Spiders, with their intricate webs and numerous eyes, often spark curiosity, especially regarding how they perceive their environment. A common question arises about their interaction with light, particularly red light. Understanding how these arachnids see provides insight into whether a red glow affects them in the same way it might affect humans or other animals.
How Spiders See
Spiders possess multiple simple eyes, known as ocelli, unlike the compound eyes of insects. Most spiders have eight eyes, though some species have six, four, or even two, with arrangements varying across families. These include a pair of principal eyes and up to three pairs of secondary eyes, each with specialized functions.
Humans have trichromatic vision, perceiving red, green, and blue light. Most spiders, however, primarily have dichromatic vision, seeing green and ultraviolet (UV) light. Their vision excels at detecting motion and changes in light intensity, useful for hunting and avoiding predators. Many spiders do not rely heavily on vision, depending more on vibrations, touch, and chemical cues.
Some spiders, particularly jumping spiders, exhibit more advanced vision. Their principal eyes offer high-resolution color vision, and some species detect a broader spectrum, including reds, oranges, and yellows. This enhanced color perception in jumping spiders is due to specialized filters within their eyes that shift green-sensitive photoreceptors to red sensitivity, or duplicated genes. Even with this capability, their sharp vision covers only a small, central portion of their field of view.
Red Light and Spider Behavior
For many spider species, red light falls outside their primary visual spectrum. Their photoreceptors are most sensitive to green and ultraviolet wavelengths, making them largely indifferent to red light. Many spiders perceive red light as darkness or a very dim environment.
Scientific observations support that most spiders are not attracted to or repelled by red light. Tarantulas, for instance, which rely on detecting movement and light changes, appear unaffected. Some studies suggest red LED lights are among the least attractive light sources for insects, indirectly making red-lit areas less appealing to spiders seeking food.
The visual world of spiders is complex and varies by species. While many spiders lack specific red light photoreceptors, some jumping spiders are an exception. These species have evolved mechanisms to perceive red, often using it in courtship displays or to identify toxic prey. For these specific spiders, red light is meaningful, but for the majority of spider species, it holds little or no visual information.
Practical Implications of Red Light
The limited perception of red light by many spider species offers practical advantages, particularly for observation. Researchers and enthusiasts use red lights when observing nocturnal spiders or those sensitive to disturbances. This allows human vision to adapt while minimizing impact on the spiders’ natural behavior, permitting close viewing without startling them.
In household environments, red light does not attract spiders. Spiders are drawn to areas where their insect prey congregates, and many insects are less attracted to warmer light colors like red, orange, or yellow compared to blue or UV light. Therefore, using red LED lights outdoors indirectly reduces the presence of spiders by making the area less appealing to the insects they hunt.
Red light is not an effective deterrent or strong attractant for most spider species. While some anecdotal evidence might suggest otherwise, scientific understanding indicates their visual system does not process red light to trigger a strong behavioral response, except for specialized species with specific red perception.