Spiders typically possess eight eyes; however, their number varies among species. One such spider was discovered living in a cave in 2012 – it lacked any!
Spiders typically possess two large, front-facing eyes that detect colors and create images with depth of field. Additional eyes may exist on either the back or sides of their heads.
The principal eyes
Spiders are one of the world’s most vision-dependent creatures. They have evolved a highly complex visual system to assist them with daily activities, from prey hunting and web construction to courtship, mate recognition, escape from enemies, and everyday activities like moving about their environment. Most species typically have eight eyes, but some species only require six.
Spiders use their primary eyes – anteromedial eyes or AMEs – as the primary visual organs, capable of focusing and judging distance. Furthermore, these large boomerang-shaped lenses also allow color vision; some spiders even possess trichromatic vision.
However, these highly sharp eyes only cover a limited area. To supplement their vision and provide a warning when something requires their attention – such as prey items or potential predators approaching – two smaller eyes known as periphery eyes scan a broader field in black-and-white and are designed to detect anything needing immediate attention (i.e., prey items or predators approaching). These periphery eyes act like radar to the anteromedial eyes to alert them of anything requiring their focus (i.e., prey items or predators coming) before alerting their counterparts (i.e., the anteromedial eyes) of anything they detect; as soon as something needs their immediate attention (i.e., prey item or predator approaching), these periphery eyes quickly alert their respective counterparts via radar systems located close-by that radar.
Researchers at the University of Massachusetts Amherst recently used a device they created to track spider eye movements, using one they tethered a jumping spider to and watched her as it watched videos with cricket silhouettes; later, they added shapes like an oval growing over time and tracked where its principal eyes focused their gaze.
Results revealed that the lateral eyes alerted anteromedial eyes of an oval’s growth, prompting their gaze to focus upon it as it expanded. This allowed principal eyes to quickly detect movement and improve their ability to capture prey while judging distance.
As well as helping researchers understand how spider eyes work, this research also provided significant insight into the structure and architecture of an arachnid retina. They discovered that an arachnid’s retina does not feature four distinct layers with differing densities of photoreceptors – contrary to what had been assumed in previous research.
The secondary eyes
Spiders possess multiple eyes for various uses, enabling them to detect movement around them. The giant pair are known as the anterior median eyes or PM eyes and catch depth perception and prey; secondary eyes on either side help detect motion detection.
Contrasting with the compound eyes of most insects, which integrate information from thousands of lenses into an intricate mosaic image, spiders’ simple eyes do not form detailed images but instead have vast fields of view that overlap one another. Behind their immobile corneal lens is a boomerang-shaped retina equipped with photoreceptor cells capable of sensing polarized light, movement, and electrical signals the spider’s brain interprets.
Studies published in Current Biology suggest that spider’s eyes may seem useless but play an essential function. When researchers blocked off the lateral eyes of jumping spiders–which typically hunt during daylight hours–they discovered they redirected principal eyes toward moving objects and hindered tracking disk movement, indicating the essential role they play in sensing prey or potential danger.
The PM and lateral eyes may have evolved from the compound eyes of spiders’ ancestor species, Chelicerata. Primary eyes specialize in in-depth perception and prey detection, while secondary eyes detect motion; further classification criteria exist between both eyes regarding location, lens optics, the field of view, sensitivity levels, and ability to see polarized light.
Day-feeding spiders that may be more vulnerable to predators rely on the lateral eyes as an extra sense. They detect movements their principal eyes cannot, such as an approaching predator or possible prey moving about – similar to how humans use peripheral vision when confronted with something as hazardous as a baseball bat being thrown at us.
The lateral eyes
Just like insects, spiders use compound eyes to see all directions. Compound eyes consist of many individual units called ommatidia with individual lenses and photoreceptor cells, which convert light to electrical signals that are sent back to their brain for interpretation, creating a powerful visual system that enables spiders to see movement across long distances and all directions simultaneously.
But spiders also possess secondary eyes on either side of their heads, allowing them to detect motion that might otherwise go undetected by their primary set. They can even see ultraviolet light – helping the spider identify objects by size and position. These secondary eyes serve an essential function: allowing distinguishing between things.
Similarly, spiders use their lateral eyes to judge whether an object is approaching or receding from them, an essential skill when trying to differentiate between coming prey items such as insects or birds and potential threats like predators or competitors.
These eyes can be distinguished from principal eyes by their smaller size and relative positioning on the head, with their optic part covering part of the base and lateral walls of the retina (Fig. 2B). While principal eyes can focus on objects at a variety of distances, only close things within several body lengths of spider can be focused upon by lateral eyes.
Additionally, the lateral eyes can move independently behind the fixed lenses of their retinal envelopes using sophisticated muscles that vary across taxa. For instance, in jumping spiders (Pyrrhocoridae), retinal muscles produce microsaccadic “twitches” of 2-4deg that lead to larger displacements of up to 15 that enable the lateral eyes to assess the relative speed and direction of movement when tracking prey items that move within its fields of view. This helps accurately evaluate moving prey items.
The ocelli
Arthropod primary eyes can detect light by sensing the direction of light; additionally, their forward-facing direct eyes contain muscles to move the retina to focus and track images. These eyes are known as ocelli, and spiders use them for several essential functions: they detect motion to identify prey and threats by sensing movement; depth perception allows spiders to determine distance; in nocturnal species, they illuminate a tapetum lucidum that helps them see better under low light conditions; they also illuminate reflective structures known as tapetum lucidum which illustrates these functions by lighting their forward-facing primary eyes a tapetum lucidum which helps them see their surroundings more easily under low light conditions.
Ocelli are sideways-pointing eyes found throughout insect orders and are most commonly seen in flying insects like bees, wasps, dragonflies, locusts, and ants. Dorsal ocelli may also be present on some terrestrial insects like cockroaches and ants but are usually less noticeable.
Ocelli are also used to detect ultraviolet and infrared light that the arachnid uses for photoreception; this helps control cell metabolism and other physiological processes. Furthermore, their shape recognition capabilities make the ocelli invaluable in visualizing shapes and colors.
Ocelli are used by animals such as spiders to detect and differentiate among various forms of prey, for instance, when tracking down potential targets such as insects. Jumping spiders use their ocelli to spot potential prey’s movements while helping determine their direction and motion relative to their body. Ocelli can also provide what’s known as a “looming response,” the ability to perceive movements that would otherwise go undetected by people or other animals.
Spider species vary, with any having two to eight eyes, though these eyes are always arranged in pairs. Family groups also differ in eye counts; for example, the Caponiidae family contains spiders with four eyes, while most spiders typically possess eight.
