Even deadly predators crave a salad sometimes. Certain orb-weaver spiders—apparent full-time carnivores who eat by trapping prey, covering it with digestive juices, and then slurping it down like an insect smoothie—have been secretly taking their meals with a plant-based side dish. Namely, pollen. Orb weavers are a family of spiders common all across the world; they're the ones that weave the classic concentric-circle webs you see in picture books. Earlier studies have shown that those webs can collect a lot of pollen on their sticky strands, and that the spiders are willing to eat pollen in the lab. Benjamin Eggs, a graduate student at the University of Bern in Switzerland, and ecologist Dirk Sanders studied two kinds of orb weavers to see what truth there was to this rumor. For the first part of the experiment, Eggs gathered 20 young Aculepeira ceropegia spiders from outdoors in the spring and brought them into the lab. He coaxed each spider to build its nest inside a cardboard box, where he supplied it with several fruit flies per week. Eggs also sprinkled half the spiderwebs with birch pollen, as they might be in the wild. After a month, Eggs broke down the spiders' bodies and examined the carbon and nitrogen isotopes inside them. Isotopes, if it's been a while since your last chemistry class, are different forms of the same element. For example, most carbon atoms in the world have 6 protons plus 6 neutrons in their nuclei, making them carbon-12. But a small percentage of carbon atoms, called carbon-13, have an extra neutron. Animals incorporate the atoms they eat into their bodies. So by comparing the ratio of lighter to heavier isotopes in spiders' bodies to the signature ratios of their various foods, the researchers could see what the spiders were eating. In the lab, orb weavers supplied with pollen had a different isotope ratio than their neighbors who only received fruit flies. This told Eggs that his spiders were, in fact, eating the pollen. But would they do it in the wild? He returned outdoors and gathered young Araneus diadematus spiders, another orb-weaver species, from the trees. He used nets and bug vacuums to gather as many other insects from the area as possible—the spiders' potential prey. And he collected samples of pollen from neighboring trees. Eggs analyzed the carbon and nitrogen isotopes in the outdoor spiders, along with all the insects and pollen that might be on their menu. Based on those ratios, he calculated that pollen made up 25 percent of the orb weavers' diet. It's convenient for orb weavers to eat pollen, because they frequently dismantle and eat their webs in order to recycle the silk proteins. As long as they're at it, they may as well suck down the vegetables trapped there. Eggs points out, though, that the pollen grains he found in his study are too big for spiders to swallow accidentally. So his orb weavers must have deliberately eaten pollen grains by first covering them in digestive enzymes, then slurping them up. Eating pollen might be most practical in the spring, when young spiders have just hatched and built webs, but insects are still hard to find. The spiders in this study were all juveniles. Yet Eggs, who carried out this research for his undergraduate thesis, thinks there's no reason adult spiders wouldn't eat pollen too. He says, "The anatomy of araneids does not change dramatically when they reach maturity." (He adds intriguingly, "Exception: genitals.")
It's even possible that other kinds of spiders nibble on pollen too. "There is evidence that other web-building spiders like the sheet weavers (Linyphiidae) also feed on pollen," Eggs says, "although they don’t eat their own web." Every diet needs a cheat day, after all. Photo by Dirk Sanders.
Eggs B, & Sanders D (2013). Herbivory in spiders: the importance of pollen for orb-weavers. PloS one, 8 (11) PMID: 24312430