Lessingia arachnoidea is a rare species of flowering plant in the daisy family known by the common name Crystal Springs lessingia. It is endemic to California, where it is known from a few occurrences in the vicinity of Crystal Springs Reservoir on the San Francisco Peninsula. It may also exist in Sonoma County to the north. The plant grows in chaparral, scrub, and other local plant communities, often on serpentine soils. This is an annual herb producing a slender, erect stem up to 80 centimeters in maximum height. It is woolly toward the ends of the stems, less so toward the base of the plant. The leaves are narrow and sometimes toothed, the lowest approaching 11 centimeters long and the uppermost reduced in size. The inflorescence is made up of a single flower head at the tip of the slender stem. The flower head is lined with tiny lance-shaped phyllaries with purplish pointed tips and sometimes a coat of woolly fibers. The head is discoid, lacking ray florets but bearing several funnel-shaped lavender disc florets with raylike lobes. The fruit is an achene with a very hairy hard body 2 or 3 millimeters long and a small, bristly pappus on top.