Exoskeletal transformations in Eriophyoidea: new pseudotagmic taxon Pseudotagmus africanus n. g. & n. sp from South Africa and remarks on pseudotagmosis in eriophyoid mites

摘要

In addition to true tagmata, various pseudotagmata are present in chelicerates. Greatly miniaturized and morphologically simplified phytoparasitic acariform mites of the superfamily Eriophyoidea demonstrate a distinct ability to form pseudotagmata. The prodorsum and opisthosoma are the primary divisions of the eriophyoid body. In more evolutionary derived lineages, there is a trend towards the formation of additional opisthosomal subdivisions (pseudotagmata). These subdivisions are termed here "cervix", "postprodorsum", "pretelosoma", "telosoma" and "thanosoma". Among phytoptids, only the telosomal pseudotagma is present in several sierraphytoptine genera. In diptilomiopids, pseudotagmata have not been recorded. The most diverse examples of pseudotagmatization concern vagrant mites from the family Eriophyidae. Remarkably, well developed and unusually shaped pseudotagmata are peculiar to phyllocoptines from palms, especially in the new vagrant mite Pseudotagmus africanus n. g. & n. sp., found on leaves of Hyphaene coriacea (Arecaceae) in South Africa. Pseudotagmosis is one form of body consolidation in Eriophyoidea, reducing flexibility and therefore decreasing the ability for worm-like locomotion. Consequently, the legs become more important for locomotion. The other form of body consolidation is strengthening of the exoskeleton via armoring with microtubercles, and topographical changes (e.g. formation of opisthosomal ridges and furrows). The data at hand suggest that ancestrally, eriophyoids had an elongate body comprising many annuli, which can be regarded as pseudosegments. Later, they convergently evolved various pseudotagmata via the apparent fusion of these pseudosegments. Two morphotypes of vagrant mites ("armadillo" and "pangolin") are proposed based on the difference in the modification of dorsal opisthosomal annuli. The minimal number of dorsal annuli (six) is equal to the number of dorso-longitudinal peripheral body muscles; however, this number is unlikely to reflect the true number of segments situated behind the prodorsum in Eriophyoidea. Although legs III and IV are absent in Eriophyoidea, the cervical pseudotagmata might be reminiscent of metapodosomal segments. Future comparative myo- and neuroanatomy studies of groups of genes involved in segmentation development are necessary to reach the final conclusion on the pattern of body segmentation in Eriophyoidea.