The conspicuous and mysterious silk decoration of a St. Andrew's Cross spider.
Credit: Andre Walter/University of Melbourne
BRISBANE: The purpose of eye-catching web decorations created by certain spiders has mystified scientists for decades, but new research suggests they help prevent unintentional damage.
Researchers from the University of Melbourne in Australia have found that the bold silk cross constructed by the St. Andrew's Cross Spider (Argiope keyserlingi) in the centre of their web is actually a visual warning of the web's presence.
"The frequency and size of the cross's increased as a response to heavy web damage, providing an explanation to the debated decorations," said the lead author of the paper published in Behavioural Ecology and Sociobiology, Andre Walter from the Department of Zoology.
X marks the spot
Orb web spiders of the genus Argiope, including the St. Andrew's Cross Spider, permanently locate themselves in the hub of their web and so are particularly vulnerable to changing environmental conditions and frequent web damage.
Web damage is costly to a spider as a lot of nutritional resources are required to produce silk for the rebuilding of the web, as well as the loss of foraging opportunities.
The function of cross decoration has been much debated, and scientists have suggested three possible explanations: that the decorations may provide a signal for prey attraction; that they camouflage the spiders by concealing their outline; or that they alert non-prey animals to the presence of the web to reduce unintentional damage.
"We have to consider that these decorations can have many functions, but it is still controversial as to exactly what the specific functions are," said Walter.
"We've turned the question on its head and asked - how do they respond to damage?" said co-author Mark Elgar, also from the University of Melbourne.
Watch out for my web
The researchers studied adult female St. Andrew's Cross Spiders. The spiders were captured and allowed to build their webs in the laboratory, with mild and heavy damage inflicted on them.
The researchers found no difference in the decorating frequency between those dealing with mild damage and those whose webs were untouched. But the spiders subjected to heavy web damage decreased the size of their web by almost 13% as well as increasing the size and frequency of the silk cross.
"The web decorations actually increased as a response to heavy damage." said Walter.
"This suggests that the conspicuous silk cross serves to make webs more visible to animals that might accidentally walk or fly into them," added Elgar. The spider's protection mechanism is similar to how we tape up new glass windows in order to prevent people from walking into them, he said. "The spiders are just trying to minimise unwanted damage."
This provides a paradox in the spider world. On the one hand, the spiders are creating webs to trap their prey, but on the other they include visual decorations to warn of the web's presence.
The decorating debate
There are still many suggestions as to why these decorations exist, and the team expect this debate to continue. "What we offer is a slightly different perspective on the question, but this is still very much an open research topic," said Elgar.
Head of terrestrial biodiversity and senior curator of Arachnida at the Queensland Museum, Robert Raven, who was not involved in the study, said that the paper provides a new line of inquiry for a number of spider questions.
"For so long people have been trying to figure out the adaptive advantages of the decorations and, as with other questions, it just needed turning around," he said.
"We will continue to ask this question to other species of spiders and try to find some resolution to this debate," said Elgar.
