Appendage development in embryos of the oribatid mite Archegozetes longisetosus (Acari, Oribatei, Trhypochthoniidae)

The development of reliable techniques to provide large numbers of fixed mi te embryos free of their tough membranes has allowed us to produce scanning electron micrographs of oribatid embryos for the first: time. Antibody sta ining for expression of the gene Distal-less demonstrates the formation of the labrum from a pair of regions in the ocular lobes, indicating a possibl e appendicular origin for the labrum. The chelicerae are followed from thei r initial postoral position through their anterior rotation to become preor al. The pedipalpal lobe, involved in the formation of the gnathosoma, is sh own to be almost the size of the pedipalp at some stages, as well as expres sing Distal-less. The fourth pair of walking limbs are not present in oriba tid prelarvae and larvae, however, their anlage are visible in later embryo nic stages but these do not express the gene Distal-less, suggesting that f ormation of distal structures is suppressed at this stage. Claparede's orga ns, present between coxae of legs I and II of larvae, are shown for the fir st time unequivocally to derive from the base of legs II. These are propose d to be homologous to the lateral organs of other arachnid groups.