In the dipoma thesis, the student showed experimentally that the black form of Sepsis flies imitate ants in a number of traits. As a result, the black form was protected from Pardosa predators. Her results further show that Sepsis flies are probably quasi-Batesian mimics. It is not known to what extent the yellow form, which appears to be non-mimetic, is protected from predators.The aims of the thesis are to investigate how selection by natural enemies affects the population structure of a dimorphic population of Sepsis flies. This includes (1) development of a computer simulation model of population dynamics of a dimorphic population, (2) measurement of phenotypic resemblance of the yellow form to ants, (3) quantification of predation pressure on two forms of Sepsis flies by two potential predators, namely Pardosa and Xysticus spiders.