The spiders look like the morphological "negatives" of some dunes often found on the bottom of Martian craters, as if they were an illusion comparable to the one where the eye can be tricked into seeing depressions where there are actually ridges, and vice-versa. These spider dunes are common on earth too, and are related to the crescent shaped Barchan dunes formed when a thin layer of sand overlays a flat rocky substrate, and where wind is predominately from one direction. The crescent shaped dune is very stable, but it moves downwind across the landscape, often in the company of others. Chesley Bonestell often depicted the Martian plains with Barchan dunes, and I believe some have been seen in imagery from the planet.