The genus Calyptoseius gen. nov. of the subfamily Platyseiinae Evans is described, based on adults and nymphs of one newly described species associated with ants of the genus Azteca occupying hollow stems of Cecropia in lowland rainforests of Costa Rica. Several unusual morphological attributes are noted, particularly the autapomorphic presence of four elongated setae on each of telotarsi II to IV. Some perspectives of these mites in the Cecropia-Azteca association are discussed, including a possible dispersal link via nematoceran flies to gain access to such an unusual habitat. The definition of the previously monobasic genus Cheiroseiulus is augmented, and also modified in view of an undescribed species at hand, and the subfamily definition is modified to account for morphological aspects of the new genus. A key to the genera of the Platyseiinae is given.