A quantitative study of the behavioural interactions of 7 European Lycaenidae and 1 Riodinidae species with 2 ant species revealed significant differences between myrmecophilous and myrmecoxenous caterpillars. A functional dorsal nectary organ is decisive for stable ant-associations. Further aspects of the function of the myrmecophilous organs, and the application of the proposed experimental method to comparative surveys are discussed.