In this high magnification electron micrograph, the surface of the spider dropping appears to consist of many similarly shaped rods. These, most probably, are crystals of guanine, a nitrogen-containing insect waste product. The size of the crystals is probably limited by proteins that coat the crystals as they are grow in the spider excretory organ.
Spiders excrete insoluble guanine crystals to conserve precious water.
Any proteins on the crystals, if airborne, could serve as allergens to sensitized individuals.