25 May 2011 Last updated at 01:11

Spiders protect webs with decorations

By Victoria Gill Science reporter, BBC Nature

The orb-weaving spider Argiope keyserlingi (Image: Andre Walter)

Previous research has shown that decorated webs tend to last longer

The delicate web decorations spun by some orb-weaving spiders are a strange but well-known phenomena, but exactly why the spiders adorn their webs is unclear.

Now a study by researchers in Australia suggests that the creatures use decorations to protect their webs from damage.

A team has made a discovery in one spider species that suggests the spiders use adornments "tactically" to make their webs more visible to animals that might accidentally damage them.

The findings are published in the journal Behavioural Ecology and Sociobiology.

"When I started studying this behaviour I had no idea that it was such a tricky area," said lead researcher Dr Andre Walter from the University of Melbourne in Australia.

"The debate about [its] function has lasted for over 100 years and is still highly controversial."

Previous research has shown that decorated webs tend to last longer than undecorated ones. But Dr Walter wanted to find out if this protective function was what motivated the spiders to spin their decorations.

He set up plastic frames in his lab, and left a group of orb-weaving Argiope keyserlingi spiders to build their webs in the frames. Once they were finished, the team carried out some controlled damage.

Dr Walter divided the spiders into three groups, left one group's webs alone, "lightly damaged" another group's and carried out heavy damage on another.

Studies have suggested that web decorations attract prey
Decorations make spiders more visible to predators, so scientists think the local environment - and what types of predators are around - influences how much a spider decorates its web

When they repaired or rebuilt their webs, the spiders increased their decorating activity following heavy damage but not mild damage, he reported.

So he thinks the spiders deliberately make their webs more visible to passing animals that might unintentionally walk into them.

"The spiders tactically use the decorations," he said, "distinguishing between normal web damage that happens every day (when the prey insects hit the web) and unusual damage by unwanted visitors."

Invisible obstacle

"Have you ever seen those cross-shaped tapes on brand-new windows?" Dr Walter asked.

"They're to warn us that there's something we might not see.

"The cross shaped decorations in Argiope keyserlingi may work in a very similar manner."