Spiders Don't Just Eat Flies: Webs Capture Pollen for Arachnid Omnivores

Spiders might not be purely predators. It turns out that they have a sweeter side. Scientists have discovered that some species of spiders make up nearly a quarter of their diet by eating pollen.

Spider webs are perfect traps. With their fine strands, they can snare insects which spiders then consume. Yet these webs can also trap aerial plankton like pollen and fungal spores. In order to see whether or not spiders were making use of this pollen and these spores, the researchers decided to take a closer look.

More specifically, the scientists conducted feeding experiments and a stable isotope analysis on juvenile spiders to see whether they incorporate plant resources in their diet. In the end, they found that 25 percent of the spiders' food intake was made up of pollen while the remaining 75 percent was made up of flying insects. More interesting was the fact that the spiders that ate both pollen and flies gained the optimal nourishment with all essential nutrients delivered by the combination.

"Most people and researchers think of spiders as pure carnivores, but in this family of orb web spiders that is not the case," said Dirk Sanders, one of the researchers, in a news release. "We have demonstrated that the spiders feed on pollen caught in their webs, even if they have additional food, and that it forms an important part of their nourishment."

In fact, the researchers found that orb web spiders regularly take down and eat their webs to recycle silk proteins. Yet as they do this, they also actively consume pollen grains. Because of the size of the grains, the spiders had to coat them in digestive enzyme before sucking up the nutrients, which shows that they specifically needed to target the pollen grains rather than "accidentally" consuming them.

"The proportion of pollen in the spiders' diet in the wild was high, so we need to classify them as omnivores rather than carnivores," said Sanders in a news release.

The findings reveal that spiders aren't the carnivores that researchers once believed. Instead, these arachnids take advantage of various food sources, including pollen grains.

The findings are published in the journal PLOS One.