Lice are booklice that moved onto warm blood
Nearly five thousand wingless parasites once ranked as their own insect order. A 2021 genetic study folded Phthiraptera into Psocodea—the booklice and barklice—as a highly modified lineage. In other words, the creatures that cement nits to hair are evolutionary makeovers of the insects that haunt dusty shelves.
Lice live as obligate external parasites on most birds and mammals, skipping monotremes, pangolins, bats, sirenians, tapirs, rhinoceroses, and whippomorphs (whales and hippos). Chewing forms scavenge skin and feather debris; sucking forms pierce for blood and secretions. Eggs—nits—are glued to hair or feathers; nymphs moult three times and mature in about four weeks, usually on a single host. Humans carry Pediculus humanus (head and body subspecies) and the pubic louse Pthirus pubis; body lice can vector typhus.
Classification now treats Phthiraptera as an infraorder inside Troctomorpha, closest to Liposcelididae booklice. Five parvorders span Amblycera (primitive chewers on birds and some southern mammals), Anoplura (mammal-only suckers), Rhynchophthirina (elephant and warthog specialists), Ischnocera (mostly avian chewers), and Trichodectera (mammal chewers split from Ischnocera). About four thousand species parasitise birds and eight hundred mammals; even Antarctic penguins host fifteen species. The oldest confirmed fossil, Archimenopon myanmarensis, is Cretaceous Burmese amber; another bird louse dates to about forty-four million years ago in Germany.
Sucking lice measure roughly 0.5–5 millimetres with retractable piercing mouthparts and a fused thorax; chewing lice reach about six millimetres with broader heads. Many species are host-specific, sometimes site-specific, and die quickly off-host—though a dead host’s lice may hitchhike on flies via phoresis. Placental mammal lice appear to share an ancestor that lived on Afrotheria after switching from an ancient bird. Culture never forgot them: Thomas Becket’s 1170 funeral famously swarmed with vermin; Robert Hooke drew a louse in Micrographia (1667); Hans Zinsser’s 1935 Rats, Lice and History blamed both head and body lice for typhus, though modern consensus points to the body louse alone.
Source: Louse
