lower range of dominance values of semiaquatic species and the higher ratio of "rare species" may be due to the singling style collectings, in contrast to more mass-collecting in the case of aquatic species. For the last set of collectings species accumulation curves were calculated, which are shown in Figure 3. It is unambiguous that most species are already captured, but there are differences between the basins (Table 2): 96% of the species living in the Siófok Basin but only 89% in the Keszthely Basin are collected. DISCUSSION Thirty-one species have been reported from Lake Balaton from the recent collectings. This is 60.8% of the hitherto mentioned 51 species in Hungary (KONDOROSY 1999). Nine further species were reported in earlier papers. Thus 78.4% of the aquatic and semiaquatic Heteroptera fauna of Hungary was or is present in the lake. From a heteropterological point of view, the lake is the best known water system in Hungary (cf. BAKONYI & VÁSÁRHELYI 2005). We have well-based knowledge about the aquatic and semiaquatic Heteroptera fauna of the lake. Further species are expected from the Keszthely Basin rather than from the Siófok Basin. Assuming that all collected species can live on the whole territory of the lake, a total of 3 or 4 further species is expected from the two Basins, respectively. This assumption is partly supported by the more natural and eutrophic nature of sites in the S W basin. Presence-absence data have relevance and provide valuable information in short-term as well as long-term monitoring. For monitoring and following the changes in the water characteristics, the new species in each exploration, and the characteristic changes between their resulting faunas emphasize the significance of future detailed and thorough investigations.