Giant eyes help spiders see in the dark

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What big eyes you have! The net-casting spider Deinopis spinosa uses it’s enormous eyes to accurately trap prey at night, according to new research.

A new study published in Biology Letters this month shows that net-casting spiders (Deinopis spinosa) use their enlarged, secondary eyes to spot prey in the dark.

Net casting spiders use their giant eyes to capture preyNet casting spiders use their giant eyes to capture prey

Net casting spiders use their giant eyes to capture prey
Image adapted from Stafstrom and Hebets (2016) Biology Letters.

Net-casting spiders are quite unusual – they spin silk like many other spiders, but rather than building a web they instead construct a small net, attached to a long thread. The spiders lie in wait for prey to move past them, and then drop their tiny net over the top.

The spiders have two dramatically enlarged eyes, which are the largest eyes of any spider, and more than 2000 times more sensitive than a human eye. To understand what their giant eyes are used for, researchers at the University of Nebraska tested the spiders’ ability to catch prey, with or without the enlarged eyes covered. The team observed the natural foraging behaviour of the spiders at night, and found that the partially blindfolded spiders were significantly less likely to catch their target, particularly for larger prey running along the ground. They found similar results when testing the spiders catching Crickets (em)Acheta domesticus in controlled lab conditions, too.

Overall, were less likely to catch target prey and caught less prey on average. Looking at different types of prey, the scientists found that it was only cursorial prey, running past along the ground, which blindfolded spiders were worse at catching. They performed equally well at catching aerial prey, but terrestrial prey seemed to stump them. Catching crickets in the lab whilst partially blind proved so difficult for the spiders that the seven successful captures they observed were where the prey was stupid enough to walk into the net!

The authors say this spider’s unusual eyes evolved to allow them to use their net-catching abilities at night, out of the sight of potential predators.

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The spiders with their enlarged posterior median eyes covered by opaque silicone

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