December 19, 2018

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How the Jumping Spider Sees Its Prey

How the Jumping Spider Sees Its Prey
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If you love spiders, you will really love jumping spiders. (If you hate spiders, try reading this article on dandelions.)

O.K., if you’re still here, jumping spiders are predators that stalk their prey and leap on them, like a cat. They are smart, agile and have terrific eyesight.

It has been clear for a long time that their vision is critical to the way they hunt, and to the accuracy of their leaps. But a lot has remained unknown about the way their eyes work together.

To find out more, Elizabeth Jakob, a spider biologist at the University of Massachusetts, led a team of researchers from the United States, Kenya and New Zealand in an investigation of spider vision.

The first step was getting a custom-built spider eye tracker, similar to ones used on humans, to follow a spider’s gaze. Actually, Dr. Jakob had two made, probably the only two in the world. She has one and her colleagues in New Zealand have the other.

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Jumping spiders have eight eyes. Two big eyes, right in the center of what you might call the spider’s forehead, are the principal ones, and they pick up detail and color.

Of the other three pairs, a rear set looks backward, a middle set is as yet a bit of a mystery, and the foremost detect motion.

The lenses of the main eyes are attached by flexible tubes to retinas. A camera was set up to look down those tubes and see the activity of the retinas, which look a bit like boomerangs. The inside of the spider’s head was lit by ultraviolet light, which penetrates the outer carapace.

But as accurate as the main eyes are, they only see what is in front of them. If they had to find prey, it would be like using a narrow flashlight beam to explore a dark room. Not very efficient.

The researchers found that the front pair of secondary eyes, the motion detectors, tell the main pair of eyes where to look.

When they were painted over temporarily and the spider was presented with moving images, it had no idea where to look.

But when they were active, the principal eyes would turn toward motion that the secondary eyes had noticed. The researchers could see the retinas moving as the spiders scanned the object for detail.

It is the “cooperation between physically separate sets of eyes” that is most remarkable, Dr. Jakob said.

The next step is to see how the spider’s tiny, but very effective, brain manages that coordination. The secondary eyes are not just informing the spider that there is something moving, they are transmitting the location of the moving object, so the brain can tell the main eyes to get a better look. All in time to plan one of the creature’s deadly leaps.

The researchers were able to conduct the experiment without harming the spiders. They affixed a Dr. Seuss style hat to the top of the spiders’ heads with wax, and that hat was attached to an apparatus to keep them in one place.

After the experiment, off came the hat. And off went the spiders.



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