This wasp spider's web is decorated with a zigzag "stabilimentum" pattern
Orb-weaver spiders attract insects to their webs with ultraviolet (UV) decorations, a study has found.
The function of the intricate patterns in the webs of orb-weaver spiders is subject to much debate in the scientific community.
Previous explanations have identified them as bird scarers, mating signs, sun shades and camouflage.
Researchers now suggest the UV light-reflecting patterns could trick flying insects into landing on them.
Scientists from the University of Incheon, South Korea, undertook the study published in the journal Behavioral Ecology and Sociobiology.
They studied wasp spiders (Argiope bruennichi), a species common throughout Europe, north Africa and parts of Asia, which are named for the females' familiar striped abdomens.
During construction of the webs, the spiders are known to sporadically add zigzag patterns leading out from the centre. These patterns are created with special white silk known to reflect much more UV light than other strands in the web.
The scientists questioned why the animals would produce a stealthy ambush in the form of a non-reflective web only to decorate it with an eye-catching design.
To test the effects of the decoration, known as "stabilimentum", researchers compared decorated webs to webs without the patterns.
Associate Professor Kim Kil-Won and his team looked at the type of insects caught in each type of web, which revealed a possible clue to the purpose of stabilimenta.
They found a link between the decorations and increased foraging success for the spiders.
"The effects of stabilimenta on foraging success appear to be due to increased interception of UV-sensitive insect pollinators," said Prof Kim.
The zigzag patterns could have multiple purposes
The team found that webs with the decorations caught twice as many large prey insects than unadorned webs.
Prof Kim explained that pollinators, such as the 20 families of flies, wasps, beetles and butterflies discovered in the decorated webs, have greater visual sensitivity to UV light.
Previous studies have shown how these pollinating insects are attracted to flowers with high UV-reflectance and Prof Kim suggested that the highly reflective web patterns capitalise on this attraction.
"We think that [by] decorating the web with a stabilimentum the spiders use a pre-existing bias in a prey animal toward UV-reflective surfaces," said Prof Kim.
However, the biologist added that his results did not invalidate earlier studies.
"Probably stabilimentum mechanically stabilise and strengthen the orb web," he said, "This property would help to maintain struggling large prey on the web."
He suggested that the adaptation could be used in different ways by different species of orb-weaver spider but further study is required to understand its original function.
"The evolutionary origin of the trait may have to be separated from its contemporary role," Prof Kim told BBC Nature.
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