Like shrivelled versions of creatures from Dune, they are pinker and slimier than their British cousins, and are surrounded by a cocoon of sticky mucus. They stretch out as long as 6in to 8in when squirming along. They are a formidable predator, devouring as many as 14 British earthworms in a week, but capable of going for a year without food by slowly absorbing their own tissue.
The background: They arrived piggyback on a seed from the Antipodes, were first discovered some 30 years ago in Belfast gardens, and were identified in London's Natural History Museum as a native of New Zealand's South Island beach forests. This small, ribbon-like creature almost certainly travelled half-way around the globe as a stowaway in the soil of pot plants.
The effect: National panic. There were sightings from Carlisle to Christchurch and there was only one topic of conversation: how could the British, brandishing pitch fork and sharpened spade, rid themselves of the Antipodean interloper? There were warlike rallying cries to protect the fine, upstanding British worm - the cornerstone of this country's ecological system and vital to the survival of plants and soil life - from the Kiwi cannibals who were blamed for waterlogged soil, dying trees and the national debt. Questions were asked in the highest chambers of the land: in the House of Lords, Lord Carter said the flatworms "go round in gangs and jump on earthworms". Earl Howe, a junior agricultural minister, reported that the official advice from the Central Science Laboratory to anyone encountering a New Zealand flatworm was: "to tread on it or pour something hot or salty on top of it".
Moments of subsequence: Government instructions to stamp on the flatworm were unsuccessful - as Lord Aberdare asked, "Are you sure it's effective to stamp on a worm that's already flat?"
Though they may have squirmed out of the public limelight, they have certainly not disappeared from gardens.
A few months after the initial media obsession with the killer worms, a second species has appeared in southern Britain - a smaller, less harmful variant. This was followed more recently by the discovery in Scotland of two more varieties of flatworm not before seen in this country.
The flatworm seems to have almost no predators, but preliminary research in Edinburgh now suggests that a so-far-unidentified beetle may eat the invader. "What we really need is funding for a post-graduate to do a thesis," says Dr Cosens. "That would revolutionise our understanding."