What Is the Head Lice Life Cycle?

Medically Reviewed by Stephanie S. Gardner, MD on February 21, 2024
5 min read

Head lice are small, wingless insects that live in human hair. Their scientific name is Pediculus humanus capitis, and they have been a human pest for thousands of years. 

Their full life cycle, from egg until death, lasts a maximum of 35 days. The eggs are called nits and hatch into small insect forms — called nymphs — which then grow into adult lice. The adult lice can begin to create more eggs as soon as they hatch and the cycle begins again. 

Each stage of the life cycle of head lice has a distinct appearance. This is how to recognize the three main stages of lice development: 

  • Nits. Head lice eggs/nits are about the size of a pinhead. They are very firmly attached to your hair and will not move around. This lack of movement is one way that you can tell them apart from dirt, dandruff, sand, and other debris — all of which can look like nits. They have darker coloration when they are alive and will pop between your fingernails. They become pale and almost see-through when they have hatched. Their sides look shriveled when they are dead. 
  • Nymphs. The head lice nymph is the intermediate stage in the lice life cycle. The shape mostly resembles an adult, with six legs coming off of a central body. The nymphs are much smaller than the adults. They begin around the size of the head of a pin and get larger each time they molt — or shed their skin. 
  • Adults. These are around the size of a sesame seed. They are brown and range from darker to lighter shades. The females tend to be slightly larger than the males. An adult louse doesn’t have wings or legs that let it jump. The only way lice can get around is by crawling.

Humans are the only host that head lice can live on. Adult lice need to eat our blood multiple times a day in order to survive. They cannot survive on pets or other animals, so the lice’s life cycle takes place only on human scalps.

The stages of the head lice life cycle are broken up into nine key steps: 

  • First, the eggs are laid. They are firmly attached to your hair — around the base of the strands.
  • They emerge from the egg around a week later.
  • They go through their first molt — or shedding of their outer layer — two days after they hatch. They get bigger with this molt. 
  • They go through their second molt three days after this and get even larger. 
  • Their third and final molt takes place 5 days later. They are now in their adult forms.
  • The adult forms of the males and females — which look slightly different from one another — begin to mate and reproduce immediately.
  • Females will lay their first batch of eggs around 2 days after mating.
  • Females can keep laying eggs for the next 16 days — up to 8 per day
  • After — at most — 35 days of life, the adult lice die. 

Head lice are most often spread by direct hair-to-hair contact. This can happen when children are playing together or are otherwise in close contact with people. Contrary to a common rumor, they are not spread or caused by bad hygiene. 

Less often, you can also get head lice from an object that was in contact with an infested head such as: 

  • Hairbrushes
  • Hats, scarves, earmuffs
  • Pillows, blankets, and upholstery

Lice, however, don’t live long when they are away from the human scalp. They need your body heat to maintain a healthy temperature and survive.

Nits are easier to locate than the adult lice because they don’t move around. You’ll need a very fine-toothed comb, or a comb specifically made for lice. You should work in a well-lit area and follow these steps: 

  • Wet the hair: It’s easier to see the lice and nits against the darker background. You can either use water to wet the whole head or comb conditioner into dry hair. The conditioner can have the added advantage of stunning the adult lice and slowing them down. 
  • Comb all sections of the hair: Move section by section through the hair, carefully combing through the strands and observing the hair. You should remove the comb after a few strokes and wipe it on a paper towel to check for lice or nits there. You should cover the entire scalp but focus on the areas of the scalp around the ears and the nape of the neck. These regions tend to have the most lice.
  • Decide what to do: If you find head lice in your search then you should begin treatment. A number of options are available and discussed below. If you don’t see any lice or nits at that time but think that there is still a risk of exposure, check again in a week.

The main focus of your treatment plan should be on the head and hair. There are both over-the-counter and prescription treatments for head lice. These are usually in the form of a cream or other product that is applied to all parts of the hair. 

It’s important that you read and follow the specific instructions that come with each of the different treatments. Some are not safe for children under age 2. This will be clearly labeled on the box. 

You shouldn’t use any conditioner on your hair immediately before or for up to two days after you apply most head lice treatments.   

You should begin treatment as soon as you notice the lice or nits. Check for lice again around a day after your first treatment. If there are still a lot of living ones, they may be resistant to the medicine you used. In this case, you’ll need to try a different over-the-counter or prescription product. 

If your treatment does seem to kill the lice, you should check again a week later. Repeat the treatment a week later if you still see signs of the lice or nits. 

Although less important, it can also help reduce the spread of head lice if you treat objects that the lice-ridden head has been in contact with. This can include: 

  • Wash all linens. Wash all pillowcases, blankets, sheets, quilts that hair has been in contact with. Your dryer should reach a hot enough temperature to kill the lice. 
  • Soak plastic items. Put hair brushes, combs, and similar objects in a pot of boiling water for 10 minutes to kill the lice. 
  • Bag objects for two weeks. Put anything that can’t be washed or boiled in a sealed plastic bag. Leave it alone for two weeks and the lice will die.   

You should only see a doctor if you need to try a prescription medication or if the lice return after treatment.