Two morphological forms of petioles and two morphological forms of cauda are defined respectively based on discrete morphological study of the male cauda and petiole in 22 species belonging to four subgenera, Arrenurus, Micruracarus, Megaluracarus and Truncaturus of the genus Arrenurus. The relationships between the 22 species were studied with a cladistic analysis using 29 morphological characters. The results show that there appear to be six phylogenetic lines in male cauda and petiole of the genus. It is considered that the development of male cauda, petiole and IV-L-4 spur, is driven by behavioural evolution of reproductive mechanism from non-mating (indirect sperm transfer) to mating (direct sperm transfer). The possible evolutionary line(s) and relationships between forms of male cauda and petiole are discussed in detail and illustrated. The results of the cladistic analysis support the monophyly of the genus Arrenurus but do not support the present subgeneric classification of the genus. Based on phylogenetic relationships among studied species in best resolved trees, a classification of seven species groups is presented, which might lead to a better and more practical subgeneric classification with further studies using more taxa.