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In caves in Brazil, there lives a newly-discovered mite that is a freeloader. Groups of these mites live on a spider’s web and steal its food. They are the first mites known to do this.

at the Federal University of Lavras in Minas Gerais first saw live mites dotting a spider web by the entrance of Brazil’s Lapa Nova cave in 2007. The relationship between mites and spider immediately intrigued him.

After observing the same thing in another cave, Bernardi and his colleagues designed an experiment. They placed live bait – a cave moth – on the web of a where mites were present.

The spider immediately attacked the moth and began feeding. But in the next 5 to 40 minutes, mites, which were previously scattered all over the web, gathered to feed on the moth.

The team has named the newly-discovered mite Callidosoma cassiculophylla.

Take what you want

“Spiders and their webs are predictable sources of food, and many animals regularly exploit this resource,” says zoologist at the National Institute of Amazonian Research in Manaus, Brazil. “These ‘thieves’ are specialised spiders, scorpion flies, flies, plant bugs, gnats and also hummingbirds.”

However, until now mites have never been reported stealing from spiders.

“The fact that the mites involved in the relationship are adult is interesting,” says at the . Adult mites in the family to which C. cassiculophylla belongs are usually free-living predators, which eat small invertebrates and their eggs. “In this regard, Callidosoma cassiculophylla is unusual.”

What’s more, these mites only eat their host spider’s freshly-killed prey, and do not scavenge on dead decaying insects.

In turn, the spider is very tolerant of mites sharing its meal. The researchers never saw any signs of aggression towards them. “I saw a mite walk under the spider’s legs and nothing happened,” says Bernardi.

It may be that the mites are too tiny to bother their host. They are about 0.14 centimetres long and 0.08cm wide, while the spider is about 5cm in size, says Bernardi.

“The mites are too small to be useful prey for the spiders, and are not large enough to be a potential predator,” says Pape. “I suspect the spiders are not adversely affected by the small amount of nutrients consumed by the mites.”