Glycacarus combinatusgen. and sp. nov. was collected from the nest of the White chinned petrel (Procellaria aequinoctialis) on Possession Island Sub-Antarctica. It can be distinguished from all other members of the Astigmata by the M-shaped prodorsal shield and by the fact that it is made up of a complete mixture of the morphological characters which separate the Glycyphagidae from the Acaridae. In order to maintain the present familial structure within the free-living Astigmata a new family Glycacaridae is proposed. It is suggested that these three families arose from three evolutionary lines which separated from a common ancestor possibly during the Cretaceous period. Ontogenetic development of the body setae is described and a new interpretation of setal homologies relative to different life-cycle stages is discussed, together with a revised nomenclatural system for the idiosomal seta.