IT鈥橲 A balmy summer evening. Lying in bed under a cotton sheet, you close
your eyes and begin the drift down into sleep. Then, through the jumbled
thoughts of your semiconscious world, you hear a noise which snaps you back to
wakefulness. A high-frequency vibration has triggered your primeval fight or
flight response. You are not alone. Somewhere in the gloom is a
mosquito鈥攁nd it鈥檚 after your blood.
Blood-sucking insects are about as welcome as those other summer scourges,
hay fever and water shortages. Nothing complements your new swimwear quite like
a batch of itchy red lumps. But spare a thought for the poor mosquito. In her
quest for blood to nourish her developing eggs, she must risk her life.
Countless insects take the same gamble every day to acquire their protein-rich
food. In Southeast Asia there is even a species of moth, Calpe
eustrigata, which has changed its lifestyle so much that it has moved on
from drinking citrus fruit juice to drinking the juice of mammals.
鈥淚s there no escaping these bloodthirsty pests?鈥� you wonder as you jump out
of bed in pursuit of your nocturnal visitor. Probably not: they are attracted by
carbon dioxide in exhaled breath, body heat and even cheesy feet (This Week, 4
November 1995, p 7). Paradoxically, vegetarians seem to make the most desirable
meal. Their breath contains more octanol, a volatile alcohol produced when plant
material is digested. Biting midges鈥攖he tiny flies known as 鈥渘o-see-ums鈥�
in the US鈥攁re particularly partial to octanol, according to recent
research by William Mordue from the University of Aberdeen.
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You are starting to regret eating that spinach quiche. Your pursuit of the
wee beastie has proved futile, and submerging yourself in a vat of ice cubes and
holding your breath seems rather drastic. Nets around your bed would have done
the trick, but it鈥檚 too late for that. Anyway, entomologists have known for a
decade that insects can become wise to them. Studies in Papua New Guinea by
Derek Charlwood and Patricia Graves from the Papua New Guinea Institute of
Medical Research in Madang, showed that where bed nets are used widely,
mosquitoes shift their biting time to earlier in the evening.
If you were a smoker, now would be the time to light up. Biting insects rely
on their olfactory system to identify a suitable host, and chemicals in
cigarette smoke cause overstimulation. The active ingredient in most commercial
insect repellents, diethyl toluamide, works in a similar way, binding to
receptors on the olfactory hairs of the antennae. Some people swear by yeast
tablets鈥攖hey believe that vitamin B blocks taste receptors the insects use
when probing for blood. And then there鈥檚 bog myrtle. Oil from Myrica
gale repels the highland midge Culicoides impunctatus, probably by
disrupting olfactory cues.
None of these repellents are to hand, however, so stifled beneath your
bedclothes, you fall into a fitful sleep. You dream of compound eyes and
slavering insect mouthparts. These come in many shapes and sizes. Over millions
of years, different groups of insects have independently evolved the taste for
blood, so it is hardly surprising that they set about their victims in different
ways.
For example, natural selection has remodelled the mouthparts of mosquitoes
and the voracious assassin bug (Rhodnius), to form a piercing
proboscis. They work with stealth and a surgeon鈥檚 precision. Others are less
subtle. Biting midges and horseflies don鈥檛 so much pierce their hosts as saw
into them. But the bully of them all is Dipetelogaster maxima from
southern California, about the size of your thumb.
As you finally drop off, the mosquito in your room has landed gently on your
exposed brow and is now prospecting for blood. Unless it鈥檚 very lucky, it won鈥檛
tap directly into a capillary鈥攕kin is only about 5 per cent blood by
volume. Instead, the mosquito repeatedly stabs a small patch of your flesh until
a pool of blood forms just below the surface. As it probes, the insect pumps out
saliva containing vasodilators, anticoagulant enzymes and platelet-aggregation
inhibitors to keep the blood flowing smoothly.
The aim now is to spend as little time as possible extracting a meal, and
insects do this by exploiting an unusual property of blood. Unlike most fluids,
the viscosity of blood varies depending on the diameter of the tube through
which it is flowing. In large-diameter tubes, the red blood cells are
distributed at random throughout the plasma, and the blood behaves much like a
normal fluid. But in tubes less than 100 micrometres across, the viscosity
starts to drop鈥攂lood cells get squashed and tend to collect in the centre
of the tube. Minimum viscosity is reached in a tube about 10 micrometres across.
Below this, blood cell diameter takes up a significant fraction of the tube
diameter and sucking blood becomes like sucking peas through a straw. Many
blood-sucking insects of all sizes have feeding tubes 10 micrometres wide.
Mosquitoes and their ilk are, quite literally, air heads. Their heads contain
cavities which expand when the insect contracts the surrounding muscles. As a
result of the drop in pressure, blood flows up through the feeding canal. The
assassin bug has the ultimate pump-action head. Its volume is taken up almost
entirely with cavities and muscles that can create a pressure differential of
between 1 and 2 atmospheres. Blood flows into the insect at a rate of about 5
metres per second.
Assassin bugs take in six times their body mass鈥攁bout 300 microlitres
of blood in 15 minutes. That鈥檚 equivalent to knocking back 450 pints of beer.
The massive Dipetelogaster maxima will take up to 4 millilitres at a
sitting. And the tsetse fly is so bloated after feeding that it often cannot fly
and is forced to glide away from its host. Your mosquito, however, sips a modest
4 microlitres of blood. Like other blood feeders, its gut contains stretch
receptors connected to the brain which tell it when to stop sucking.
Tomorrow morning you will wake up to find an itchy red blotch on your
forehead. Your immune system has kicked in, producing histamine to start the
healing process. Meanwhile, over the next two or three days, the female mosquito
will digest her meal. Later she will lay between 50 and 200 black-brown ovoid
eggs and as your skin returns to its flawless state, a whole new generation of
the little bloodsuckers will be growing in that stagnant pool at the bottom of
your garden.