Most of their victims are insects but the largest tropical species occasionally make a meal of vertebrates such as frogs, lizards, fish and small mammals, said experts.

There are more than 45,000 species of spider living in all parts of the world with a collective weight of about 25 million tons.

Together they kill between 400 million and 800 million tons of prey annually, a team of Swiss and Swedish scientists has calculated.

In comparison, all the humans on Earth consume about 400 million tons of meat and fish each year, according to the United Nations Food and Agriculture Organisation.

The appetite of spiders even exceeds that of whales, which get through an estimated 280 million to 500 million tons of prey a year.

Lead researcher Dr Martin Nyffeler, from the University of Basel in Switzerland, said: "Our calculations let us quantify for the first time on a global scale that spiders are major natural enemies of insects.

"In concert with other insectivorous animals such as ants and birds, they help to reduce the population densities of insects significantly,.

"Spiders thus make an essential contribution to maintaining the ecological balance of nature."

Ninety per cent of spider prey consists of insects and springtails, small insect-like arthropods, the study published in the journal The Science of Nature found.

The team showed that spiders killed many times more insects in forests and grasslands than in other habitats.

Their impact was lower in agricultural areas because intensively managed farmland is not favourable to spiders, the researchers said.