Scanning electron micrograph of a head louse egg (green) attached to a strand of human hair (reddish-brown). Head lice (Pediculus) feed on human blood and live in close proximity to the scalp. They lay eggs (nits) in sacs which firmly attach to individual strands of hair near the base of the shaft. Horizontal image width is 1.5 mm. Supplied by Kevin Mackenzie, courtesy of Wellcome Images.