Curiously, the soft-bodied, fast-living astigmates are most closely related among other mites to the heavily-armoured, long-lived Oribatida. In fact, both morphological and molecular phylogenetic studies have indicated that astigmates are derived from within oribatids (though recovering this result in molecular analyses is dependent on the analytical method used due to the much faster evolutionary rate of astigmates; Dabert et al., 2010*). Astigmates have been derived from oribatids by a process of neoteny where the characters of nymphal oribatids have been carried over to the adult astigmate (OConnor, 2009). Astigmates have also developed a highly modified deutonymph (the second nymphal stage of development) that is specialised for dispersal through phoresy (hitching a lift on some flying insect). The astigmate deutonymph (referred to by many authors as a hypopus) is generally non-feeding and the well-developed mouthparts present in the earlier protonymph become rudimentary, only to reappear when the mite moults through to the next stage, the tritonymph. In many species, if conditions are favourable and dispersal unnecessary, a protonymph may moult directly into a tritonymph, bypassing the deutonymph stage. Other species will only develop into deutonymphs if a suitable host for dispersal is available. The Psoroptidia, the main vertebrate-associated lineage of astigmates, have dropped the deutonymph from their life cycle entirely.
*And a thank-you to Macromite for notifying me of this paper).
While some phoretic astigmates will attach themselves to any old host, others may be very specialised. The members of the subfamily Ensliniellinae (family Winterschmidtiidae) associate solely with nest-building wasps and bees. The early stages of the enslinielline life cycle occur in the host's brood cell and the mites reaches their phoretic stage when the host larva has matured and is ready to leave the brood cell as an adult wasp (or bee). At that point, the mites cluster in specialised pockets on the host's body called acarinaria. In the wasp Ancistrocerus antilope, only the male wasps emerge from the cell carrying mites in acarinaria behind the wings (the females kill any mites in their brood cell while larvae); when the male mates with a female, its mite passengers abandon him to enter acarinaria around the female's genitalia (Houck & OConnor 2001). In other wasp species, the females carry mites in acarinaria from when they emerge. When the female lays its eggs, the mites leave the acarinaria to be sealed in the new brood cells where they will mate and lay their own eggs.
You might be wondering what the wasp gets out of this arrangement as it is hard to see why it would have developed specialised structures to transport the mites if it was not benefiting somehow. And yet, at best, the mites seem to have no significant effect on their hosts; at worst, they are actively harmful, feeding on the food stores left for the developing larva or on the larva itself (though no parasitic ensliniellines have been known to cause the death of their host). Klimov et al. (2007) have suggested that acarinaria have developed not to facilitate the mites' development but to contain them. The mites cannot break through the walls of the brood cells themselves; they can only be carried by an emerging host. If the mites cluster into acarinaria before the host emerges, they remain with already-infected individuals rather than spreading to their potentially mite-free siblings. Perhaps adaptation is not always a matter of achieving an optimum; perhaps it is sometimes simply a form of damage control.
Dabert, M., W. Witalinski, A. Kazmierski, Z. Olszanowski & J. Dabert. 2010. Molecular phylogeny of acariform mites (Acari, Arachnida): strong conflict between phylogenetic signal and long-branch attraction artifacts. Molecular Phylogenetics and Evolution 56 (1): 222-241.
Houck, M. A., & B. M. OConnor. 1991. Ecological and evolutionary significance of phoresy in the Astigmata. Annual Review of Entomology 36: 611-636.
Klimov, P. B., S. B. Vinson & B. M. OConnor. 2007. Acarinaria in associations of apid bees (Hymenoptera) and chaetodactylid mites (Acari). Invertebrate Systematics 21 (2): 109-136.
OConnor, B. M. 2009. Cohort Astigmata. In: Krantz, G. W., & D. E. Walter (eds). A Manual of Acarology, 3rd ed., pp. 565-657. Texas Tech University Press.