Metrioppia Grandjean, 1931

Lindo, Zoë, 2018, Diversity of Peloppiidae (Oribatida) in North America, Acarologia 58 (8), pp. 91-97 : 93-94

Metrioppia Grandjean

Metrioppia is one of two genera within the Peloppiidae that possess pelopsiform chelicerae, being differentiated from the other, Paenoppia , by the shape of the lamellae (converging but not touching, and without translamellae). Worldwide, eight species of Metrioppia have been described; four of these (and one subspecies) are known from North America. The type-species, Metrioppia helvetica Grandjean , has a Holarctic distribution and occurs across the western arctic and subarctic of North America (Alaska, Yukon, Northwest Territories, and Nunuvut). The second, M. serrata (Sellnick) is known from Mexico and seems a different species based on original descriptions. However, type specimens of M. helvetica and M. serrata were unfortunately lost or destroyed ( Marshall et al. 1987) and some confusion has developed in the literature. Hammer (1952) redescribed M. helvetica , originally under the name C. microseta Hammer , which she later synonymised ( Hammer 1955); however no type-specimen of C. microseta exists ( Marshall et al. 1987). This is unfortunate as both M. helvetica and M. serrata appear to have considerable morphological variation, and it is unclear whether M. atlantica Jacot found in the eastern United States (Virginia, Illinois, North Carolina) represents a true species or a subspecies of M. serrata as suggested by Jacot (1938).

There are two west coast species of Metrioppia : M. oregonensis Woolley & Higgins , and M. walbranensis Lindo. Extensive sampling and museum records suggest they do not co-occur, with M. walbranensis having a very restricted distribution endemic to only a few valleys (<3000 km 2) of intact old-growth temperate rainforest on Vancouver Island, British Columbia ( Lindo 2015). Metrioppia oregonensis has been found both north and south of M. walbranensis , extending to northern California, where the southernmost specimens display morphological variation in body size ( Walter and Norton 1984).