Moths possess highly specialized visual systems that allow them to navigate and function effectively in low-light conditions. Unlike humans, whose vision struggles significantly in dim environments, moths have evolved unique adaptations enabling them to perceive their surroundings with remarkable sensitivity. This specialized vision allows them to find food, locate mates, and avoid predators under the cover of night.
The Unique Structure of Moth Eyes
Moths navigate their nocturnal world using compound eyes, which differ from the single-lens eyes found in humans. Each compound eye is a mosaic made up of thousands of individual light-sensing units called ommatidia. The number of ommatidia can vary widely among species, ranging from approximately 1,000 to as many as 30,000 in some larger moths.
Each ommatidium is typically hexagonal in shape and contains several components. At the outermost surface is a transparent cornea, followed by a crystalline cone that acts as a lens. Beneath these are photoreceptor cells, which contain light-sensitive pigments. These cells collectively form a central light guide called the rhabdom. The signals gathered by each ommatidium are sent to the moth’s brain, where they are combined to create a mosaic-like image of the environment.
Nocturnal moths typically possess a type of compound eye known as a superposition eye. This design is suited for dim light as it allows light from multiple ommatidia to converge onto a single photoreceptor, enhancing light capture compared to other eye types. This optical arrangement contrasts with apposition eyes found in many day-flying insects, where each ommatidium is optically isolated and focuses light from only one direction.
How Moths See in the Dark
An adaptation is the ability of light-absorbing pigments within the photoreceptor cells to migrate. In bright conditions, these pigments move to block light, protecting the sensitive cells, while in darkness, they retract to allow maximum light transmission to the retina. This pigment migration adapts vision to varying light intensities.
Many nocturnal moths also possess a reflective layer behind their photoreceptors called a tapetum lucidum. This layer acts like a mirror, reflecting any light that passes through the photoreceptors without being absorbed back through them, providing a second chance for light detection. This “light recycling” amplifies the signal in low-light conditions and is responsible for the characteristic “eyeshine” in many nocturnal animals.
Moths further enhance their light sensitivity through neural summation, where signals from multiple ommatidia are pooled together in the brain. While this pooling improves the overall sensitivity, allowing some hawkmoths to see at light levels 100 times dimmer, it comes at the cost of reduced visual resolution. This trade-off prioritizes detecting faint light over sharp image detail, advantageous in dimly lit habitats. Moths are also sensitive to ultraviolet (UV) light, which aids in finding flowers that reflect UV patterns and navigating under moonlight.
Why Moths are Drawn to Light
Moths fluttering around artificial light sources stems from their natural navigation strategies. Moths traditionally rely on distant light sources, such as the moon and stars, for orientation, a behavior known as transverse orientation or celestial navigation. They maintain a constant angular relationship to these distant celestial bodies, which allows them to fly in a straight line.
When a moth encounters a nearby artificial light source, this natural navigation system is disrupted. Unlike the distant moon, a close artificial light causes the angle to the light source to change rapidly as the moth flies. The moth attempts to correct its flight path to maintain a constant angle, leading to a spiraling trajectory that draws it closer to the light.
A related theory suggests the “dorsal light response,” where moths try to keep their back oriented toward the brightest light source. In natural environments, the sky is the brightest area, helping moths maintain proper flight attitude. However, with an artificial light below them, this instinct causes them to tilt their bodies and continuously steer around the light, trapping them in looping flights. Moths are also attracted to certain wavelengths of light, notably ultraviolet (UV), which many artificial lights emit.