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Thursday, January 16, 2014

Are spiders omnivores?

Western spotted orbweaver (Neoscona oaxacensis).

When you think of mealtime for a spider, what comes to mind? Flies? Beetles? Aphids? How about pollen? According to Benjamin Eggs and Dirk Sanders of the University of Bern, pollen could make up a quarter of the diet of orb-weaving spiders.

Orb-weaving spiders (Araneidae) not only spin webs, but they also dismantle and eat those webs at regular intervals. Very often the sticky silk threads are covered with pollen, which the spider ingests along with the web. Aha! you say, the spider isn’t eating the pollen on purpose so it doesn’t count. That might be true except for one thing.

Most pollen grains are too large for spiders to eat. I know that’s hard to believe, but spiders have really tiny mouth parts. In order to consume hard objects like pollen, they have to first dissolve the outer coating and then slurp up the insides, just like they do with insects. In other words, they have to want to eat the pollen.

And sure enough, they do want to eat pollen. Chemical tests showed that wild caught spiders consume both flying insects and pollen in a ratio of three to one. That’s a lot of pollen.

To find out what the benefit could be of eating all that pollen, the researchers compared two groups of captive spiders. Group A were fed fruit flies three times a week. Group B got the flies but also had their webs dusted with birch pollen. While group B spiders definitely ate both pollen and flies, they didn’t grow any larger or faster than group A spiders who only got flies.

So why do spiders purposefully chow down on pollen if it doesn’t help them grow? One possibility is that pollen does help wild spiders who aren't getting fruit flies delivered to them at regular intervals. When flying insects are scarce or wily, pollen may mean the difference between starvation and survival. The researchers suspect that pollen is a particularly important supplement for young spiders. 

For whatever reason, the spiders clearly are consuming pollen, and in large quantities. Other animals with diets that are one quarter vegetable are considered omnivores, therefore, spiders should be reclassified as omnivores. 

Eggs B, & Sanders D (2013). Herbivory in spiders: the importance of pollen for orb-weavers. PloS one, 8 (11) PMID: 24312430.

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