Lice (sg.: louse) are wingless, obligate ectoparasitic insects that constitute the infraorder Phthiraptera. They are equipped with claws on their legs that help attach to the bodies of most birds and mammals. These insects prefer living in darkness, within the feathers or fur of their host, and thus have considerably poor vision.
Around 5,000 species of lice are found worldwide. They are classified into five parvorders: Anoplura, Amblycera, Ischnocera, Trichodectera, and Rhynchophthirina. Members of Anoplura are referred to as sucking lice, since they puncture the skin of the host and suck on their blood. Most species of the other parvorders are chewing lice that live among hair or feathers and chew on skin, debris, or sebaceous secretions from the skin of the host.
They are hemimetabolous insects that lay eggs, which hatch into nymphs and eventually become adults. Many species are vectors causing several diseases in humans, including typhus, trench fever, and relapsing fever.
Although size varies with species, they typically range from 0.01 to 0.2 in (0.5 to 6 mm).
They have a dorsoventrally flattened body, a shape that allows them to slip between host hair or feathers. Depending on the species, lice range in color from gray and tan to pale brown or off-white. Their cryptic coloration helps them blend seamlessly into their surroundings, making them difficult to detect.
Their body is divided into three segments: head, thorax, and abdomen.
In chewing lice, the head is wider than the thorax, whereas in the sucking type, it is narrower than the thorax. The head bears a pair of short antennae, having 3 to 5 segments.
While compound eyes are reduced or absent in sucking lice, they are present in chewing lice. Both types of lice lack simple eyes or ocelli.
Since sucking lice feed on liquid blood, their mouthparts are piercing-and-sucking type, comprising a toothed proboscis and a set of stylets arranged in a cylinder inside the proboscis. In contrast, chewing lice only have chewing-type, strong mandibles that are adapted for biting.
The thoracic region has three subsegments: prothorax, mesothorax, and metathorax. However, these segments are fused in sucking lice.
Each thoracic segment bears a pair of short legs. In chewing lice, each leg typically ends in one or two small claws, while in sucking lice, the legs terminate in a single, robust claw adapted for grasping host hair.
The abdomen bears 8 to 10 distinctly visible segments. It houses the digestive and reproductive organs, as well as multiple pairs of respiratory pores or spiracles.
The name Phthiraptera was coined by Ernst Haeckel in 1896. Although traditionally recognized as a distinct order, a 2021 study reclassified Phthiraptera as an infraorder within the suborder Troctomorpha.
The five parvorders under this infraorder are the following.
The earliest confirmed fossil of lice is that of Archimenopon myanmarensis, an amblyceran from the mid-Cretaceous Period (about 99 million years ago).[1]
These insects are found on every continent. As obligate ectoparasites, they live on the bodies of warm-blooded hosts, including most birds and mammals. However, they have not been documented on monotremes, pangolins, and bats.
Of the nearly 5,000 known species of lice, approximately 4,000 parasitize birds, while about 800 are found on mammals. In Antarctica, 15 species of lice in the genera Austrogoniodes and Nesiotinus live on penguins.[2]
Three different types of lice parasitize the human body: the head (Pediculus humanus capitis) and body louse (Pediculus humanus humanus), and the pubic or crab louse (Pthirus pubis).
While sucking lice feed on the blood of their host, chewing lice derive nourishment from feathers, hair, scales, dead skin, and occasionally sebaceous secretions of their host’s body.[3]
Although lifespan varies with species, most lice survive for about 30 to 35 days. However, they do not survive if they are removed from the host’s body. For instance, a head louse may die in just 2 days if it fails to have a blood meal.[4]
The males actively seek mates as they reach adulthood. They mount the females from above to copulate, a process that may last several minutes to ensure sperm transfer to the female’s reproductive tract.
The females begin laying eggs within 1 to 2 days of copulation, typically laying about 3 to 8 pale-white eggs or nits per day. They cement these eggs to hair shafts or feathers using a specialized, sticky saliva. Those living on birds attach their eggs to parts of the body inaccessible to preening, such as within the feather shafts.
As hemimetabolous insects, lice eggs typically hatch into nymphs in about a week. These nymphs undergo three molts to become fully-grown adults.
A few species, such as Bovicola bovis, have been documented to undergo parthenogenesis, meaning females produce offspring from unfertilized eggs.[5]
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