The oldest orb spiders to weave a spiral web of silk have been found trapped in a fragment of amber 120 million years old.
A study of the amber-trapped spider suggests that it must have lived at the time of the dinosaurs - long before the rise of the warm-blooded mammals.
Many spiders use webs to catch prey or to protect their young but it is only the true orb-weavers that spin the spiral-shaped aerial structures for trapping flying insects.
David Penney ofManchester University and Vicente Ortuno of Alcala University in Madrid found the spider in a piece of amber unearthed at Alava in northern Spain.
The fossil was preserved because amber is fossilised tree resin, which helps prevent biological degradation. The find suggests this group of spiders had already evolved by the time that flowering plants and insects were undergoing an explosive phase of co-evolution more than 100 million years ago, the scientists say in the journal Biology Letters.
It is the oldest fossil species of orb-weaving spider and its existence shows that this family of spiders was set to exploit the rapid growth in the diversity of pollinating insects.
"One modification is quite fantastic," Dr Penney said. "Picture a spiral orb web and running down from it a ladder-type structure which is also made from sticky silk. This has evolved to trap moths, which have scales that rub off. When a moth flies into a normal orb web, it's the scales that stick and the moth tumbles out of it.
"But with the ladder structure, the moth tumbles down until all the scales come off and eventually it gets caught."
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