Many animals employ various shapes, patterns, and colors to blend into their backgrounds or mimic other (usually more dangerous) animals. The body color of many spiders are often a compromise between being attractive to prey and inconspicuous to predators. Well, the orb-web spider Cyclosa ginnaga disguises itself as bird poo. Juveniles have a silver body less than 6 millimeters long, and they weave a decorative, silky white disc on their web to appeal to prey. Unfortunately, those very same characteristics seem to make them stand out to predatory wasps.
Now, a team led by I-Min Tso from National Chung-Hsin University, Taiwan, shows that the amazingly clever combination of the spider’s body coloration and its web decorations make it indistinguishable from bird poo to -- not only humans -- but even to wasps.
First, they measured the surface area of 125 C. ginnaga web decorations and 27 bird droppings in a forest at Wu-Shy-Keng in Taichung, Taiwan. They found no statistically significant difference.
Then, after collecting 10 spiders, the team measured the spectral reflectance of their body and the web decoration and of 6 randomly encountered bird droppings -- all in the field against a natural background of shrubs and leaf litter. They also measured the color contrast between the bodies and the decorations and found that the values were well below 0.1 hexagon units -- that’s the wasps threshold for discriminating between colors. In other words, the spiders and their webs were indistinguishable when viewed by predatory wasps.
The team then blackened the spiders’ bodies with pen ink and the web decorations with carbon powder. They placed video cameras near webs and recorded hundreds of hours of footage of various combinations of experimental conditions using 156 spiders: bodies and decorations exposed, bodies concealed, decorations concealed, and bodies and decorations concealed. Wasp attacks were far greater when only the decorations were blackened.
Here are photographs of randomly sampled bird droppings (rows 1 and 3) compared with C. ginnaga web decorations (rows 2 and 4) at their study site. Notice how the droppings and the webs are similarly oriented, and even the backs of the spiders were designed to be a part of the masquerade.
The work was published in Scientific Reports this week.
Images: Min-Hui Liu, National Chung-Hsin University
