Cat fleas can see. However, their eyes serve only as light sensors. They can detect changes in light and dark, but can’t see visual images. Flea eyes are able to sense wavelengths between 300 and 600 nanometers, being most sensitive to green-yellow light (515 nm).
Structure & Function of Flea Eyes
Fleas don’t Have Compound Eyes
Adult fleas have two eyes, one on each side of their head. Instead of compound eyes, fleas possess simple, black eye-spots which resemble the dorsal ocelli of other insects Img 1.
Img 1 Adult cat fleas have a black, round, simple “eyespot” on each side of the head.
The simple eyes of fleas are believed to be reduced compound eyes. As fleas evolved into parasites, visual sensitivity became less important, and this led to a reduced eye structure. Some species don’t have eyes at all and are blind, specifically those which parasitize nocturnal hosts living in nests or caves, such as bat fleas.
Fleas Eyes are Light Sensors
Fleas eyes have a single biconvex lens. These simple eyes can detect changes in light intensity, but are unable to form acute visual images. The eyes include the prerequisites for vision (e.g. photosensing, photoshielding, and photorefracting). However, they lack the typical photoreception structures of insects, such as rhabdoms, crystalline cones, and corneal lenses.
Since fleas don’t have acute vision, prior to jumping, a flea will wobble its head and body from side to side to increase depth perception.
Sensitivity to Light
Fig 1 Percent response of cat fleas (y-axis) to light at different wavelengths in nanometers (x-axis) compared against light of 345 nm.
Cat fleas are positively phototactic, meaning they’re attracted light and will move towards it. Fleas can sense light in wavelengths between 300 and 600 nanometers. They can’t see above 600 nm, in the red spectrum. Green-yellow light is most attractive to cat fleas, with wavelengths between 500 and 530 nm eliciting the best response Fig 1.
Sensitivity to Movement
Cat fleas can detect when a target is moving, and are attracted to the movement. Obviously, this response greatly decreases in darkness or red light, because they can’t see. Altering flea traps to use intermittent light significantly boosts their efficacy. When the light goes off and on again, it’s perceived as a shadow cast by a host traveling in front of the light. As a result, fleas jump towards it.
Sensitivity to Color & Size
Experiments show that fleas are most attracted to red and blue targets. While white consistently proves to be the least responsive color. White is likely difficult for the fleas to see. Fleas can’t differentiate between different color patterns on moving targets.
Cat fleas can assess the size of targets with their vision. They aren’t attracted to small targets. In a natural setting, these would be unsuitable hosts, such as rodents.