Louse (pl.: lice) is the common name for any member of the infraorder Phthiraptera, which contains nearly 5,000 species of wingless parasitic insects. Phthiraptera was previously recognized as an order, until a 2021 genetic study determined that they are a highly modified lineage of the order Psocodea, whose members are commonly known as booklice, barklice or barkflies. Lice are obligate parasites, living externally on warm-blooded hosts, which include most species of bird and mammal, notably excluding monotremes, pangolins, bats, sirenians, tapirs, rhinoceroses, and members of the clade and suborder Whippomorpha (Cetaceans and Hippopotamids). Chewing lice live among the hairs or feathers of their host and feed on skin and debris, whereas sucking lice pierce the host's skin and feed on blood and other secretions. They usually spend their whole life on a single host, cementing their eggs, called nits, to hairs or feathers. The eggs hatch into nymphs, which moult three times before becoming fully grown, a process that takes about four weeks. Humans host two species of louse—the head louse and the body louse are subspecies of Pediculus humanus; and the pubic louse, Pthirus pubis. Lice are vectors of diseases such as typhus. Lice were ubiquitous in human society until at least the Middle Ages. They appear in folktales, songs such as The Kilkenny Louse House, and novels such as James Joyce's Finnegans Wake. The body louse has the smallest genome of any known insect; it has been used as a model organism and has been the subject of much research. They commonly feature in the psychiatric disorder delusional parasitosis. A louse was one of the early subjects of microscopy, appearing in Robert Hooke's 1667 book, Micrographia. The oldest known fossil lice are from the Cretaceous.
Classification Lice are classified as an infraorder called Phthiraptera, named by Ernst Haeckel in 1896. Phthiraptera was previously recognized as an order, and was considered the sister taxon to the order Psocoptera, which contains booklice, barklice and barkflies, all within the greater group called Psocodea. However, a 2021 genetic study determined that Phthiraptera lice are a highly modified lineage within the Psocoptera group, thus rendering Psocoptera paraphyletic and necessating a classification change. Phthiraptera is now nested within the suborder Troctomorpha, belonging to the now-ranked order Psocodea. Within Psocodea, the Phthiraptera lice are most closely related to family Liposcelididae. A cladogram showing the position of Phthiraptera within Psocodea is shown below:
Internal classification Phthiraptera is a monophyletic grouping, united as the members are by a number of derived features, including their parasitism on warm-blooded vertebrates and the combination of their metathoracic ganglia with their abdominal ganglia to form a single ventral nerve junction. The order has traditionally been divided into two suborders, the sucking lice (Anoplura) and the chewing lice (Mallophaga). However, subsequent classifications indicated that the Mallophaga are paraphyletic, and four suborders were then recognized. Upon finding that Phthiraptera was nested within Psocoptera in 2021, de Moya et al. reduced the rank of Phthiraptera to infraorder, and the suborders were reduced down to parvorder. Also, based on the paraphyletic nature of one of the parvorders - Ischnocera - they recognised a fifth parvorder Trichodectera, which had been split off from Ischnocera two years earlier. Here are the five proposed parvorders within Phthiraptera:
Amblycera: primitive chewing lice, widespread on birds, and also occurring on South American and Australian mammals Anoplura: sucking lice, occurring on mammals exclusively Rhynchophthirina: parasites of elephants and warthogs Ischnocera: avian chewing lice, and contains the large family Philopteridae Trichodectera: parasitizing mammals, split off from Ischnocera, and contains the family Trichodectidae
Evolutionary history The oldest confirmed fossil louse is Archimenopon myanmarensis, an amblyceran from the Cretaceous amber from Myanmar. Another early representative of the group is a bird louse, Megamenopon rasnitsyni, from Eckfelder Maar, Germany, which dates to the Eocene, around 44 million years ago. Saurodectes vrsanskyi from the Early Cretaceous (Aptian) Zaza Formation of Buryatia, Russia, has also been suggested to be a louse, but this is tentative. Placental mammal lice had a single common ancestor that lived on Afrotheria with this arising from host-switching from an ancient avian host.
Description Nearly 5,000 species of louse have been identified, about 4,000 being parasitic on birds and 800 on mammals. Lice are present on every continent in all the habitats that their host animals occupy. They are found even in the Antarctic, where penguins carry 15 species of lice (in the genera Austrogonoides and Nesiotinus).
… excerpt ends here. Continue reading the full article.
