Spider diet goes way beyond insects

Spiders eat insects. That’s why some of us are reluctant to kill spiders we find at home — we figure they’ll eat the critters we really don’t want around. But a new study reveals that the spider diet is far more diverse than we learned in elementary school. Spiders are insectivores, sure, but many also have a taste for plants.

Only one species of spider is known to be completely vegetarian. Bagheera kiplingi jumping spiders of Mexico survive mostly on bits of acacia trees, Science News reported in 2008. And while scientists have yet to find any other vegetarian species, plant-eating appears to be very common, particularly among jumping spiders and spiders that make webs outdoors.

Martin Nyffeler of the University of Basel in Switzerland and colleagues combed books and journals for reports of spiders consuming plant material. There is evidence of veggie-eating among more than 60 species of spiders, representing 10 families and every continent but Antarctica, the team reports in the April Journal of Arachnology.

Perhaps past scientists can be forgiven for overlooking the plant-eating behavior, as spiders can’t eat solid material. They have a reputation for sucking the juices out of their prey, but that’s not quite the right description. Instead, a spider covers its prey with digestive juices, chews the meat with its chelicerae and then sucks the juices in. This eating style means, though, that spiders can’t just cut a piece of leaf or fruit and chow down.

Some spiders feed on leaves either by digesting them with enzymes prior to ingestion (similar to prey) or piercing a leaf with their chelicerae and sucking out plant sap. Others, such as the vegetarian Bagheera kiplingi, drink nectar from nectaries found on plants or in their flowers. More than 30 species of jumping spiders are nectar feeders, the researchers found.

“During such [feeding], the spiders were seen pushing their mouthparts deep into flowers to drink nectar, similar to the way nectar-drinking insects feed,” the researchers write. And this isn’t accidental behavior — some spiders can feed on 60 to 80 flowers in an hour.

Pollen is probably another common plant-based food source for spiders, especially those that make webs outdoors. That’s because spiders eat their old webs to recycle the proteins. And when they eat their webs, they eat anything that might be caught on the sticky strands, such as calorie-rich pollen. Spiders might also be consuming tiny seeds and fungal spores this way, though the latter may be a risky meal as there are many fungi whose spores will kill spiders.
The researchers also found some cases of spiders intentionally eating pollen and seeds, and they also note that many spiders are eating plant material when they munch on plant-eating insects. Just how common plant-eating is among spiders isn’t yet known, but it could be even more common, especially among species that create webs outdoors.

“The ability of spiders to derive nutrients from plant materials is broadening the food base of these animals,” Nyffeler says. “This might be one of several survival mechanisms helping spiders to stay alive for a while during periods when insect prey is scarce.”

And with reports of spiders eating a whole menu of other non-insect foods — including crustaceans, earthworms and small vertebrates in the wild; and sausage and soy milk in the lab — it’s clear that we need to call them something other than insectivores.