Bold jumping spider (Phidippus audax).
Illustrations by Adelaide Tyrol
During my first lecture in front of a large room of students, a small spider put me – and my 220 students – at ease. As I flicked on the overhead projector, I spotted a ¼-inch spider with large eyes and fuzzy-looking body under the glass. This was a jumping spider of the family Salticidae, and, right on cue, it jumped the width of the projector. From the students’ perspective, that jump seemed to span the entire width of the room.
The three most common jumping spiders in the northeastern United States are the bold jumping spider (Phidippus audax), which graced my projector and calmed my nerves that day; the tan jumping spider (Platycryptus undatus), which has been moving north as the climate warms; and the Eurasian zebra jumping spider (Salticus scenicus). You might spot any of these species either in the woods or climbing the walls of your house – inside or out.
Like most spiders, jumping spiders boast eight eyes, arranged in pairs. But jumping spiders’ eyes are different from those of their arachnid cousins – and essential to the spiders’ hunting endeavors and to escaping perceived danger. Most spiders have eight eyes of roughly equal size placed in rows or clusters, helping them detect light and motion in most directions.
Jumping spiders, however, have one pair of eyes significantly larger than the others and forward-facing, adding to the visual appeal of these tiny creatures, and providing color vision that extends past the human range into the ultraviolet end of the spectrum. Another set of eyes adjacent to these, together with the pair farthest back, provide nearly 360-degree peripheral monochromatic vision. A final set of very small eyes point upward and, as far as scientists can tell, these eyes do not appear to do much of anything.
Jumping spiders’ forward-facing “principal eyes” have prominent round corneas at the front, fixed in place like car headlights. These eyes extend from the corneas like long tubes that taper back into the spider’s cephalothorax. The principal eyes function like telescopes: high-resolution, but with a narrow field of view. Imagine looking through a pair of toilet paper tubes taped together; to change your field of vision, you’d have to turn your head. But a jumping spider can’t turn its head, because a spider’s head and thorax are fused into a cephalothorax, to which all eight eyes (and legs) are attached.
The retinas of those primary eyes, however, are located deep in the cephalothorax and can move. To gaze at a particularly juicy fly, a jumping spider first turns its body to face in the right general direction. If the fly moves slightly to the left or right, the spider shifts the retinas of its primary eyes to line up a more focused view of the target.
What the spider loses in field of view with its principal eyes, it gains in resolution. The retinas are shaped like deep funnels so that the images viewed by a jumping spider strike many more light-detecting cells than would be the case with the slightly cup-shaped retina of a human. And the spider’s remaining eyes pick up movement in all directions – of predators or prey – missed by the principal eyes.
Jumping spiders rely on information from each of these eyes not only to locate prey and to identify danger, but to adeptly perform the defining trait of this spider family: their jumps. Visual cues are essential to appropriately judge distances to landing points. While many two-eyed creatures subconsciously triangulate distance to what they are seeing, jumping spiders appear to use an entirely different mechanism.
Takashi Nagata and colleagues from the Graduate School of Sciences in Osaka, Japan, manipulated light wavelengths and made behavioral observations demonstrating that jumping spiders use both image focus and lack of focus to manage depth perception. When Nagata blocked one of the forward-facing eyes, jumping spiders still jumped and landed successfully, demonstrating that triangulation between two eyes was not needed to judge distance.
Light receptors in jumping spider retinas are stacked in four distinct layers, and each layer detects a different wavelength of light. The researchers discovered that jumping spiders performed best under green light. That wavelength is always out of focus in the spider’s third retinal layer. In the deepest retinal layer, however, green light is sharply focused. Researchers believe jumping spiders use the amount of fuzziness perceived in the third layer compared to the focused green light in the fourth, deeper retinal layer to judge distance – and to take off and land their jumps successfully. With focal distances reflexively calculated, the spider uses hydraulic pressure to powerfully flick its third and fourth pairs of legs backward just enough to launch a jump, which may carry it as far as 30 times its body length.
The spider hiding in the dark recesses of my overhead projector during my first lecture was poised and ready when I flooded its world with dazzling light. It seems that amazing eyes and gymnastic jumping ability are adaptations useful for more than pouncing on fruitflies; they can also help a jumping spider escape from biology lectures – entertaining students and professor in the process. Hopefully my students are not so eager to avoid class!