Calcium carbonate and calcium oxalate as cuticular hardening agents in oribatid mites (Acari: Oribatida)

Hardening of adult oribatid mite cuticle by the deposition of crystalline calcium salts is demonstrated by a combination of X-ray diffraction and X-ray dispersion methods, in conjunction with experimental acid decalcification. Calcite (calcium carbonate) is the mineral deposited by mites representating the Ptyctima, but three species of Enarthronota (Eniochthonius minutissimus, Archoplophora rostralis, and Prototritia major) deposit whewellite, a form of calcium oxalate. The latter is deposited even in individuals living in base-poor environments such as sphagnum bogs, and probably derives from crystals originally precipitated by the fungal food of these mites. Our observations provide the first strong evidence of cuticular hardening by mineralization in the Arachnida, and the only known instance of the use of whewellite as a general intracuticular hardening agent in arthropods. A brief survey of representative oribatid mites, examined for strong birefringence under polarized light, suggested that each of the three ptychoid lineages (Ptyctima, Mesoplophoridae, Protoplophoridae) evolved cuticular mineralization independently.