Spiders Use "3-D" Webs As Shields From Wasp Attacks

John Pickrell
for National Geographic News
February 5, 2003
Spiders are often considered top-dog predators of the creepy crawly
world. However, a new study suggests that some types of complex webs may
have evolved to help the arachnid weavers fend off attacks from
carnivorous mud-dauber wasps.

Most scientists have regarded webs as food-snagging structures and little else, says Todd A. Blackledge, a behavioral ecologist at Cornell University in Ithaca, New York. However, three-dimensional or "3-D" webs might also "provide protection by surrounding the spiders with silk…and acting as an early warning system against attack," he said.

Web Wonders

Spider-spun architectural masterpieces come in two major varieties: the two dimensional orb webs that resemble a delicate circular fishing net of silk, and the dome-shaped or haphazard masses of silk fanning out in three dimensions. 3-D webs are the sort you might find nestled in crevices on a cliff-face or suspended between the rafters of your house, said Blackledge.

Although known spider species number more than 37,000 worldwide, compared to approximately 4,200 known mammal species, most spiders live in burrows or are opportunist prey-snaggers. Of the 10,000 web-crafting spider species, 60 percent build webs of the 3-D design.

Scientists now believe that 2-D web-weaving group of spiders represent the more primitive variety from which 3-D web weavers evolved. It seemed plausible that the complex and energy-consuming 3-D webs "might be involved in defense as well as foraging," said Blackledge.

"Silk is expensive and can be seen as limiting because spiders go to so pains to recycle it, eating old, disused webs," commented William Shear, an entomologist at Hampden-Sydney College, in Virginia.

In fact, spiders suffer a surprising amount of insect predation. Other studies have shown that island outbreaks of some species of spider-preying wasp can reduce local populations of web-building spiders by up to 77 percent. In addition, "a single wasp species in the Caribbean has been shown to seize 30 times the number of spiders captured in a day by a total of 15 different…bird species," said Blackledge.

Mud-dauber wasps capture and paralyze their prey, said Blackledge, carrying up to 30 of the still-living spiders back to tiny mud or paper nests. A single egg is laid in the nest, which hatches 10 days or so later to reveal a larva yearning to snack on the still-fresh spiders.

Blackledge and his insect biologist colleagues: Rosemary G. Gillespie at the University of California, Berkeley and Jonathan A. Coddington of the Smithsonian Institution in Washington D.C., decided to test the idea that 3-D webs evolved partially as a defensive adaptation. The researchers amassed tens of thousands of figures from spider-predation studies published in the last 100 years.

The team collected data on the ratio of 2-D to 3-D web-weavers found in thousands of wasps' nests examined, for a large group of related spider species. They also collected data on the ratio of 2-D to 3-D web weavers in the natural environment, for the species group. A comparison of these two figures revealed whether or not proportionally more undefended 2-D web-weavers end up on the wasps' menu.

Flip-Flopped Numbers

Blackledge's results provided compelling evidence that 3-D web-weavers are better defended against attack than their 2-D web-weaving relatives. The researchers found that 3-D web-weavers accounted for fewer than one in five spiders captured by wasps. However, this proportion is "flip-flopped" around from the proportion of spiders available to wasps, said Blackledge.

In fact, 3-D web-weavers comprise an average of four in five web-building spiders actually found in the in the environment.

"3-D web-builders appear to be somehow protected from predation," said Blackledge. "Perhaps something about 3-D webs make them less attractive to predators," he added.

The classic, flat, 2-D orb web, though efficient at catching flying insects with little silk, leaves spiders more readily exposed to predators, said Blackledge. On the other hand, the 3-D web provides physical protection and early warming of attack. The concept that the predators might have influenced the evolution of web architecture attracted much attention in recent years, said Shear, who is the Charles Patterson Distinguished Professor of Biology at Hampton-Sydney College. However, this study is one of the first tests of the theory that "defense against predators is an important selective force on spider web architecture," he said.

Blackledge is "testing experimentally…assumptions that have been taken for granted by spider folk for a long time," said Shear.

The study is documented in last month's issue of the journal Ecology Letters.

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