"How quickly do lice start laying eggs?"
If a head louse transfers onto hair if it has already been fertilized it can lay eggs immediately. Eggs once laid take approximately six to seven days to become viable and hatch. If more than one head louse has transposed to the hair they could all be at different stages thus meaning that eggs are all hatching at different times. Head lice eggs are notoriously hard to remove from the hair as they are glued to the hair with a very strong bonding agent that the mother louse produces when laying the eggs.
"Why are head lice eggs so difficult to remove?"
A head lice egg is made by structure to protected the louse by making the nit sheath similar in composition to the hair; thereby, agents designed to unravel the nit sheath may also damage human hair. So if treatments were made to destroy the egg they would also destroy the hair.
The colour of the egg is such that is camouflaged by the hair as the sheath has a reflective quality and though usually a white or cream colour, the reflective sheath takes on the colour of hair it is bonded to.
Research and analysis from University of North Carolina at Chapel Hill, North Carolina, USA has revealed that the nit sheath of the head louse is composed of 4 bands of protein. As the proteins bond together this makes the egg extremely strong and safe place to grow and unfortunately a very difficult thing to remove. The egg is so small 0.1mm that in many cases the egg is smaller than the width of the average space between teeth of most popular nit combs is 0.3mm making it virtualy impossible to remove. The majority of the time theyare also laid very close to the scalp so that they have an easy food source, this in turn of course make it even more difficult to remove. However when hair is tied back they could end up anywhere on the head - this is why it is so important to remove nits and eggs.
"What is the difference between an egg and a nit?"
An egg is a shell with a viable louse growing to the stage of a nymph. A nit is the shell left behind when the nymph has hatched.
A viable egg
Nymph hatching from egg