The insect has been quickly breeding - and sucking blood - along the rivers and reservoirs of Catalonia and Aragon, causing alarm in small towns.

Only two to three millimetres long, the fly is much smaller and harder to spot than most mosquitoes, but its voracious bite sent more than 2,000 people to hospital last year in Catalonia alone. Its vigorous jaw, which releases a cocktail of chemicals, can produce allergic reactions.

"If the mosquito is a neurosurgeon that bites with a probe, the black fly is a butcher that scratches the skin and makes you bleed," Raul Escosa, member of an Ebro river environmental board, told El Pais.

"We had to take my 18-year-old daughter to the dermatologist and the allergist because she had a dozen swellings of eight to 10 centimetres," said Jesus Llop, a town council member in the town of Mequinenza.

The black fly, an umbrella term for several Simulium species, was first detected in the region in 1997, and it has been making its annoying presence increasingly felt. Unlike the mosquito, it breeds in clean river water. Regional experts believe the current outbreak stems from improvements in water quality and new irrigation channels, which created a new habitat.

The insect injects an anaesthetic, an anti-clotting agent and a vasodilator into the skin of its host, who belatedly notices the damage after the fly has moved on. In Switzerland an attacking swarm reportedly killed a calf.