The sensory world of a spider is far removed from the human experience, leading many to wonder if these eight-legged predators can detect light invisible to humans, specifically infrared radiation. Spiders possess specialized organs to process their environment, but their ability to sense the infrared spectrum requires a clear distinction between visual perception and thermal detection. Understanding how spiders sense their surroundings provides insight into their remarkable ecological success.
Spiders and the Infrared Spectrum
Spiders do not possess the biological mechanisms to visually perceive infrared light, which is radiation with a wavelength longer than visible red light. The infrared spectrum includes the thermal radiation, or heat, emitted by warm-blooded animals and objects. Unlike pit vipers, spiders lack a dedicated visual heat-sensing apparatus. Visual perception relies on light-sensitive pigments in the eyes, but these pigments in spiders are tuned to different wavelengths.
Anecdotal reports suggest spiders react to infrared beams from devices like night vision cameras. This is likely a reaction to near-infrared light at the edge of the visible spectrum, or a response to the light’s intensity, rather than true thermal vision. Researchers often use infrared lights to illuminate spider retinas because the light does not stimulate the visual system, confirming their general insensitivity. Any thermal sensitivity spiders exhibit is a non-visual reaction to temperature changes, possibly mediated by specialized thermo-sensitive hairs or sensilla on their legs and body.
Spider Vision: Specialized Ocular Systems
Since spiders do not perceive infrared light, their visual world focuses primarily on the visible and ultraviolet (UV) spectra. Most species possess up to eight simple eyes, categorized into two main types: the principal eyes and the secondary eyes. The principal eyes, always the anterior median pair, are responsible for acute vision and detailed image formation, sometimes possessing a movable retina. The secondary eyes function more like motion detectors and often have a light-reflecting layer called a tapetum, which enhances sensitivity in low-light conditions.
The principal eyes of many hunting spiders show high sensitivity to ultraviolet light. This UV sensitivity is significant for their behavior, allowing them to detect UV patterns on flowers to locate prey or recognize species-specific UV markings used in courtship and communication. This intricate ocular system is adapted for their specific predatory and social needs.
Sensing the World Without Eyes
Spiders rely heavily on an array of non-visual sensory organs. Mechanoreceptors, which detect mechanical stimuli, are important for their survival. Thousands of fine hairs, called trichobothria, cover the spider’s legs and function as highly sensitive air current detectors. These hairs sense minute air displacements caused by the movement of prey or predators, allowing the spider to orient itself toward the disturbance.
Unique to arachnids are the slit sensilla, tiny, stress-sensing organs located on the exoskeleton, particularly near leg joints. These sensilla, sometimes grouped into lyriform organs, detect minute strains and vibrations in the cuticle caused by substrate vibrations. They allow a spider to pinpoint the location, size, and type of prey caught in its web by analyzing the frequency and intensity of the vibrations.
Spiders also use chemoreceptors, sensory hairs often found on their legs and mouthparts, to “taste” and “smell” their environment upon contact. These receptors allow them to identify potential mates, judge the quality of a food source, or navigate by following chemical trails. The combination of these ultra-sensitive mechanical and chemical senses provides a detailed, localized map of their surroundings.