[Animals] Jumping spiders: Facts about the cutest arachnids on the planet

[EVIL BABY.]

Ever seen a teeny, furry spider scampering across the floor? There's a good chance you've glimpsed a jumping spider. 

Jumping spiders are the largest family of spiders, with more than 6,380 species identified by science, according to the World Spider Catalog. The scientific name for the jumping spider family is Salticidae. With so many species, it's no surprise that jumping spiders vary widely in appearance, habitat and preferred prey. The largest, Hyllus giganteus, can be 0.98 inches (2.5 centimeters) in length, while smaller species, like the colorful Habronattus pyrrithrix, range from 0.19 to 0.3 inches (5 to 8 millimeters) in length. 

But what jumping spiders do have in common is their impressive eyesight. These arachnids always have four pairs of eyes, including a large, forward-facing principle pair that make the spiders look rather Muppet-like and cute. These large eyes make jumping spiders a standout among spiders."They make their living on foot," said Nathan Morehouse, a biologist at the University of Cincinnati who studies jumping spider vision. "They have to stalk and jump to capture their prey. … To do all of that, they need really exceptional vision. And their vision, depending on how you measure it, is the best for anything as small as they are." 

Jumping spiders are a diverse bunch. Some are drab, while others sport a dazzling array of colors and patterns, from the blue, red and yellow abdomen of the peacock spider (Maratus volans) to the black-and-white stripes of the zebra spider (Salticus scenicus). Saitis barbipes, a European species of jumping spider, sports a red "headband" of coloration and similarly hued stripes on its third pair of legs. Bagheera kiplingi, found in Central America, is striped with a brilliant emerald green. In many species, males are colorful, while females are often more drab. 

Jumping spiders do have a few things in common. They're usually small, often 0.5 inches (12.7 mm) or less in length. They're often fuzzy. And they're distinguishable by their distinct eye pattern of four pairs of eyes, in which the large middle pair sits close together at the front of the face. The face is fairly flat. Jumping spider eyes are, in a word, incredible. The spiders, despite their tiny size, are better at seeing patterns than elephants are, Morehouse told Live Science. Their pair of central eyes, known as the principle eyes, are almost like a pair of tiny binoculars: They have a large outer lens, then a smaller inner lens that magnifies the image from the outer lens and projects it onto the retina. The spiders are also unusual in that they have muscles attached directly to their retinas. That means they can move retinas up and down, side to side, focusing on different segments of the world without a hint of movement on the outside, Morehouse said — a handy ability for a hunter. Meanwhile, the spiders' secondary pairs of eyes are less acute, but they provide crucial peripheral vision, Live Science previously reported. 

Most species of jumping spider can see ultraviolet light and blues and greens. Some species, though, have evolved special tricks to expand their color vision. These, Morehouse said, tend to be the flashier species, such as the paradise jumping spiders (genus Habbernatus), which are often decorated in shades of red and orange. In some species, a random genetic mutation has introduced an extra copy of the gene that makes the proteins that allow for blue/green vision. This extra copy has then acquired mutations capable of detecting red and orange. Similar mutations allowed primates to develop color vision, Morehouse said. Other jumping spiders have hacked color vision by developing pigments that sit in front of their blue/green color receptors. These pigments let only red wavelengths of light through. Though the receptors aren't most sensitive to these wavelengths of light, they react to the stimulus anyway, cuing in the spider that red or orange is present. Birds and reptiles often use this strategy to expand their color vision.

Jumping spiders can also see depth, in a "totally unusual" way, Morehouse said. According to 2012 research, the spiders use something called "depth via image defocus." Here's how it works: Jumping spiders' retinas have four layers of photoreceptors. Because different colors of light have different wavelengths, each layer catches a different color of light in precise focus. In a jumping spider retina, the bottom two layers are both chock-full of green color receptors. But green light that hits one of those layers perfectly in focus will appear a teensy bit blurred on the other layer, and vice versa. The 2012 research found that the spiders use this difference in focus to detect depth. 

https://www.livescience.com/jumping-spiders