А highly regressed prelarva, specialized into а ecdysis organ and consisting of two sclerotized bosses, is present in numerous families of both free-living and parasitic Astigmata. It has been observed in the following families: Acaridae, Glycyphagidae, Chortoglyphidae, Pyroglyphidae, Psoroptidae and Epidermoptidae. It is absent in some parasitic Astigmata such as the Knemidokoptidae, the Cytoditidae, the Sarcoptidae, the Teinocoptidae and the Gastronyssidae. killiпg mechanically ovigerous females of reared Lepidoglурhиs destrиctor, a:nd- by-maintaining these dead females under 75 % relative humidity, the authors observe that the eggs contained in these females continue their development until the prelarval and larval stages. This experience proves that the ovoviviparity observed previously in some females found dead in house dust (Dermatophagoides pteronyssinиs) is not а true ovoviviparity but results simply from the continuation of the development of the egg after the dead of the female mite.