As I’ve mentioned a thousand times on this blog, I’m scared of spiders. Since I was a little kid anything with eight legs and eyes that moved gave me the shivers. Jumping spiders have a particular place of fear in my heart, however, mainly because my parents always told me stories about the mythical ‘Martha’s Vineyard leaping spider’. Whenever we went to vacation there, they teased me with horrible stories about the leaping spider, and I lived in constant fear that spiders would jump at me from all directions. In reality, jumping spiders are a lot less scary and much more fascinating than my childhood memories would suggest.
Jumping spiders comprise the family Salticidae, which is the largest family of spiders. There are over 5000 known species, and they live almost everywhere in the world, except at the poles. Despite their immense diversity there are a few characteristics that are universal to all jumping spiders.
The most prominent feature of jumping spiders is their eyes. Like all spiders, they have eight of them, but the first two pairs are the most important. The eyes on the front of the spider’s head are huge, and have excellent vision. They contain up to four kinds of cone cells, suggesting that jumping spiders have tetrachromatic vision, and can see ultraviolet light. Unfortunately for the spider, these eyes are too close together for depth perception, which is exceptionally important when you jump though the air to hunt insects. They have a solution for this though, which is called image defocus. There are two layers in the eyes, one which is receptive to UV light and the other which can sense green light. Any incoming green light makes fuzzy images on the UV layer, and the amount of blurriness tells the spider how far away the object is. It seems like a really neat solution to the problem. Kudos to you, evolution.
The eyes directly to the sides of the front middle are very useful for hunting, and can move around in response to stimuli. In experiments where researchers covered all eyes but the front two, jumping spiders were still able to successfully leap and catch prey. When these eyes were covered and the other three pairs were visible, motion in front of the spider did not trigger a predatory response.
With these two pairs of eyes (and help from their other two pairs, which gives the spiders near 360 degree vision), jumping spiders are formidable hunters. Their back legs have a hydraulic system which allows them to jump more than their body length. The spiders attach themselves to strings of silk before they jump, just in case they misjudge it completely and fall. Once a jumping spider detects a prey item, they stalk it, approaching it until they are close enough to leap on the prey. Sometimes this stalking behaviour takes the spider on a path where it can’t see the prey, but the spider is still able to find the prey.
Jumping spiders are often brightly coloured and use these markings to communicate with conspecifics. Males are generally brighter than females, and can have markings on their legs that are only visible in ultraviolet light. Jumping spiders often perform elaborate courtship dances, which involves walking in zigzags and waving the forelegs. If a female sees one and is suitably impressed, she crouches down and permits the male to mate with her.
Jumping spiders are pretty impressive little creatures; I’m just glad they are little. Otherwise I would be terrified at all times – after all these spiders are pretty vicious and have some scarily complex behaviour. Maybe they lend themselves to a cheap hollywood horror movie? I wouldn’t watch it, but only because I’d have nightmares.