Scanning electron micrograph of flour mites, Acarus siro, feeding on the surface of damp soil. The picture shows three mites (yellow), on soil covered with diatoms (short rods, some green or yellow). The foreground mite is 3.5mm long. A. siro is a common pest of granaries and mills. In kitchens it may infest stored flour. The mites require warm temperatures and high humidity to thrive; a female can lay 20-30 eggs each day, and at normal room temperatures, adults taken 2-3 weeks to reach full development. Infested flour is rendered unpalatable to humans, but if fed to livestock, such as pigs, causes impaired weight gain. Mites are not insects; related to spiders, they lack wings, and have four pairs of legs. They are widespread and numerous, with 50000 described species. They are beneficial as decomposers of organic material, although some may cause allergic reactions in humans.