New Delhi, Oct. 14: A spider has been caught supping on camera.

This is news not because the spider is exercising a choice that is denied to many citizens in several Indian states but because it is said to be only the third such instance to be documented in the country. The first was recorded six decades ago in a village near Calcutta.

Scientists in Kerala have photographed a giant spider, the size of an adult human palm, feeding on a tiny bat in an unusual predator-prey relationship recorded elsewhere but rarely.

In the pure-chance observation, the scientists who were on a bird-watching assignment in Thrissur district spotted a giant wood spider feeding on an evening bat caught in its web on a tree about two metres from the ground.

"This is an extremely rare event - a predator-prey relationship between spiders and bats is not something we would expect to see," Pyngamadom Ommer Nameer, an associate professor of wildlife at the Kerala Agricultural University, told The Telegraph.

Nameer and a team of KAU research scholars spotted the spider feeding on the bat a little past noon on November 25, 2013, but have described their observations only this week in Current Science, a journal published by the Indian Academy of Sciences.

The spider moved away as the KAU team members approached the web. The scientists extracted the bat and identified it as belonging to an insectivorous family of evening bats called Vespertilionidae, but could not pinpoint the species because the spider had consumed the bat's head.

"This is an unexpected observation because bats are vertebrates and flying mammals, and could damage spider webs," Nameer said. "But this has also been documented elsewhere in the world, so it's possible this is something that has just gone unnoticed."

Biologists Mirjam Knornschild from the University of Ulm, Germany, and conservation scientist Martin Nyffeler at the University of Basel, Switzerland, had two years ago catalogued a set of 52 reports of bat-catching spiders between 1842 and 2013.

The two scientists, who had published their review of bat predation by spiders in the journal PLOS One, said it was unclear how many of the incidents were genuine predation and how many were just deaths by web ensnarement "without active involvement of the spider".

G.C. Bhattacharya, a scientist at the Bose Research Institute in Calcutta, was the first in India to document a spider preying on a bat in March 1941. He published a graphic account of the bat's last moments in a crevice in a cowshed surrounded by a matted wall "in a neighbouring village of Calcutta".

A house spider had gripped the bat's neck with its powerful mandibles. "There was intermittent gasping and screaming of the bat... the bat tried to drag itself away a distance, crawling with its peculiar habitual gait with the help of its forearms, the spider all along keeping its hold," Bhattacharya had chronicled in a 1941 issue of the academy journal, Current Science.

The second Indian report of bat predation by a spider emerged only three years ago, also from a Kerala forest. A theraposid spider was seen feeding on a bat. Knornschild and Nyffeler say such spiders are "fully equipped with toxins and enzymes to subdue and devour small vertebrate prey".