The net-casting spider has eight eyes. But two of those peepers – the spider's secondary eyes – are particularly enormous, and scientists had never been able to explain exactly why they came to be. According to a new study published in Biology Letters, the reason is that those amazing eyes help the creature hunt not only prey that flies, but also prey that crawls.

The spider is a lethal predator that spins a web that looks like a net. When prey arrives, the arachnid strikes within a thousandth of a second, stretching the net to ensnare its victim. A team led by Jay Stafstrom from the University of Nebraska-Lincoln camped in Paynes Prairie Preserve State Park in Florida for months to watch the net-casting spider in action and test the power of its vision. 

The team made a spider blindfold out of dental silicone to see whether the creatures could still hunt with their enormous secondary eyes covered, and used four closed-circuit cameras to watch their progress. If the spiders hunted flying prey, the blindfold has no significant effect on their hunting ability. But net-casting spiders could not catch crawling or walking prey if they couldn't see out of their secondary eyes. In fact, none of the blindfolded spiders managed to catch walking prey when they were blindfolded, whereas one in four snagged a catch when the researchers didn't cover their eyes.

The findings seem to suggest an answer to a vexing question about the spiders, which is why they evolved such large secondary eyes when most web-building spiders did not. It takes a lot of energy to keep eyes healthy, and it takes disproportionately more energy as those eyes get bigger. "It's a really good example of the Law of Diminishing Returns, where you're spending way too much to get just a little bit more information," Stafstrom said a statement.

The study suggests that it's worth it for the spiders to grow big eyes because it helps them find ground-based prey. He also found the walking prey that net-casting spiders snared was larger than the flying prey it caught, and more nutritious. In addition, Strafstrom and his colleagues also believe that the oversized eyes allow the spiders to hunt solely at night and hide during the day by camouflaging themselves against the palm fronds where they live. That nocturnal survival strategy is yet another reason that may explain what big eyes they have.