Ultrastructure of attachment specializations of hexapods (Arthropoda): evolutionary patterns inferred from a revised ordinal phylogeny Author R. G. Beutel Author S. N. Gorb text J. Zool. Syst. Evol. Research 2001 39 177 207 journal article 10.1046/j.1439-0469.2001.00155.x 95b3d62f-8fcc-4088-873f-b33f4a63d3fc 2360323 1 Protura ( Janitschek 1970 ) The tarsus is undivided as in all other entognathous hexapods. A very short, ring­shaped pretarsus is present. It bears a single claw, an unmodified empodial seta and a ventral empodial appendage. The claw is usually knife­shaped. Mid­ and hind claws have a navicular shape in protentomids and acerentomids and are equipped with lateral teeth, which are connected by a membrane in members of the latter group ( Janitschek 1970 ). Specialized adhesive structures such as arolia, pulvilli, or dense fields of hairs are absent as in all other apterygote hexapods. However, empodial structures may be potential preadaptations for formation of pulvilli.