Correlation between feeding and heteromorphic life stages of Astigmata through rearing

Metamorphosis is a key innovation allowing the same species to inhabit different environments and accomplish different functions. In Astigmata, metamorphosis is prophesy-related, that is, the function of dispersal on hosts is accomplished by a heteromorphic nymph. This stage is highly specialized for phoresy -- it lacks oral feeding and is morphologically divergent (heteromorphic) in comparison to other stages, which usually live in a concealed but patchy habitats and feed using mouthparts. The origin of the heteromorphic nymphal instar is enigmatic although some fossils (Levantoglyphus) may retain chelicerae, suggesting a gradual evolution of metamorphosis in Astigmata. In contrast to most other Astigmata, the heteromorphic stage of the "living fossil" family Schizoglyphidae retains the mouth and pharynx and is represented by a tritonymph rather than deutonymph. Because the morphology of the heteromorphic phoretic nymphs is very divergent, the systematics of Astigmata has challenges associated with the need of correlating the heteromorphic and homeomorphic stages. As an example, in most species-rich families, Acaridae and Histiostomatidae, most described genera are based on heteromorphic stages rather than on homeomorphic stages, which impedes further systematic research. Here, we discuss various techniques allowing correlating these stages in the lab. Rearing experiments involving live homeomorphic or heteromorphic stages provide direct evidence for such correlation and the entire life-cycles can be studied in this way. In contrast, molecular evidence may be useful when both stages are available for sequencing.