About 3,800 species of spiders are known to exist in North America. These species fall into 68 different families, but some of the families contain many more species than others. The Sheetweb Weavers (family Linyphiidae - pronounced "lin-uh-FEE-eh-dee") is a very large family of mostly very tiny spiders. In some parts of the country, a quarter of all the spider species may be Sheetweb Weavers. Many of them are only a couple of millimeters long, or less. And a lot of them look alike -- particularly the Dwarf Spiders (subfamily Erigoninae). In fact, the female Dwarf Spiders are actually the only spiders in North America with no key to genus, which means they can't be identified using the standard spider identification guide, Spiders of North America.