There is no morphological "key" to honey bee subspecies, no simple logical tree based on a sequence of single discriminating characters. Instead, measurable morphometric characters show gradual changes and their ranges mostly overlap between subspecies. Thus, subspecies often differ only slightly in the mean values of several body characters, and therefore advanced statistical methods are required for discrimination of groups. The concept of numerical taxonomy was introduced into honey bee taxonomy by DuPraw (1964, 1965) and further elaborated by Ruttner et al. (1978).