Head lice are adapted to live on the scalp of humans and do not voluntary leave the head, as this is where the food is, and the temperature and humidity is just right. A head lice that “falls” off the hair will typically be ill or dying. The probability of finding their way back to a head and re-establishing is very unlikely. If you find a head louse in the bed or anywhere else in your surroundings, it will not constitute a risk of infestation. It is nothing but an old myth that you must spring clean your house when you have head lice.
Head lice gently removed from their host are able to stay active for more than 24 hours without food. However, this would normally require that the temperature is lower and the humidity higher than is the case in most homes. Studies have shown that at normal indoor temperature and humidity, most head lice die 40 hours after their last blood meal. They are only able to suck blood and re-establish if they find their way back to their host or find a new host within a very few hours. Furthermore, a head louse that do not have a strand of hair to grab onto, can barely move around.
Eggs removed from the hair may develop completely and theoretically constitute an infestation risk. In practise, however, the eggs will die due to the lower temperature and lower humidity away from the head. The eggs do not fall off by themselves.
Consequently, though head lice can survive outside the scalp for a short period of time under very special conditions, there is no reason to worry about this theoretical risk.
Head lice spend time sucking blood, mating and laying eggs, but for most of their lives, the head lice are inactive. They sit quietly and hold on to the hair, with the claws on all of their legs. When they rest, their bodies are situated along the hair strands with their heads pointing towards the scalp. They move along the hair shaft and they easily move both forwards and backwards. Furthermore, they can move sideways over the hair, and when doing this, they move almost like a crab, as they transfer their legs from one hair to another.
Transfer of head lice to other hosts usually takes place via ‘interruptions’ in the hair. Such interruptions may stimulate head lice in certain stages of their lives to move towards the outer layer of hair. During the first and second nymph stage, head lice are less active than the older stage head lice and more are inclined to stay close to the scalp. Therefore, it is less likely that small nymphs transfer to another host. The adult males are the most mobile – probably because of their constant search for a possible mate. Consequently, it is not uncommon that the first head louse to invade a head is male. On average, however, more female head lice than males are found.