 A louse 's color varies from pale beige to dark gray ; however , if feeding on blood , it may become considerably darker . Female lice are usually more common than males , and some species are parthenogenetic , with young developing from unfertilized eggs . A louse 's egg is commonly called a nit . Many lice attach their eggs to their hosts ' hair with specialized saliva ; the saliva / hair bond is very difficult to sever without specialized products . Lice inhabiting birds , however , may simply leave their eggs in parts of the body inaccessible to preening , such as the interior of feather shafts . Living louse eggs tend to be pale whitish , whereas dead louse eggs are yellower . 
