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Giant Spider Eyes See Better at Night

The net-casting spider gets a boost to its nocturnal foraging with some of the biggest eyes in the arachnid world.

<p>University of Nebraska-Lincoln</p>

For the first time, scientists say they have made a direct link between the enormous eyes of a net-casting spider and its ability to hunt at night, something implied in the past but never tested.

In a new study, researchers from the University of Nebraska-Lincoln wanted to see how the spider Deinopis spinosa fared without its two biggest eyes (it has eight, but the ones you see in the picture are its biggest, the posterior median eyes).

They temporarily covered the arachnid's large eyes and then compared its foraging success at night with spiders whose prodigious peepers had not been covered. It turned out the spiders with all of their eyes available for duty were able to capture prey 3.5 times more often than the spiders whose vision was impaired.

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Net-casting spiders, the genus Deinopis, get their name thanks to their hunting style. They make small, detached webs they hold (see picture below) and drop down from above onto unsuspecting prey - "casting" their nets, as it were, to ensnare their meals.

The net-casting spider Deinopis spinosa is shown holding the band of wooly silk that it uses to engulf and capture prey. | University of Nebraska-Lincoln

Their eyes are among the biggest, relative to their bodies, in the arachnid world. While it had previously been suggested that they were key to nighttime hunting, the researchers say their study is the first to test specifically that notion.

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The scientists say the spider's evolution of big, night-vision eyes gave the arachnid access to a wider variety of food, particularly walking food.

In the study, only land-based meals were lacking when vision was impaired. The spiders don't need the large eyes to catch flying things, but evolving a nocturnal ground game - finding new victims that walked on land - really opened up the menu.

click to play video

"By having vision in these enlarged eyes, not only are they getting things off the ground, but those things are bigger and probably more nutritious," explained study co-author Jay Stafstrom in a statement.

Not only that, but the larger eyes also allowed the spiders to become exclusively nighttime hunters, say the researchers. The night-owl life helps them lay low and avoid daytime predators.

Stafstrom and co-author Eilleen Hebets have just published their findings on the spider in the journal Biology Letters.

Deinopis spinosa

You almost feel for them, spiders. They can't help being what they are, and yet almost no one is happy to see them. Especially true when they have exceedingly long legs, thick bodies and a general mien that makes you turn quickly in some other direction. Photos don't bite, though, so let's take a look at some honking-big spiders -- with Halloween on the way, we may as well get started freaking ourselves out. Shown here is the Brazilian wandering spider (a.k.a.

Phoneutria

), a feisty and venomous crawler from South America. Just four years ago it took home an award from the Guinness World Record people for the title of "most venomous" spider. This spidey's legs can span nearly 6 inches, its body just shy of 2 inches. It gets its name thanks to its preference for strolling along the tropic floor at night seeking out prey, rather than building webs or hiding out someplace waiting to strike. During the day, it lays low wherever it's convenient -- even inside banana plants, which is how it get its nickname "banana spider."

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Not to be outdone is a spider that's been making a big splash of late, with an entomologist's

blog about his encounter with one

. It's called the Goliath bird-eater (a.k.a.,

Theraphosa blondi

). It can weigh in at almost 6 ounces and it's been known to reach nearly a foot in leg-span. The "bird eater" moniker must be there to warn birds away, though, because this spider doesn't typically eat birds as a matter of, er, course. It will regularly eat small land animals such as frogs, lizards, and snakes, however.

Spiders Have Personality Too

Meet the golden-silk orb weaver spider. Step into its parlor, if you must. Don't be fooled by its deceptively gentle-sounding name. The female golden-silk orb weaver's body alone can reach 2 inches, its legs can stretch to more than 5 inches, and it's even been observed killing and eating tree snakes. What's more, a study published earlier this year found that these spiders, when living in urban areas,

are growing even bigger

than usual. Interesting side-note: The golden-silk orb weaver also belongs to the oldest surviving genus of spiders,

Nephila

, which has a fossil in the record that dates to 165 million years ago.

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The Brazilian salmon pink bird-eating tarantula has a leg-span that can reach 11 inches and weight that can tip the scales (well, for a spider) at about 3.5 ounces. Despite its name, it's not confirmed that they actually eat birds any more than do the Goliath bird-eaters. Instead, they dine on insects or the random small amphibian or reptile. Instead of making a web, it takes its prey by quick-strike ambush in the open.

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The giant huntsman spider is so big it even took the trouble to have a size descriptor built into its name (given that Goliath was taken). The huntsman is neck and neck, or leg and leg, with the Goliath bird-eater for the title of biggest spider, by leg-span (in sheer body mass, though, the Goliath is more like an offensive lineman, while this spider is a lanky cornerback). A giant huntsman's legs can stretch out to 12 inches, and its speedy, crab-like gait makes it a fast hunter that excels at chasing down its meals. It hails from caves in Laos.

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