Comparative ultrastructure of the integument in adult mites of the Parasitengona and its phylogenetic implication

Abstract

The integument of adult Hirsutiella zachvatkini and Euschoengastia rotundata (Trombiculidae), Platytrombidium fasciatum and Camerotrombidium pexatum (Microtrombidiidae), Piona conglobata (Pionidae), and Teutonia cometes (Teutoniidae) (water mites) were investigated by transmission and scanning electron microscopy. The soft integument of adult trombiculids is built up of specific epithelial respiratory tissue, provided by long, branched mechanoreceptor setae organized as neotrichia functioning as a plastron. The seta pits with clear cuticle are surrounded by cuticular folds with tightly packed ridges composed of an electron-dense substance. The underlying epidermal tissue is formed of separately scattered compact epidermal cells intermingled with large polymorphic so-called ‘intra-epithelial cells‘ with clear cytoplasm devoid of organelles. The integumental folds with the intra-epithelial cells filled with metabolic and sorptional water are thought to function as air gills, which selectively absorb and transport oxygen from the outside to internal tissues. In microtrombidiids, also an internal cuticular meshwork is expressed, formed of thick electron-clear strands immersed into the epidermis and crossing at right angles. This meshwork is separated from the external cuticle, comprising a thick lamellar procuticle covered by thick electron-light epicuticle. Flat uniform epidermal cells contain numerous pigment granules. In water mites, a very thick lamellar procuticle penetrated by pore canals is covered by a thick dense epicuticle. Setae are arranged following the orthotrichous type. The epidermal layer underlying the cuticle consists of flat uniform epithelial cells devoid of pigment. The organization of the integument in stocks of Trombiculoidea and Trombidioidea appears strongly apomorphic, whereas it is plesiomorphic in water mites.