Most spiders get a bad rap. Few would harm you, and only rarely are spiders aggressive towards humans. Most will defend themselves if threatened, of course, and a few are venomous. Most spiders, however, would prefer to ignore humans and be ignored by us. A few species are curious, quite interesting and most appealing. Jumping spiders, for example., reveal visual mechanisms unknown in the rest of Animalia.
Jumping spiders are positively charming creatures, and you will know that to be true if you have ever watched one closely. These are common spiders and range from approximately 3 to 17 mm in length and will watch you closely as you approach them. They have four pairs of eyes, with the large anterior median (AM) set the most obvious (Figure 1). These circular eyes provide an “attentive child” appearance because they are fixed and are relatively large based on body size, but are tiny on an absolute scale. These placid eyes belie the organized complexity and evolutionary genius that lies beneath the carapace.
The AM eyes are Galilean telescopes with a corneal lens fixed to the carapace, and a second “lens” at the end of a small tube immediately in front of the retina. This second lens deserves a closer inspection. It consists of a steeply sloped pit at the retinal level (Figure 2). This pit is analogous to our fovea, or area of sharpest vision. There is an index of refraction gradient between the amorphous fluid in the tube (analogous to vitreous in the center of our eye) and the wall of the foveal-like pit. The tissue lining the pit is the retina. This pit and the index of refraction gradient create a minus lens and a Galilean telescope, much like the fovea of many raptorial birds. A creature this small cannot have much retinal area, so to maximize the number of photoreceptors struck by photons, the retina is tiered. This means that there are 4 layers of retinal cells for light to traverse. Each layer of retina extracts information from the light passing through the photoreceptive portion of the cell, called a rhabdom, into the next tier until it strikes the fourth and final layer. The angle of separation from the center of one rhabdom to another perpendicular to incident light is approximately 1.7 micron, creating a mosaic slightly more than three times the wavelength of visible light rays.
This visual system is indeed unique and begins with the aforementioned corneal lens connected to an elongated tube within the cephalothorax (first segment of the spiders body). There are six muscles to move the tube. Since the external cornea/lens is fixed to the carapace, it creates an image that is fixed at one point within the tube. This compact telephoto lens system combined with the tiered retina achieves excellent acuity, but only a very tiny field of vision. So, to increase this field of acute vision, this optical marvel moves the tube housing the retina with six muscles per eye by mostly scanning movements. This is akin to a raster scan similar to those seen on a TV or computer screen. Jumping spiders scan their world much like painting a wall with a fine brush although the retina is not linear, but shaped more like a boomerang. The other pairs of eyes do not scan and are principally used as motion detectors to find other animals for the AM eyes to decipher.
With the AM eyes, jumping spiders have the finest discrimination of all arthropods, and probably all invertebrates as they are visual hunters, whereas most other spiders use the tools of silk.
There is more to this story, though, as recent work by Nagata has shown. Focusing on a prey item would be key to the success of any visual predator. Jumping spiders cannot focus as easily as most vertebrates because they cannot accommodate, or change the shape of their lenses as most vertebrates do. Furthermore, they do not have stereopsis as humans do because the field of view does not overlap as it does in humans or other animals that have stereopsis. Some few insects, and, vertebrates to some extent, can also tell distance and a form of focusing by parallax when the prey item or the predator moves. The mental gymnastics of distance measurement of a telephone pole seen from a moving car window as compared to the scene behind it is an example of such motion parallax. Jumping spiders cannot accommodate and cannot move their eyes. If the spider moves, it may frighten the prey, so the spider needs another mechanism. Nagata and his fellow investigators have shown that jumping spiders use defocused green light from the third of the four layers of visual pigments mentioned above and compare it to green light that is properly focused on the visual pigment in the layer directly beneath that third layer. In some ways, this would resemble parallax but would be a quite different, and very clever, mechanism.
Evolution has found a simple, elegant to measure distance using only color. This permits this tiny eye to be an excellent visual instrument especially at short distances, with surprisingly good acuity that is far better than any compound eye found in other arthropods.
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Harland, D.P. Jackson RR (2004) pp. 5-40. In: Complex Worlds from Simpler Nervous Systems (F.R. Prete, ed.). MIT Press, Cambridge, Massachusetts.
Nagata R, Koyanagi M, Tsukamoto H, et al: Depth perception from image defocus in a jumping spider. Science 2012;335: 469-471.