A tan jumping spider (Salticidae: Platycryptus undatus) makes its way inside my lab to escape the imminent winter. This species dwells on wide vertical surfaces such as tree bark, lying inconspicuously with a dull suit of gray, white, and black colors. In contrast to typical perky jumping spiders, Platycryptus adopts a flattened cryptic posture with the legs splayed out. Like most salticids, they will construct silken refuges to lay eggs in and provide protection during the incubation period. Spiders will also conceal themselves within these abodes during the night and while braving the winter season. Sexes of Platycryptus undatus are easily distinguished at maturity; females like this individual have a white “moustache” (band under the eyes) while males have an orange moustache.
Salticids are perhaps the most visually-adept family of spiders. Their eye arrangement allows them to achieve both a wide field of vision and incredibly precise motor function. The two posterior pairs of eyes are reduced in most salticids, but retain rudimentary light-sensing capabilities for motion detection behind the spider— literally eyes in the back of their heads. The anterior lateral eyes provide a similar function, but are much more developed, capable of distinguishing details and sufficient in eliciting predatory behaviors. They allow for a ~120° visual field with stereoscopic overlap, critical for spiders in perceiving and navigating their immediate surroundings. When experimentally covered, spiders were entirely unable to track the movement trajectories of objects. (Jakob et al., 2018)
The two front-facing eyes are called the antero-median eyes. This pair is enlarged, and by giving them a more humanoid appearance, they are entirely responsible for saving jumping spiders from much of the insensible malice that is thrust upon almost all arachnids. But back to their function. The antero-median eyes are by far the most specialized, having high spatial acuity and allowing perception of a breadth of colors across the electromagnetic spectrum. These two eyes are long and tubular shaped, and through muscular control, salticids can move the structures independently through lateral and rotational movements. If you’ve ever gazed long enough at a jumping spider, you’ll notice the bizarre lens movements going on inside their heads. While the corneal lens just at the surface remains fixed, another flexible lens lies in the interior just in front of the retina, giving them a telephoto lens system which results in a magnified image.
The ‘boomerang-shaped’ retina possesses four layers of photoreceptors: two layers with ultraviolet-sensitive pigments, and two deeper layers with green-sensitive pigments. Remarkably, it’s been hypothesized that the difference between focused green light on the deepest layer and an unfocused image on the second deepest layer contributes to depth perception in salticids; the amount of image defocus is proportional to distance. Through evaluating the ability of jumping spiders to successfully target prey under green and red light conditions, behavioral experiments support the image defocus hypothesis: spiders under green light conditions produced the highest ambush accuracy, while spiders under red light consistently underestimated the distances of their jumps.
Another discovery in the salticid visual system is the presence of a long-wavelength filter in a subset of retinal photoreceptors, shifting their peak sensitivity ~100 nm from green to red. Despite solely having green and UV-sensitive photopigments, a red filter makes salticids functionally trichromatic, which may be important in prey selection or complex courtship displays. Because the filter covers a small portion of the retina, trichromatic vision is only present in a narrow visual field. Localized trichromatic vision could therefore be an explanation for salticids’ intricate head motions to face targets head-on, working complementarily with the interior movements of the anterior-median eye tubes.
A subadult male white-spotted jumping spider (Salticidae: Phidippus audax) pulses its pedipalps down and up, flashing the dazzling blue-green iridescent colors on the chelicerae. Jumping spiders are a morphologically diverse family, with thousands of species sporting iridescent and conspicuous color patterns for use in courtship displays and conspecific aggression. In some salticids, the position of iridescent hues across the carapace and abdomen signals fighting ability among males. These so called status signals allow spiders to assess relative fighting ability, minimizing the energetic and health costs of predictable contests that would have otherwise escalated into physical aggression. Although it remains to be tested, the iridescence might also have a function in antipredator behavior. I was able to elicit a charming defensive response in this individual by waving my finger around. The spider raised its front legs high above its body and moved its pedipalps aside to completely expose the iridescent patches and bright red fangs. The display was accompanied by jagged slow movements in a backwards retreat, eventually culminating in a mad dash to the underside of a leaf.
All photographs in this post were taken under controlled conditions after capture [6]
