The main task of this thesis was to investigate the influence of the preservation by ethyl alcohol and dry mounting on the morphometric characteristics of water bugs´ and to test the hypothesis that live individuals and individuals preserved by ethyl alcohol and dry mounted (both groups deal with individuals of the same species, sex and developmental stage) have the same morphometric parameters. The next task was to determine the effect of preparation by ethyl alcohol and dry mounting on the change of weight caused by different way of individuals´ preparation. Two model species of water bugs were chosen for this study: creeping water bug Ilyocoris cimicoides (Naucoridae) and backswimmer Notonecta glauca (Notonectidae). Weight and twelve other morphometric body characters were measured in adults of both sexes of both species. All data and their differences were tested by Student's paired t-test or nonparametric Wilcoxon test. The level of significance p = 0,05 was used for the decision whether the difference before and after preservation by ethyl alcohol and dry mounting is or is not statistically significant. Both preservation by ethyl alcohol and dry mounting influence only the body weight of treated individuals. Other morphometric characters are more or less independent on the way of preservation of water bugs´ individuals in collections. In this work, we also tested the effect of the Dyar rule for the increase in weight and size between different developmental stages of both species studied. Dyar rule for the evolution of both species was not verified. This project was supported by the grant of the Grant Agency of the Czech Republic No. P505/10/0096.