One of the challenges for pilots is determining the position of the horizon. Horizon detection is important during visual flight to maintain proper attitude of the aircraft and correct the pitch (Nose Up; Nose Down) and roll (Right and Left Wings level, or one wing higher than the other.) Failure to accurately detect the horizon can result in a crash.
Flying insects must also be able to detect the horizon to stabilize their flight. The insect organs used for horizon detection are the ocelli. Ocelli are simple eyes with a single lens. (The larger compound eyes contain multiple lenses.) Most insects have 3 ocelli in a triangular array with one medial and 2 lateral ocelli. The medial ocellus faces forward and the lateral ocelli are at an angles to the sides. Together the 3 ocelli provide the insect with a wide angle view of the landscape.
How do insect ocelli detect the horizon?
Unlike our eyes with lenses that focus an image on our retina, insect ocelli have lenses that do not focus the image on the retina. The ocelli cannot “see” details but are capable of determining the direction of a light. The three ocelli have overlapping fields of vision that provide a wide angle view of the sector that contains the horizon.
Ocelli in locusts and dragonflies contain multiple photoreceptors that have 2 peaks of sensitivity. Some receptors are most sensitive to UV light and some are most sensitive to green light. UV light is very intense in the sky and mostly absent on land. Thus, light in the UV range provides a sharp contrast between “bright” sky and “dark” land. However, relying on UV light alone would not compensate for bright objects in the visual field (such as the sun). Light in the green range is of similar intensity on the land and sky. Thus, processing the differential in brightness between the UV receptors and the green light receptors will compensate for the sun and give an accurate indication of the horizon. The line in the field of vision where there is the sharpest change in the differential between UV and green receptors will be the horizon.
Humans do not see light in the UV range so the use of UV sensors in horizon detection might escape our attention. Insects have been flying for over 400 million years with plenty of time to evolve sophisticated strategies for horizon detection. Learning that insects are using UV light in horizon detection suggests that UV sensitive detectors may be better for horizon detection than visible light detectors. In this way, studies of insect vision may lead to better sensors for detecting the horizons. Such sensors can provide pilots (or drones) with additional information that improve flight safety and navigation.