EGG-SHELLS IN MITES - VITELLINE ENVELOPE AND CHORION IN ACARIDIDA (ACARI)

This study deals with the formation of vitelline envelope (VE) and cho rion compartments in several free living and parasitic acaridid mites. In all investigated mites, the VE is of primary origin (produced by o ocyte itself), whereas exochorion material is of tertiary origin (ovid uct or chorion gland secretion). In acarid mites Acarus siro and Tyrop hagus perniciosus, VE formation starts with the oviductal oocytes in w hich vitellogenesis already proceeds. It is characterized by stratific ation (Acarus) or coarse fibrillar texture (Tyrophagus). Oocyte microv illi penetrating VE material were not observed. When the vitellogenesi s terminates, VE becomes homogeneous and is transformed into chorion. This is the only layer protecting the deposited egg in A. siro, wherea s in T. perniciosus the chorion-coated eggs passing through the distal portion of the oviduct are additionally covered by exochorion materia l deposited in three distinct forms: dense patches, granules, and most conspicuous locular chambers. In Tyrophagus longior, the egg surface closely resembles that of T. perniciosus, but the locular chambers are smaller. In Aleuroglyphus ovatus the exochorion material forms tiny s pherical patches instead of locular chambers. In Sarcoptes scabiei, No toedres cati and Falculifer rostratus, flocculent VE appears on vitell ogenic oocytes in the oviduct. VE development is characterized by form ation of numerous lenticular perivitelline spaces, which initially gro w to disappear later. Then VE material transforms into fully homogeneo us chorion. Chorion glands in Sarcoptes and Notoedres produce multives icular secretory bodies; their content is released onto the egg surfac e to form a vesicular monolayer (exochorion) during the egg passage. T he chorion gland in Falculifer is composed of two secretory cell types . Its secretion possibly glues the eggs to the host feather barb durin g highly ordered deposition, and forms the appendage ending with a rib bed plate, here considered to be a print of female undulate lamina act ing as an ovipositor. The hatching suture is present. Neither distinct micropyle nor aeropyles have been found in eggs of species under stud y.The exochorion is proposed to be an adhesive layer which fixes the e ggs to the substratum. The same role plays the chorion gland secretion in F. rostratus. It can be argued, however, that locular chambers of Tyrophagus exochorion may participate in reduction of water loss rathe r than in egg adherence or plastron respiration, as previously suggest ed in the literature.