The "Daring Jumper"
Phiddipus audax

Home  UWL  Multiple Organisms  Contact Me

Eight-legged cats!

    Like a cat the salticids will locate, track, stalk down, and leap onto active prey. All of these actions are under optical control. Due to this optical control they can distinguish between mates and rivals, as well as living and non-living organisms. Most spiders have poor vision but salticids have acute vision. No other spider has such intricate eyes and such complex vision-guided behavior. Salticids vision is unusual, and highly evolved. Many neurobiologists have extensively studied this stand-out trait of the salticidae family. Salticids have 4 sets of eyes. The antero-median (AM) eyes are responsible for their acute vision. These eyes are quite large and give the spider an almost cat-like resemblance. The other three sets of eyes are located on the side of the AM eyes, and the back and top of their head. These additional three sets detect movement. Here is a diagram of their eye layout (Harland 2000).

    Interestingly, when attacking its prey the spider may take a detour if a direct route is not available. This detouring ability has been extensively studied because the fact that they can perform these detours shows that they have the ability to problem solve. These studies have been done by placing a spider in artificial vegetation and observing its hunting tactics. When the spider chose between two routes, of which only one led to prey, the spider always chose the right one. The spider chose correctly even if it meant loosing sight of the prey momentarily and/or moving away from the prey. This has been compared to similar detours in lions’ hunting tactics that although have not been studied experimentally are demonstrations of planning ahead. However, not many question how this is possible in lions because they have much bigger brains and are also mammals that have evolved tremendously.

    Studies have shown that salticids can distinguish between insects and spiders, even if the spider is outside of its web. Also, they can rely on their eyesight to differentiate between egg-carrying spiders and eggless spiders. In order to do this, the spider must have extremely good spatial acuity. Sympetrum striolatus, a dragonfly, has the highest acuity, 0.4, of any insect. The salticids much smaller eyes have an acuity of 0.04, this is ten times better than that of the dragonfly! To give an idea of just how much smaller the salticid's eyes are compared to the dragonfly, the compound eyes of the dragonfly are comparable in size to the spiders entire combined, abdomen and head! The humans acuity of 0.007 is only five times better than that of the spiders. Acuity of 0.04 would mean that at a distance of 200 mm, the spider can distinguish between two objects that are no more than 0.12 mm apart (Harland 2000).

    How in the world is this possible? Well, to put it simply the layout of the AM eyes, which are responsible for the acute vision of the spider, are an almost exact resemblance to that of binoculars. The AM eyes of salticids have a long eye tube that has a small four-layered stack of retina at the end of it. The unique stacked layers of retina function in the ability to perceive color. The differing wavelengths of light, the different colors, come into focus at different layers on the retina. Allowing the spider to distinguish what the color is of the object it is looking at. The salticids actually have the ability to move their eye-tubes side to side. This allows the image to be focused and gives the spider the ability to sustain its complex vision-guided behavior (Harland 2000).

An example of such vision-guided behavior would be its ability to accurately leap to attack its prey.