The first thing anyone notices about a jumping spider is its eyes. Specifically, the two large forward-facing ones that give it a face-like appearance. Those eyes are the reason jumping spiders have become so popular online, and the reason they behave so differently from other spiders. But what's actually going on behind those oversized lenses is stranger and more interesting than most people realise.
Eight eyes, four jobs
Like all spiders, jumping spiders have eight eyes. They're arranged in three rows on the front of the cephalothorax (the head-thorax body section). But unlike most spiders, where all the eyes are fairly similar, jumping spider eyes are heavily specialised. Each pair does a different job, and together they create a visual system that's unique among arthropods.
The two large front-facing eyes are the anterior median eyes (AME). These are the ones you notice. Behind and to the sides sit the anterior lateral eyes (ALE), and further back are the posterior lateral eyes (PLE) and posterior median eyes (PME). The PME are often small to the point of being functionally reduced, especially in Salticidae.
The ALE and PLE are motion detectors. They have wide fields of view and low resolution, which means they're good at spotting that something moved but not at telling you what it was. When these eyes detect motion, the spider turns to point its AME at the source. You've probably seen this behaviour. Something moves at the edge of a jumping spider's vision and it snaps around to face it. That's the lateral eyes triggering a redirect.
The anterior median eyes: a telephoto lens on legs
The AME are where things get remarkable. Each one is essentially a long tube behind the lens, with the retina at the back. The tube shape gives them a very narrow field of view (about 5 degrees) but surprisingly high spatial resolution for eyes this small. They can resolve detail at distances that seem absurd for an animal measured in millimetres.
The retina itself is unusual. It has four layers of photoreceptors, and the layers are sensitive to different wavelengths of light. The deepest two layers are sensitive to green light, one to UV, and one to blue. This layered structure is thought to help with both colour vision and depth perception, because objects at different distances focus on different retinal layers. It's a bit like having a built-in range finder.
The AME can also move independently within the head. The retinal tubes are attached to muscles that can scan the retina across the visual field, a bit like moving your eyes without moving your head. When a jumping spider sits still and stares at you, it may actually be scanning its retinae back and forth to build up a more complete image of what it's looking at. From the outside, you can sometimes see this as a slight darkening or shifting of the eye colour as the retinal tube moves behind the lens.
Colour vision
Jumping spiders see colour, and they see it well. The layered retina of the AME provides sensitivity across a range from UV to green wavelengths. Some species have been shown to perceive red as well, likely through a filter mechanism where red pigment in the retina shifts the sensitivity of underlying green receptors.
Colour vision matters for these spiders because many species use colour in courtship displays. Males of species like Maratus (the peacock spiders from Australia) have elaborately coloured abdominal flaps that they raise during mating dances. If the female couldn't see those colours, the display would be pointless.
Even in the commonly kept Phidippus species, the iridescent green chelicerae of males are a visual signal. When a male displays to a female, he faces her head-on and waves his front legs, showing off those metallic mouthparts. The female watches with her AME and makes a decision that the male very much hopes goes well for him.
Depth perception and hunting
Jumping spiders need accurate distance judgment to land their pounces. They hunt by stalking prey, creeping closer, and then making a final leap. If they misjudge the distance, they miss. Missing costs energy and alerts the prey. So their depth perception has to be good.
Research published in Science in 2012 (Nagata et al.) demonstrated that jumping spiders use the defocused images on different retinal layers to calculate distance. Because the retinal layers are at different depths behind the lens, an object at a given distance will be in focus on one layer and out of focus on another. The spider uses the degree of defocus to determine how far away the object is. It works without the binocular overlap that mammals rely on for depth perception.
This explains something keepers have noticed: jumping spiders seem to judge distances accurately even with a head that's only a few millimetres wide, where binocular parallax would provide almost no useful information. They're not using binocular vision for distance. They're using image defocus within each individual eye.
What this means for keepers
Understanding jumping spider vision has some practical implications for how you keep them.
Light matters. These are visual animals that depend on good lighting to hunt and navigate. Keeping them in a dim corner suppresses natural behaviour. They don't need intense light, but they do need a clear day/night cycle with enough brightness during the day to hunt effectively. A spot near a window (out of direct sun, which would overheat the enclosure) is ideal.
They will watch you. This isn't anthropomorphism. When a jumping spider turns to face you and tracks your movements, it is gathering visual information about you using some of the most sophisticated eyes in the invertebrate world. Whether it processes that information in any way we'd recognise as "interest" or "curiosity" is debatable, but the visual attention is real.
Avoid sudden movements near the enclosure. The lateral eyes are sensitive to motion, and sudden movement triggers a startle response. If the spider frequently bolts into its web sac when you approach, try moving more slowly. They habituate to regular, predictable movement over time, which is why spiders kept on busy desks often seem calmer than those in quiet rooms where any movement is unexpected.
Finally, their vision is one reason jumping spiders feel different from other invertebrate pets. Most invertebrates react to vibration, chemical cues, or touch. Jumping spiders react to what they see. When you interact with one, there's a sense that the animal is visually engaged with you in a way that a millipede or beetle simply isn't. It's not mammalian cognition, but it's something, and it's what makes these spiders so unusually compelling.