Acarinaria are specialised structures on the bodies of insects that harbour dispersing mites, providing a secure attachment place for the mites. The structures are best known among bees and wasps. Their presence remains enigmatic, however, since the associated mites often have negative or neutral effects on their hosts. A new hypothesis explaining the origin of the acarinarium as a specialised defence mechanism is proposed. In nests with partitions (as constructed by many bees and wasps), parasitic or cleptoparasitic mites are rarely found in all cells. They negatively interact only with host larvae developing in infested cells and apparently cannot disperse within the nest to attack others in the developing brood before bee emergence. Only when emerging bees break the partitions can the mites reach other hosts. We propose that acarinaria serve to concentrate unwanted mites, reducing the chance that they will disperse to other members of the brood as the infested host leaves the nest. Development of special mite pouches (acarinaria) by hymenopteran hosts presumably increases the likelihood that all mites will stay with the individual(s) with reduced fitness, thereby reducing their effect on other bees in the brood. This paper reviews the associations between chaetodactylid mites and long-tongued bees (Apidae and Megachilidae). Only apid bees (Apidae) have acarinaria; megachilid bees, which harbour species of Chaetodactylus that usually kill the bee larvae, do not possess acarinaria. The following associations involving previously undescribed acarinaria or mite species are reported: Achaetodactylus ceratinae (axillar acarinarium on Ceratina nigriceps); Roubikia latebrosa, sp. nov. (metasomal acarinarium on Tetrapedia sp.), Sennertia argentina (genital acarinarium on Xylocopa fimbriata), Sennertia devincta, sp. nov., Sennertia sayutara, sp. nov. (metasomal acarinaria on Ceratina (Zadontomerus) spp.), Sennertia lauta, sp. nov. and Sennertia ratiocinator, sp. nov. (incipient scutellar–metanotal acarinarium on Xylocopa (Zonohirsuta) spp.). In the latter case, the mites display a remarkable difference in the attachment sites between male and female hosts. In females, the mites are phoretic in a groove between the scutellum and metanotum (scutellar–metanotal acarinarium), whereas on males, mites attach to the hairs of the anterior scutum.