have the biggest eyes of any known spider on earth.
Scientists who study spiders (arachnologists) have long suspected the two deep black orbs adorning Deinopis spinosas helped them hunt at night. University of Nebraska-Lincoln doctoral student Jay Stafstrom put those assumptions to the by spending two months in a Florida forest equipped with cameras and spider-sized blindfolds.
The net-casting spider earns its name by spinning a rectangular net of wooly silk that it holds between its legs while rappelling upside-down from a single thread. When the time comes, the spider lunges forward stretching the net to capture prey in about one-thousandth of a second.
The critter has eight eyes, like most arachnids. But the net-casting spider has evolved two of its eyes into giant peepers that are 2,000 times more sensitive to light than human eyes. With the adaption, the spider can expand its menu beyond flying bugs to include a smorgasbord of plump ground-dwelling insects, as well as hunt at night, which means it can hide during the day to avoid becoming a meal itself.
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While large eyes can be found on some ground-dwelling arachnids like wolf spiders, the net-casting spider is the only one that lives in a web above the ground to develop such large eyes.
“Pretty much all the other spiders are paying attention to vibration signals in the web,†he said.
To test how it uses those big eyes, Stafstrom camped out at Paynes Prairie Preserve State Park in Florida, where he captured several spiders, held them down and put dental silicone on their eyes with wood toothpicks.
He found the blinded spiders still could catch flying prey but bugs that were on the ground were a problem. Spiders without the silicone blindfold snared prey on the ground about 3½ times more often than those with their secondary eyes covered.
He and his co-author, Professor of Biological Sciences Eileen Hebets, repeated the experiment in the lab at UNL and found similar results.
Vision is an expensive adaption, Stafstrom said. Keeping photoreceptors healthy and functioning takes a lot of energy. If the spiders can still catch things out of the air without their eyes, why do they need them?
“If you see an animal with really big eyes they’re almost definitely investing a large amount of metabolic energy into just keeping the eye there. You would expect there to be some pretty big benefit for having eyes that are so big,†he said.
Stafstrom believes it’s because prey on the ground, which the eyes make possible to catch, are bigger and more nutritious. The eyes also likely let the spider become a solely nocturnal hunter.
The researchers' study, "," appeared in the journal Biology Letters.