Thallus: areolate or verruculose, rarely forming rimose areolate areas, sometimes squamulose, overall up to 10 cm wide areoles: round to irregular, up to 3 mm in diam. but often reduced and less than 0.5 mm in diam., up to 0.2 mm thick, discrete to contiguous; rim: down-turned or undulate, lobed or not upper surface: brown with sometimes blackish tints or white or bluish gray (when pruinose), dull, plane to convex, smooth and without fissures or lumpy and cracking, often with cross-hatched or radial fissures around apothecia, sometimes marred with black lines of a fungal parasite, epruinose, or more usually light to heavily pruinose lateral cortices: paraplectenchymatous, 45-165 µm thick; cells: 2-3(-5) µm wide; syncortex: (20-)40-50(-100) µm thick, with hyphal cells sometimes visible, sometimes splitting down to eucortex, often in cross-hatch pattern; eucortex: sometimes inspersed, upper layer often with a distinct horizontal reddish line in cross section, 10-15 µm thick; lower layer hyaline and 15-50 µm thick (increasing in thickness with size of areoles) algal layer: thickness and distribution various, 80-160 µm thick, sometimes reduced to below apothecia; upper stratum level; algal cells 7-15 µm wide medulla: hyaline to brown or pale red-brown, obscure in K with abundant crystals of varying sizes, thickness of medulla varying to over 100 µm thick lower surface: ecorticate, appearing whitish or pale from medullary plectenchyma attachment: broad, sometimes becoming elevated, but without a stipe Apothecia: 0.1-1 mm wide, punctiform to round, angular or stellate from radial fissures, one or more per squamule, sometimes confluent, sometimes one apothecium filling whole areole, sometimes apothecium elevated above areole with a lecanorine margin disc: black to reddish brown, plane to concave, smooth to rough, pruinose or not, rarely with interascal sterile plectenchyma parathecium: 10-20 µm wide, usually expanding to 50-70 µm around the disc, sometimes forming a ring around disc, sometimes elevated above areole or thallus eroded away by erosion and turning black, or sometimes barely developed epihymenium: often brown, c. 10 µm thick in pigmentation layer hymenium: clear to pale yellow, (60-)100-110(-170) µm tall; paraphyses:. 1-2(-2.5) µm wide at base, apices usually not expanded, but occasionally expanded up to 5 µm subhymenium: hyaline to pale yellow, 20-55 µm thick; hypothecium: 10-20 µm thick asci: narrowly cylindrical to clavate, 80-120 x 17-27µm, 100+-spored ascospores: hyaline, simple, broadly ellipsoid but globose in early ontogeny or appearing globose, (3-)4-6.5(-7) x (2-)2.5-3(-3.7) µm Pycnidia: not seen Spot tests: UV- (often appearing white, but occasionally pale green, or orange from inclusion of minerals), all spot tests negative Secondary metabolites: none detected. Substrate and ecology: on limestone or granite, in full sun to shade World Distribution: China, North America, South America (Chile and Bolivia), Mexico and Siberia Sonoran distribution: common in Arizona, southern California, Baja California, Baja California Sur, Chihuahua, and Sonora. Notes: Acarospora strigata is highly successful at populating harsh microhabitats, but its best developed specimens are usually collected from protected, more mesic microhabitats. The areoles can be quite various in size and shape. A number of taxa have been described by Hasse, Bouly des Lesdain, Magnusson, and Zahlbruckner, but so far no supporting evidence for accepting these segregations from A. strigata has been found. Thus, the description above encompasses the full range of morphological variation. These morphotypes do not appear to be stable. The most constant characteristics of A. strigata are its broadly ellipsoid spores and spore size, which can be very useful in determining epruinose forms. I do not recognize var. longispora because long spores are quite common. The holotype of Acarospora utahensis (UPS!) had spores in the upper range of length but they were shorter than the measurements given in the English description accompanying the protologue; consequently, it is placed in synonymy with Acarospora strigata. Some observers have reported longer spores (up to 12 µm), but I have not seen any this long. The atypically long measurement may have resulted from optical artifices, that can eliminated by using a filter. Acarospora strigata can also be confused with some pruinose specimens of A. veronensis, but the latter species has narrowly ellipsoid spores that are only 1.9-2.2 µm wide. Based on collections seen so far, A. strigata does not appear to be very common in the mountains of South America where it was first collected.