Poison: cold, coiled and lurking. An enamelled skin, sloughed off annually. A flickering, forked tongue and eyes that never blink. All this makes "viper" a byword for treachery, for prettily camouflaged wickedness, a lethal danger you never see. "If a snake or a viper cross your path, watch out for false friends", goes an English proverb. Cherishing a viper in your bosom is showering kindness on someone who turns out unworthy, ungrateful. "How sharper than a serpent's tooth it is, to have a thankless child", says King Lear.

Yet snakes have also been potent sacred symbols. Egyptian Pharoahs had cobras on their crown, the Indian god Vishnu sleeps on the serpent of eternity, Aztecs worshipped the plumed serpent. Ancient symbolisms like these, evolved in hot countries with many lethally poisonous snakes like Israel, Egypt, the Mediterranean, profoundly shaped Western imagination. John the Baptist calls the Pharisees a "generation of vipers"; the serpent who poisons Eve morally brings about the Fall of Man; Eurydice, Orpheus's beloved, is bitten in the heel and dies; Philoctetes' festering snake-bite never heals; Cleopatra uses Nile asps for suicide.

Britain's only poisonous animal is the Common Viper (Latin vipera) also known here as the adder (from Old and Middle English naddere). Its bite is unpleasant rather than fatal, but we tend to elide it with myths from more dangerous climes. In Thomas Hardy's Return of the Native, the Eden of the hero's love is poisoned when his mother is turned away at the door by his wife, is bitten by an adder and dies on Egdon Heath. Snakes also have a Freudian phallic aura, and vipers' mythic victims are usually female. In Milton's Paradise Lost, Satan 's tempts Eve first visually, by his sexy surface glamour. His sleek enamelled neck "lures" Eve's eye, showing off "many a wanton wreath", in "rising folds that tow'red/ Fold above fold a surging maze; his head/ Crested aloft, and carbuncle his eyes;/ With burnished neck of verdant gold, erect/ Amidst his circling spires, that on the grass/ Floated redundant."

But Greek mythic females also collude with snakes. Medusa has vipers in her hair, Furies in their hands; Lamia the seductress is a snake in disguise. Again, surface shimmer camouflages lurking poison. As a snake, Lamia in Keats is "a gordian shape of dazzling hue,/ Vermilion-spotted, golden, green and blue". As a woman, she lures an innocent young man into fatal love.

In Greek myth, every negative has its positive. Pharmakon, "poison", also meant "healing drug" (hency "pharmacy"). Snakes were lethal, but sacred; they had power for good. In myth, they guarded sacred places and gave prophets power to understand bird speech. In the home, people worshipped them and built little shrines like the snake shrines of South India. Records from the healing temple at Epidaurus describe patients cured by snakes. Asclepius, Greek medicine's mythic father, began work after seeing one snake resuscitate another with herbs, and our modern medical emblem is still Asclepius' staff, entwined with those two snakes. For snakes, "children of earth", were daemonic intermediaries to the underworld (Hermes, divine messenger, also had snakes on his staff) and had the underworld power to kill or cure, bring good luck or bad.

The snake's double tongue also suggested the doubleness of good and bad. English folk medicine used "viper's flesh" as a cure; in Swallows and Amazons, Young Billy keeps an adder in a cigar box under his bed "for luck". As Edward Fitzgerald puts it in Omar Khayyám, God "devised the Snake" as well as Eden. The viper reminds us that good and bad belong together, both in the world created by God, and in ourselves.

D. H. Lawrence wrote a famous snake poem which uses all this tissue of reverence and fear. Watching a gold-coloured snake emerge "from a fissure in the earth-wall" to drink at a Sicilian trough, the poet feels he should kill it, for gold snakes are venomous. But he can't: "I felt so honoured". The snake looks round "like a god, unseeing, into the air". The poet throws a log at him, and then is deeply ashamed. "He seemed to me again like a king,/Like a king in exile, uncrowned in the underworld". As the snake vanishes, he feels "I missed my chance with one of the lords /Of life."

The snake's skin-shedding suggests transformation, both for the earth (which "like a snake", says Shelley, renews "her winter weeds outworn"), and – again - for ourselves. People take snajkes very personally, somehow. In American Indian myth, snakes symbolize rebirth: power to shed one's past self, start anew.

In imagination, vipers hint that everything we most fear - Furies, Gorgons, poison, the underworld, death – is sacred, with power to help, transform, bring luck. In reality, venomous snakes (ten percent of all snakes) developed venom to paralyze quick-moving prey (some venoms also help digestion). It had to be strong, to act fast. Snakes "smell" prey with tongue and mouth: the tongue's two prongs collect air samples, constantly flickering these back to the Jacobson's organ, a chemo-receptor in the roof of the mouth. Pit vipers evolved even more sophisticated equipment: heat-sensitive facial pits which "see" heat, homing in on warm-blooded prey. Margaret Atwood's poem "Bad Mouth" addresses a pit viper whose "nasty radar" locates "the deep red shadow/nothing else knows it casts."

Our adder, the round-nosed little European viper, is a more modest operator than these heady New World relations. Adders are internationally protected: the most northerly, and one of the most widespread, snake species in the world, and the only snake inside the Arctic Circle. Unlike Milton's Satan (and many snakes in other countries) they are timid, not aggressive. They get away, or pretend to be dead, rather than attack. To get bitten, you have to step on them or annoy them by trying to pick them up or (if you are a dog) sniffing at them. They live throughout Britain, but Cornwall is their national stronghold, and 1998 was a bumper year.