Head louse

Small bugs live in human hair.

They spend their whole lives on the scalp. These bugs cannot fly or jump. They have short legs with claws. The claws help them hold onto hair.

They eat tiny bits of blood to live. A mother bug lays eggs on the hair. The eggs are called nits. The eggs stick to the hair with glue. They hatch in about one week.

These bugs move when heads touch. They like to hide in dark spots.

Head lice are tiny insects that live on human scalps.

They spend their whole lives on people. These bugs cannot fly. They also cannot jump. This is because their legs are short and stumpy. Instead, they use claws to grab hair. These claws help them climb hair very fast.

Head lice eat human blood to stay alive. They bite the skin many times each day. They use mouthparts to pierce the skin. To make new lice, females lay eggs. We call these eggs nits. A female lays three or four eggs every day. She uses a special glue to stick them to hair. This glue is made of proteins.

Nits hatch in six to nine days. The baby louse is called a nymph. A nymph must molt three times to grow up. It takes about 12 to 15 days to become an adult. Lice move from person to person through close contact. The most common way is head-to-head contact.

The head louse is a tiny insect that lives on the human scalp. These insects are called obligate ectoparasites. This means they must live on the outside of a human host to survive. They spend their entire lives on people and feed only on human blood.

While other types of lice live on birds or mammals, this specific kind only lives on humans. This makes them very specialized for living on our heads.

To survive, the head louse has a very specific way of moving and eating. They do not have wings, so they cannot fly. Their legs are also short and stumpy, which means they cannot jump. Instead, they use six legs that end in a claw and a thumb-like part. This allows them to grip hair strands very tightly and climb quickly. When it is time to eat, they use mouthparts to pierce the skin. They inject saliva to help them suck blood several times every day.

Life for a louse begins as a tiny egg called a nit.

A female louse lays about three or four eggs every single day. She uses a special glue made of proteins to stick the eggs to the hair. These eggs are usually placed near the scalp, about 3 to 5 mm away. After six to nine days, the eggs hatch into baby lice called nymphs.

These nymphs must molt, or shed their skin, three times before they become adults.

Scientists have studied how head lice are different from other insects. For example, they are closely related to body lice, which live in clothing.

Genetic studies suggest these two groups split apart about 30,000 to 110,000 years ago. This happened around the time humans began wearing much more clothing. While they look almost the same, head lice prefer hair while body lice prefer fabric. Adult head lice are very small, measuring only about 2.5 to 3 mm in length.

Most people encounter lice through close social contact. Because they cannot jump, they move from one person to another by crawling.

Head-to-head contact is the most common way they spread. This is why children in schools are often the most affected group. In the United States, about 6 to 12 million people are treated for lice every year. In the UK, many children will have lice at least once before they finish primary school.

The head louse, known scientifically as *Pediculus humanus capitis*, is an obligate ectoparasite of humans.

Being an obligate ectoparasite means this insect must live on the outside of a human host to survive. They spend their entire life cycles on the human scalp. They feed exclusively on human blood, a behavior called hematophagia. While other lice species infest various birds and mammals, this specific species is known only to host humans. This makes them highly specialized for a life spent entirely on the human head.

To survive on a host, the head louse has developed unique physical adaptations. They are wingless insects, meaning they cannot fly to find new hosts. Their legs are short and stumpy, which makes them unable to jump or walk efficiently on flat surfaces.

Instead, they use six legs that end in a single claw and an opposing "thumb." This specialized anatomy allows them to grasp hair strands tightly. They can climb up strands of hair very quickly to move between hosts. Their bodies are dorsoventrally flattened, which helps them stay close to the scalp.

An adult head louse is quite small, typically measuring between 2.5 and 3 mm in length.

Their bodies are generally grey, though their color can change based on their environment. After a meal, the consumed blood can cause the louse to take on a reddish color. The head features one pair of antennae with five segments and one pair of eyes. Their mouthparts are highly specialized for piercing skin and sucking blood. These parts are usually retracted into the head and only emerge during feeding. They bite the skin four to five times daily, injecting saliva containing an anticoagulant.

The life cycle of the head louse begins with the laying of eggs, often called nits.

A female louse is oviparous, meaning she lays eggs. She typically lays about three to four eggs every day. To ensure the eggs stay in place, the female secretes a glue from her reproductive organ. This glue hardens into a "nit sheath" that covers the hair shaft. Recent studies show this glue is made of proteins similar to hair keratin. The eggs are oval, roughly 0.8 mm long, and are usually placed 3–5 mm from the scalp.

After the eggs hatch, the insect enters the nymph stage.

Head lice undergo hemimetabolous development, which is a type of gradual metamorphosis. A newly hatched nymph must molt, or shed its skin, three times before becoming a sexually mature adult, also called an imago. Each nymphal stage, or instar, lasts about four to five days. The entire process from egg to adult takes between 18 and 24 days. Nymphs behave much like adults and must feed on blood to survive. They cannot survive for more than 24 hours away from a human host.

Reproduction is a vital part of their survival and requires sexual copulation.

Males and females can begin pairing within 10 hours of becoming adults. During mating, the male uses specialized larger front legs to hold the female. This process can last for more than an hour. A single female can lay between 50 and 150 eggs during her four-week lifespan. Interestingly, the head louse is closely related to the body louse, *Pediculus humanus humanus*. Genetic studies suggest these two subspecies diverged 30,000 to 110,000 years ago when humans began wearing significant amounts of clothing.

Infestations, known as pediculosis capitis, are common among certain groups.

Children between the ages of 4 and 14 are the most frequently infested. Research shows that girls are two to four times more likely to be infested than boys. Because lice cannot jump, they usually spread through close head-to-head contact. In the United States, between 6 and 12 million people are treated for head lice every year. In the United Kingdom, it is estimated that two-thirds of children will experience at least one case of head lice before leaving primary school.