The experts at London's Natural History Museum pride themselves on being able to identify species from around the globe, from birds and mammals to insects and snakes. Yet they can't figure out a tiny red-and-black bug that has appeared in the museum's own gardens.

The almond-shaped insect, about the size of a grain of rice, was noticed in March 2007 on some of the plane trees that grow on the grounds of the 19th century museum, collections manager Max Barclay said Tuesday.

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Within three months it had become the most common insect in the garden, and it was also spotted in other central London parks, he said.

The museum has more than 28 million insect species in its collection, but none is an exact match for this one. Still, Barclay is cautious about calling it a new discovery.

"I don't expect to find a new species in the gardens of a museum," he said. "Deep inside a tropical rainforest, yes, but not in central London."

The bug resembles the Arocatus roeselii, which is usually found in central Europe but is a brighter red and lives on alder trees. Entomologists suspect the new bug could be a version of the roeselii that has adapted to live on plane trees but acknowledge it could be a new species.

Either way, it appears the museum's tiny visitor, which appears harmless, is here to stay.

"We waited to see if the insect would survive the British winter," Barclay said. "It did and it's thriving, so now we had better figure out what it is."