It looks like spiders may not be the pure predators we generally believe them to be. Dr Dirk Sanders of the University of Exeter has demonstrated that orb web spiders choose to eat pollen, even when insects are available. His research appears in a PLOS One article called “Herbivory in Spiders: The Importance of Pollen for Orb-Weavers.”
“Most people and researchers think of spiders as pure carnivores, but in this family of orb web spiders that is not the case,” Dr. Sanders said. “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.”
Spider webs snare insect prey, but they can also trap aerial plankton like pollen and fungal spores.
Dr Sanders, along with Benjamin Eggs from the University of Bern, conducted feeding experiments and a stable isotope analysis on two species of juvenile spiders (Aculepeira ceropegia and Araneus diadematus) to see whether they incorporate plant resources into their diets. They discovered that 25% of the spiders’ food intake was made up of pollen, with the remaining 75% consisting of flying insects. The spiders that ate both pollen and flies gained optimal nourishment, with all essential nutrients delivered by the combination.
“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 Dr. Sanders.
Orb web spiders regularly take down and eat their webs to recycle the silk proteins, and it has been suggested they may “accidentally” consume the pollen during this process. But the study found this to be impossible due to the size of the grains ingested.
“Most pollen grains (so are birch, pine and spruce pollen) are too large to pass through the spider’s pharynx and therefore cannot be swallowed accidentally but have to be actively consumed,” the authors write. “Spiders dissolve the outer coating of a pollen grain via extraoral digestion and suck up the dissolved nutrients afterwards.”
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