Good vibrations (courtesy of Dr K W Kim)

It is like something out of a horror movie: baby spiders devour their own mother, then climb aboard her web, and make it throb in a series of pulsating vibrations.

But that is exactly what black-lace weaver spiderlings do, a biologist in South Korea has discovered.

After cannibalizing their mother, up to 160 spiderlings gather and contract their bodies in synchrony, collectively pulling at the web to make it vibrate.

Ultimate sacrifice

Most spiders are solitary animals.

But some are social animals, living in communal webs throughout their lives, cooperating in nest construction, brood caring and prey hunting.

Other species are subsocial, where individual spiders come together and cooperate at certain stages of their lives.

They belong to the subsocial spiders, with the young baby spiders displaying a range of intriguing behaviours.

For example, female A. ferox spiders produce a single clutch of 60-130 spiderlings, which she feeds by laying eggs for her offspring to eat.

But then she makes the ultimate sacrifice: she encourages her spiderlings onto her body, and allows them to devour her alive (see video below).

After she has died, the spiderlings then form a social group for 3-4 weeks until they disperse from the social nest.

During this time, the spiderlings are known to cooperate by going hunting together.

By cooperating on a hunt, the spiderlings are capable of attacking and subduing prey up to 20 times bigger than themselves.

Web shake

But Dr Kil Won Kim of the University of Incheon of the Republic of Korea, who has researched this cooperative hunting behaviour, has discovered that the spiderlings gang together in another bizarre, and previously unknown way.

"After matriphagy [the eating of the mother], A. ferox spiderlings show synchronous movement, contracting their bodies simultaneously," Dr Kim told the BBC.

This behaviour emerged the day after the death of the mother, and was triggered by intruding insects, mites or worms approaching the web.

An individual spiderling contracted its body, pulling at the web as it did so.

Almost immediately, other spiderlings joined in, also contracting their bodies.

That created a bigger effect, which was to make the whole web throb in a series of rhythmic vibrations.

At its peak, up to 60 per cent of the spiderlings engaged in this behaviour at any one time.

The denser the group of spiderlings, the stronger the vibrations of the web, and the presence of other spiderlings nearby encouraged others to also contract their bodies.

The spiderlings also performed contractions at the highest frequency four days after eating their mother, with the behaviour declining as they aged.

It also occured during the period before the baby spiderlings were old enough to go hunting.

Scary tactic

It is unclear why the baby spiders make their web throb in this way.

"Contractions may function as antipredatory behaviour," says Dr Kim.

A few lines of evidence point to this.

The baby spiders do not do it in the presence of the mother, which likely protects them before sacrificing herself.

They also vibrate only when a large intruder is nearby, suggesting it is a defence mechanism, as the vibrations would be transmitted to any intruder touching the web.

The visible movements of the web may also give an intruder the impression that there is a much larger organism nearby, again deterring them from approaching the vulnerable baby spiders.

Few other collective defence responses have been recorded in spiders.

One other example is in the territorial social spider Cyrtophora moluccencis. When a bee or wasp flies over a female's cocoon, she will shake it vigorously, an action that prompts other females nearby to also shake their cocoons, perhaps to deter the invader.