Rare 110-million-year-old fossil spiderweb
Credit: American Museum of Natural History
An American Museum of Natural History entomologist has described the oldest spider web--a rare 110-million-year-old fossil preserved in a pencil-thin rod of amber along with the insect and arachnid prey snagged by the silk. It is uncommon to find fossilized spiders or their silk. In this case, the amber preserved 26 silk strands (many connected to one another), glue droplets produced by the spider to better trap insects, and a diverse array of prey captured in the web, including a fly, a mite, a beetle, and a wasp. Scientists already accepted that the evolution of spiders and insects were related. This finding confirms that spiders and complex, sticky webs date back early enough to have affected the early evolution of the most diverse groups of flying insects. The new finding is described in the journal Science by David A. Grimaldi, Curator in the Museum's Division of Invertebrate Zoology; Enrique Peer, a postdoctoral fellow at the University of Valencia who completed his graduate degree while studying at the Museum; and Xavier Delct the University of Barcelona.
The amber piece was collected in Spain. An orb-weaving spider likely made the web, which has some features that resemble part of a sticky spiral. Two strands in the amber formed loops, which indicate that the silk was highly elastic and contracted when it came into contact with the resin that eventually hardened into amber. (Elasticity allows webs to absorb the impact of flying prey.) The longest main strands originally ran in the same downward direction of the amber, tree resin that, millions of years ago, under the force of gravity, formed into the shape of a small icicle and eventually hardened. This orientation indicates that the web was strung between objects and hung in the air, rather than being cast on the ground or over another surface. In one segment of the amber, five strands run in basically the same direction, and three of those are connected to another strand. And two little perpendicular strands connect two other main strands. A comb-footed spider also could have made the web, the authors say, but in any case, the web and its geometry has preserved far more detail than the earliest known spider silk, a single strand also from the Early Cretaceous Period.
Rare 110-million-year-old fossil spiderweb
Credit: American Museum of Natural History
Small flies, bees, wasps, and beetles are the most common prey of today's orb-weaving spiders, and this new fossilized web preserved insects representative of three of the four orders of flying insects with the greatest diversity today--Coleoptera (which includes beetles); Diptera (which includes flies); and Hymenoptera (which includes bees and wasps). A fly in the genus Microphorites and the mite (an arachnid) are trapped in strands found in the amber section with the most complex web fragment. In another section, a thick thread snagged the leg of a wasp in the genus Cretevania. A looped strand snared a beetle in the genus Cucujidae. So this amber piece suggests that the relationship between spiders and insects long ago was just as finely adapted as it is today.
"The advanced structure of this fossilized web, along with the type of prey that the web caught, indicates that spiders have been fishing insects from the air for a very long time," said Dr. Grimaldi. "Spiders today have a huge impact as predators on insect populations, along with birds and bats. This new finding suggests that spiders exerted a similar selection pressure on insects 110 million years ago, around the time when certain groups of insects were radiating to become major pollinators of plants." Spider predation could have affected evolutionary changes in insects' ability to navigate and to forage from flowers, he added.
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