Getting Into a Spider’s Head
This cross section through the head of a jumping spider shows off the interior structure of four of its eight eyes. Jumping spiders have the most acute vision of any terrestrial invertebrate. In fact, their ability to perceive detail almost matches that of humans, and they accomplish that feat with eyes that are only a tiny fraction of the size of our eyeball.
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DownloadGetting Into a Spider’s Head
This cross section through the head of a jumping spider shows off the interior structure of four of its eight eyes. Jumping spiders have the most acute vision of any terrestrial invertebrate. In fact, their ability to perceive detail almost matches that of humans, and they accomplish that feat with eyes that are only a tiny fraction of the size of our eyeball.
What am I looking at?
This is a cross section through the cephalothorax (fused head and torso) and the front four eyes of a jumping spider. The left half of the image shows the distribution of the filamentous protein actin; you can see that it is highly concentrated in the muscles (1) and neurons, as well as in the retinas of one of its two principal eyes (2) and of one of its six secondary eyes (3). The right half of the image reveals the cells’ nuclei and shows the organization of the cells within the spider’s cephalothorax (4).
Click on the right arrow to see some additional views of this spider’s eyes.
Biology in the background
Unlike insects that have compound eyes with multiple lenses and light-sensing receptor fields, spiders – like humans – have simple eyes with only one lens each and a layer of light-sensitive cells. Each of the two large principal eyes of a jumping spider has a tubular appearance and a narrow acceptance angle, making it similar to a telephoto lens. But the spider’s lens is fused with its exoskeleton, so spiders cannot move their eyes to track a target. To keep a target within their narrow field of vision, jumping spiders would need to rotate their body, risking revealing their position. But evolution provided an elegant solution to that problem. To compensate for their narrow field of vision and inability to move their eyes, they evolved a strip-like, movable retina. It scans the image projected by the lens using six muscles. In certain species with a translucent exoskeleton, the movement of the retina can even be seen though the cuticle.
The retinas of the principal eyes are made of four layers of stacked photoreceptors that are sensitive to different wavelengths on the electromagnetic spectrum, from near ultraviolet to red. The six pairs of secondary eyes have a wide field of view and broad, cup-shaped retinas. They offer low resolution and monochromatic vision, but all six fields combined create a nearly 360-degree panorama. The role of the secondary eyes is to alert the spider to movement around it. This visual system of principal and secondary eyes is reminiscent of humans’ fovea (a part of the eye with color sensitivity and a high density of photoreceptors) and peripheral retina (which contains a lower density of receptors and lacks color perception).
The head of a jumping spider is about 1.2 millimeters across, or roughly the width of a pencil lead.
Technique
These images were created using confocal microscopy.
Igor Siwanowicz, HHMI's Janelia Research Campus