Big spider eyes put more insects on the menu

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The net-casting spider’s secondary eyes – the largest of any arachnid – likely evolved in part to help it capture walking prey.
Credit: Jay Stafstrom

The biggest eyes of any known spider may have evolved for night-time hunting prowess, say researchers.

The link was uncovered by a new study, in which net-casting spiders were effectively blindfolded, allowing researchers to gauge the value of their enormous peepers.

The net-casting spider, Deinopis spinosa, is a unique web-building spider found in forest areas in Australia, the Africas and parts of America.

Unlike most spiders, this species is known to have excellent eyesight, thanks in part to an enlarged pair of secondary peepers which round out a total of eight eyes.

The species is also known for its unique predation methods. At night, the net-casting spider dangles from a triangle-shaped web, holding a net made of woolly silk. When prey passes, the spider propels itself downwards, ensnaring the target in its silky trap.

The spider then wraps its prey, paralyses it with a bite, and feeds.

Jay Stafstrom, a biologist at
University of Nebraska-Lincoln, set out to find why these enlarged eyes – the biggest in the spider world – may have evolved.

To effectively analyse the arachnid’s hunting behaviour, Stafstrom observed the species in its natural habitat, as well as in a series of laboratory tests.

In fact, he camped out in a Florida state park for two months for field-testing. Stafstrom studied 29 net-casting spiders in the state park, excluding mature males, which don’t engage in the net-casting behaviour.

To ascertain the value of those enlarged eyes, Stafstrom used dental silicone to blindfold the spiders during some of the tests.

According to the findings published in Biology Letters, the massive eyes are crucial to nocturnal D. spinosa’s ability to hunt its ground-dwelling prey, as opposed to its flying prey.

The findings show that when blindfolded, the spider was just as capable catching flying critters. But its predation of ground-dwellers such as crickets suffered.

Stafstrom points out that a lot of energy is spent maintaining eyes on any animal, let alone eyes as big as those on the net-casting spider. This caused the researchers to hypothesise on why such an energy-costly trait may have developed.

“Vision is really expensive,” says Stafstrom. “Simply keeping photoreceptors healthy and functional requires a lot of energy. Now if you wanted to grow the size of the eye to gain more visual information, it would become disproportionately more expensive as the mass of the eye increases.

“[The spiders] can still catch things out of the air [without those eyes]. Why are they, presumably, investing so much in these large eyes? One of our hypotheses is that it’s because there’s a lot of prey on the ground, and 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.”

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The net-casting spider Deinopis spinosa holding a band of woolly silk to engulf and capture prey.
Credit: Jay Stafstrom

The findings from the lab revealed a similar result: spiders with their enlarged eyes covered took 10 times longer to catch a cricket in their vicinity than those without the blindfold.

Stafstrom also reflected on the incredible camouflage tactics of D. spinosa, which made it difficult for him to spot the spiders out in the field during daylight.

“Not only are they really looking like a stick and [lying] completely motionless, but on their first pair of legs, they actually have some hairs that puff out a bit and cover their eyes somewhat.”

This camouflage ability may be a valuable defence against predators, which could explain why this nocturnal creature evolved such powerful night-vision.

Co-author and fellow University of Nebraska-Lincoln biologist Eileen Hebets says the findings are both significant, and well-earned.

“The fact that such a large component of this was done in the field, in pretty harsh conditions, is amazing,” says Hebets.

“It was a tonne of work and physical hardship and technical hardship. The data came out beautifully, but the effort behind it was monumental.”

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