The merciless mother spiders would consume new suitors after they had already mated and were expecting spiderlings.

The team said their study was the first 'natural' experiment to prove correct the old folklore about spiders, and said it also shows why such behavior might be beneficial.

The Mediterranean tarantulas in the study did not eat their mates, but instead ate males before courtship - and usually after the females had already mated with another male, the researchers found.

Some studies had suggested that studying spiders in the lab produced skewed results, perhaps because the creatures were stressed or perhaps because they could not obtain all their needed prey or nutrients.

So the researchers set up a field experiment in which they watched the spiders, sometimes snatching the males from the jaws of females before they were devoured.

'At natural rates of encounter with males, approximately a third of L. tarantula females cannibalized the male,' they wrote in their report, published in the Public Library of Science journal PLoS ONE.

'The rate of sexual cannibalism increased with male availability, and females were more likely to kill and consume an approaching male if they had previously mated with another male,' they added.

'We show that females benefit from feeding on a male by breeding earlier, producing 30 per cent more offspring per egg sac, and producing progeny of higher body condition.

'Offspring of sexually cannibalistic females dispersed earlier and were larger later in the season than spiderlings of non-cannibalistic females.'

One theory had also held that females who ate males were simply more aggressive and perhaps better hunters - but when the males were saved just in time, those females did not produce superior broods, suggesting that the male meals were an important source of nutrition.

Source: Daily Mail