Profitable Wonders
‘The leech falls off and can survive three years before its next bloody meal’
The marvel of red blood cells is a commonplace as they effortlessly transport, millisecond by millisecond, billions of molecules of oxygen and carbon dioxide to and from the tissues.
The fluid plasma in which they are conveyed is, if anything, more marvellous still. It’s a liquid treasure trove, brimming with over 4,000 chemical compounds necessary for the fabrication and functioning of the body’s 70 trillion cells. They include water, 13 vitamins (A, B, C, D, E etc), ten essential metals (sodium, potassium, magnesium, zinc etc), the 20 amino acids from which all proteins are made (alanine, glycine, lysine, methionine, valine etc), dozens of hormones (cortisol, insulin, thyroxine, oestrogen etc), fatty acids, triglycerides and cholesterol, the sugars glucose and fructose – and much else besides.
Blood, in short, contains the full range of nutrients – and so is an abundant source of pre-packaged ‘ready meals’ available to any organism with the means of siphoning it off from the veins of unwitting hosts. This requires considerable ingenuity, though, with the distinct advantage that the expenditure of energy involved is vastly lower than (for example) that of bees and butterflies when gathering nectar, herbivores when chewing grass or carnivores in pursuit of their prey.
This habit of drinking the blood of fellow creatures (haematophagy) is indeed the preferred form of feeding for an impressive variety of diverse species. They include, inter alia, ‘vampire’ bats, birds and moths; leeches and hookworms; fleas, bedbugs and ticks; blackflies, horse and deer flies and (most significant of all for humans) the mosquito.
While their lifestyles could scarcely be more different, the practicalities of bloodsucking require that they share several features in common. They are all small enough that their hosts are usually unaware of their clandestine intentions.
Better still, the hosts should be asleep – so these haematophages tend to be nocturnal. They must thus possess highly sophisticated, sensory organs able to detect (and locate in the dark) the source of their next meal by the smells and heat they generate. Their saliva must contain a medley of complex anticoagulants to prevent their hosts’ blood from clotting. And their mouth parts must be adapted to form razor-sharp, needle-like or cutting instruments.
These common features apart, the fascinating (and sinister) connotations of vampirism are inescapable, prompting a grim interest in the specifics of the gory details.
Take that Gothic staple the vampire bat, whose distinctive feature, the one that sets it apart from its insect- and fruit-eating fellow species, is its chiselshaped incisors (so sharp that handling their skulls can result in serious injury), with which it makes a broad, shallow incision into its victim’s flesh.
The bat then drinks its fill of the blood oozing from the wound, facilitated by the piston-like movement of its grooved tongue and copious amounts of saliva infused with three types of blood-thinning agent.
The feeding adaptations of the leech (a close relative of the humble earthworm) are quite different, though similarly ingenious. Firmly attaching itself with what Professor of Biology Bill Schutt describes as a ‘finely tuned suctorial device’ – a circular disc of muscle glued in place with sticky, glandular secretions – the leech can enjoy a more leisurely meal.
The saw-like movement of its three jaws, each lined with a hundred teeth, abrades the capillaries in the skin, oozing blood, which is then sucked under negative suctorial pressure into its gut – capacious enough to hold ten times its body weight. Once satiated, the leech releases its grip and falls off, able to survive for a further three years before its next bloody meal.
The most notorious of bloodsuckers is the mosquito – the unwitting transmitter of three mortal illnesses (yellow fever, filariasis and malaria). The mosquito’s elegantly designed mouth parts consist of six interlocking rapier blades. It advances four of these blades, sharp and serrated at the tip, into the skin by moving its head back and forth. The fifth blade encloses a narrow tube, down which it injects its blood-thinning saliva – while the sixth, at the centre, is a rigid, strawlike, bloodsucking tongue.
Together, the combination of these six rapier blades, readily discernible in the fossilised remains of mosquitoes preserved in amber from 80 million years ago, is by far the most lethal weapon ever invented.