We aren't the only species that decorate our homes. Some orb web weaving spiders cover their webs in decorations including egg sacs, meal remnants,silk motifs and pieces of vegetation. The question of why spiders decorate their webs has puzzled scientists for more than a century, explains Daiqin Li from the National University of Singapore and Hubei University. It has been suggested that the decorations could deter predators by making the web's occupants look threatening, advertise the web's presence to protect it from damage, or attract tasty prey, but it wasn't clear why Cyclosa mulmeinensis decorated their webs. Curious to find out why these spiders decorate their webs with egg sacs and pieces of vegetation, Li and his student, Eunice Tan, decided to track predator and prey activity around the webs to find out what effect the decorations had(p. 1832).
Travelling to Pulau Ubin island close to Singapore, Tan and Li searched for unadorned webs, webs decorated with egg cases and webs decorated with the remains of meals before filming them to find out how many insects flew in the vicinity of the web, how many were trapped and how often the web came under attack. After months of analysis of more than 170 h of film footage, it was clear that the decorated webs were far more effective at trapping prey than the unadorned webs. Webs decorated with meal remains trapped almost 2 insects per hour and egg sac decorated webs snagged more than 2.5 insects per hour. However, unadorned webs barely managed to trap 1 insect per hour.
But why were the decorated webs more successful at trapping prey than the plain webs? Either the insects were attracted by the decorations, or the decorations were behaving like camouflage, causing unsuspecting insects to blunder into the death trap. Tan and Li decided to take a look at the webs and their occupants from an insects' perspective.
Collecting spiders and their webs, the duo brought them back to the lab where they could measure the way that the spiders and their decorations reflected light. Based on their reflection measurements, the pair calculated the contrast between the spiders and a background, and the web decorations and the same background to see how well they stood out. The spiders, egg cases and meal remains were almost invisible. It was clear that the decorated web's success in trapping prey was due to camouflage
But spiders aren't always the predator. Sometimes they are the prey too, so Tan and Li wondered what effect the ornaments had on the number of attacks the webs endured. Looking back at the film they found that none of the undecorated webs were attacked, the webs adorned with prey remains endured two attacks while the egg sac decorated webs suffered one attack.
And when the team calculated how well bird predators could see the spider and web decorations, it was clear that the decorations provided good camouflage from hungry birds too, although they aren't sure why the undecorated webs remained safe from predation without their camouflage protection.