註釋 The ant genus Mystrium in the Malagasy region is revised and six new species are described from Madagascar and its surrounding islands. The discovery of the division of females into major and minor forms was the key to solving this omplicated taxonomic puzzle. We found that in some species of Mystrium , major or minor workers develop as reproductives. In Mystrium , morphological comparison among individuals from the same phenotype, not from their reproductive or functional role is important. Here we propose a new taxonomic framework for interesting and mysterious genus. All species are reclassified into three subgroups based on the reproductive function, and innovative pictorial keys to the species for all castes and sexes are developed. The associations between existing names and males are reexamined, and males of eight of the ten Malagasy species are described or redescribed. 