Head lice, nits, and nymphs are three different things, and understanding the difference is the first step to actually dealing with an infestation properly. Here's a full breakdown of what each one is, what they look like, and how to tell them apart from things that only look similar.
What is a head louse?
A head louse (scientific name Pediculus humanus capitis, though it's often shortened to Pediculus capitis) is a parasitic insect that lives exclusively on the human scalp and feeds on human blood. Head lice cause itching through their bites, but they don't carry or transmit disease. The only real health risk comes indirectly: scratching a bite with a dirty fingernail can occasionally introduce bacteria and cause a minor skin infection, not from the bite itself.
Head lice are genuinely common. In fact, head lice infestation is the second most common communicable condition affecting schoolchildren, behind only the common cold.
Louse, nit, and nymph: what's the difference?
These three terms get used interchangeably, but they actually describe three different life stages:
Louse (plural: lice) refers to the insect itself, at any age.
Nit is the egg laid by an adult female louse.
Nymph is a young, newly hatched louse, before it reaches adulthood.
Knowing which term applies to what you're looking at matters, because each stage looks and behaves differently.
What does an adult head louse look like?
An adult louse resembles a dark sesame seed or a small grain of rice. Colour ranges from light brown to grey to almost black, and colour tends to darken with age, a younger louse (nymph) is often lighter and slightly transparent. Lice are sensitive to temperature and humidity, and the human scalp provides close to ideal conditions for them. They actually regulate their own temperature by moving to different parts of the scalp, staying warmer under a ponytail and cooler near the hairline.
How head lice move (and why they can't jump or fly)
Lice move quickly across the scalp using six specially adapted legs, each ending in a tiny claw designed to grip a hair shaft. This grip is strong enough to make lice genuinely difficult to dislodge, and it's also why they can move rapidly around the scalp, which makes them hard to spot on dry hair even when you're looking carefully. The same claw structure means lice cannot fly or jump between heads. They can only crawl, and only through direct contact between hair. Head lice also don't burrow into the skin, they live and feed entirely on the surface of the scalp.
What do nits look like?
Nits are small, oval-shaped eggs, brownish or greyish in colour, and firmly cemented at an angle to the side of the hair shaft. Female lice lay nits as close to the scalp as possible, since that's where the temperature and humidity are most favourable for the egg to develop. A simple rule of thumb: eggs found more than 1 to 2 centimetres from the scalp are usually already empty casings left behind after the nymph has hatched, often lighter or white in colour rather than the darker shade of a viable egg.
There's a reliable physical test to check what you're looking at: a genuine nit is cemented on and won't dislodge easily. If you can blow it off, flick it away, or crumble it between your fingers, it isn't a nit.
Nits vs. dandruff and "hair casts": how to tell them apart
This is one of the most common points of confusion, and it's a genuinely reasonable one. Dandruff flakes are irregular, dry, and dislodge freely with the lightest touch, nothing like the firmly cemented structure of an actual nit.
A less commonly known look-alike is the hair cast (sometimes abbreviated HC), occasionally called a "pseudo-nit" because of how easily it's mistaken for the real thing. A hair cast is thin, elongated, firm, and whitish, and it encases the hair shaft rather than attaching to its side. Unlike a genuine nit, a hair cast can usually be slid off the hair shaft without much force at all. There are two recognised types, peripilar casts and parakeratotic keratin casts, both of which are a scalp-related keratin buildup rather than anything to do with head lice.
The practical takeaway: if something slides freely down the hair, it's very unlikely to be a nit, regardless of colour or shape.
Frequently asked questions
What is the scientific name for head lice?
Pediculus humanus capitis, commonly shortened to Pediculus capitis.
Is a nit the same as a louse?
No. A louse is the insect itself. A nit is the egg it lays. A nymph is a newly hatched, immature louse.
Do head lice spread disease?
No. Head lice cause itching from their bites but don't transmit disease. Infection risk comes only from scratching a bite with a dirty finger, not from the louse itself.
Can head lice jump or fly?
No. Their legs are adapted for gripping hair, not jumping or flying. They spread only through direct, close contact.
How do I know if it's a nit or just dandruff?
A genuine nit is firmly cemented to the hair shaft and won't dislodge with a light touch. Dandruff and hair casts both slide or brush off far more easily.
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