KEY TO THE GROUPS OF PRIMITIVE ORIBATID MITES 1. The legs of the adults have 2 femora; in some or in all immature stages the legs are not monodactyle. Gnathosoma visible from above. Absence of notogaster and latero-abdominal gland.............. Palaeacaroidea - The legs of the adults have i femur, and are monodaetyle in all immature stages. The notogaster is present ...............2 2. Notogaster divided by dorsal and superpleural "coupures". Gnathosoma visible or invisible from above. Absence of the latero-abdominal gland. One family (Protoplophoridae) is ptychoid............. Enarthronota - Notogaster divided in a different way, or undivided........3 3. Gnathosoma visible from above. Notogaster transversely divided, although ill-defined. Presence of latero-abdominal gland. Aptychoid.............. Parhypochthonoidea - Gnathosoma not visible from above............4 4. Ptychoid....................5 - Aptychoid...................6 5. Genital and anal openings of the adults separated by a part of the ventral plate. Nymphs scleritized and ptychoid......... Mesoplophoroidea - Genital and anal openings of the adults not separated by a part of the ventral plate. Nymphs not scleritized............ Phthiracaroidea 6. Protero-hysterosomatic articulation......... Perlohmannoidea - Absence of protero-hysterosomatic articulation.......... Nothroidea

Published as part of van der Hammen, L., 1959, Berlese's Primitive Oribatid Mites, pp. 1-93 in Zoologische Verhandelingen 40 on pages 3-4