These rare, fist-sized spiders do not make webs, but rather excrete secretions which harden upon contact with air. These "droppings" resemble barley-sized spider eggs, or even lustrous pearls, once the slime coating them, dries up. In fact, dried Pearl Spider "drops" are indistinguishable from the marine varieties produced by mollusks, and hence of identical value on the open market!
Several centuries ago, they were studied by naturalists, and several observations were made. Firstly, was that these spiders "lay" these pearls for no apparent or discernible "natural" reason, and secondly, the naturalists had discovered that the more these spiders ate or were fed--and they were true omnivores--the larger the spider pearls came out.
A cottage industry began. Enterprising merchants hunted and collected these creatures across the lands, erecting spider-farms for the manufacture of Spider Pearls. It wasn't long before someone got the idea to force-feed the spiders, ala foie gras geese, and soon, the fattened spiders began pooping out pearls of great size! (relatively speaking). The regular pearl market came to disarray, and prices and value fluctuated wildly.
[b]Plothook[/b] The Mermen Mercantile Alliance hires the party to eradicate all terrestrial Pearl Spider Farms!