Head lice suck blood. Quite a lot of other insects like our blood, too. There are fleas, bed-bugs, mosquitoes, midges, and a small brown moth called Calyptra eustrigata. Not to mention the non-insect groups like ticks, chiggers (skin-burrowing mites) and leaches.

Blood is good food. It is high in protein, with all those red blood corpuscles full of haemoglobin, and the serum fluid filled with complex biochemicals. And as it’s a watery liquid, it is easily sucked up without any need to bite or chew.

But contrary to ideas put about in romantic teen vampire fiction, blood is not the perfect diet and eating blood alone will lead to nutritional deficiencies. Blood-feeders like mosquitoes eat blood only as adults. As larvae, they get a whole other set of nutrients from the decaying organic matter on which they feed. Blood is just a top-up food to mature their eggs. But head lice feed on blood alone and they need extra help. They get this from micro-organisms living inside their bodies.

The head louse has special pockets called mycetomes attached to its guts. These are full of dedicated bacteria. They digest the blood differently and supply the louse with vitamins and other chemicals it cannot produce on its own. Depriving a louse of its live-in bacteria severely limits its growth, and may kill it. Surely there’s a louse-remedy research project there somewhere?

Not all blood is equally acceptable to a louse, though. Alligator blood cells are too big to pass up the louse’s snout, for example. Guinea pig haemoglobin crystallises in the gut and rips the louse intestines open. Human blood, though, is just right

Critters by Ivydale Primary School, Natural History Club.

You have to respect an animal that bites something 100 million times larger than itself. But you also have to question how sensible it is to try and tap blood pumped under pressure from a muscular heart. Surely the louse would inflate like a balloon? Or would it drown in leaking blood?

The louse’s mission is a logistical nightmare, but it is armed with the right tools for the job. Its piercing mouthparts are very thin and sharp. As the biting tube is inserted into the skin, small hooks around it extend and anchor the louse in place. There is no danger of a blow-out.

There was a time when medical textbooks and louse monographs worried about the effects of blood loss. Could severe louse infestation cause anaemia?

For many years, blood loss was guesstimated. No one could accurately weigh a louse, or even discover how often they fed, so it was all a bit haphazard. Finally, Australian phthirapterist Rick Speare did some proper measurements in 2006. Lots of head lice were collected from helpful schoolchildren. They were kept in humane conditions, warm and moist, but were starved for six hours (the lice, not the children). Batches were weighed on super-accurate electronic scales, allowed to feed on the back of a hand (whose we are not told) and then reweighed

Blood meals were minuscule: 0.0001579 (that’s 1/6000) of a millilitre in an adult female, 0.0000657 (1/15,000) of a millilitre in a male. These are truly trivial amounts. Thousands of lice biting all day long could not cause serious blood loss.

The real worry, though, comes not from what the louse takes away in its meagre blood meal, but what it leaves behind in its saliva.

If you make a Lego head louse, remember to use some red bricks to show its blood-filled intestines snaking through its translucent body.

Human blood clots easily. Otherwise, even the slightest nick shaving or chopping the vegetables would put us in mortal danger as it all leaked away. A complex biochemical reactions starts in a fraction of a second if even the tiniest capillary blood vessel is damaged. Even if it is bitten into by a head louse. By all reports, the louse should just get a mouthful of wet scab. But the louse is ahead of the game, and instead drinks its fill

To stop our blood clogging up their fine sucking mouth-parts, head lice wage chemical war on us. As they suck, they also inject saliva full of anticoagulants to stop the blood cells clotting together. It works perfectly.

In seriously heavy infestations (that’s many hundreds of lice), a person may be receiving thousands of bites each day. The human immune system eventually starts reacting to all the injected louse saliva. The victim has a slightly raised temperature, they feel achy, lethargic, irritable and genuinely unwell; they feel, in a word, lousy

Children suffering in this way become tired, sullen and withdrawn. They can’t concentrate and in less enlightened times were branded nitwits by their teachers. It was thought that head lice preferred less intelligent heads, but it now turns out that louse saliva was interfering with the children’s education.

All that biting on the head – and the scratching that goes with it – takes a toll on the skin, too. It makes the scalp feel tight and dull, feeling and sensation are reduced and the victim is becoming, quite literally, a numbskull.

Louse researchers still have to be careful of allergic reactions. The dry, dusty louse faeces can cause coughing and sneezing (just like hay fever) if too much gets into the laboratory air.