Isabel Cantú
If you were to turn a human eye inside-out, you could expect to find something functionally similar to a compound eye, the type of eye that many insects possess. They are eyes perfect for seeing wide swaths of vision, individual frames put together as a mosaic letting an insect see where any food may be hiding. Human eyes offer convenience, but not perfection. We have corneas and lenses that help focus light on our retinas, giving us vision albeit with some blind spots. It is our solution for sight, but for insects who are constantly on the lookout for predators and prey, a single-chamber camera eye such as our own is not sufficient. The compound eyes of insects form the array of teeny hexagonal shapes we see in simple flies and honey bees, leading them along a world of invisible patterns and wrap-around vision.
600 million years ago marked when insects and crustaceans began to develop these unique eyes. Convex structures with pigment cells and small lenses make up compound eyes, the perfect combination for a large field of view. Ommatidia, hexagon-shaped tubes, funnel light to receptors and number in the thousands. Each has a lens, cone, and photoreceptive organ, and all ommatidia are pointed in different directions. They are responsible for the rough-looking texture that we see in most insect eyes. The ommatidia form images that compose the mosaic of sight that insects use to see the world. Some insects, like the dragonfly, have around 30,000 ommatidia that grant them the ability to see small details in high spatial resolution, all while zipping through the air.
An ancient component of compound eyes is the dorsal rim area, which is an area also composed of ommatidia. This area is entirely specialized for detecting the electric field of polarized light, using this polarization of light to find water surfaces, to navigate, and for basic orientation. As light filters through the atmosphere it gets scattered, becoming plane-polarized and producing patterns. Patterns of polarization aren’t visible to humans, but insects see them and use them to their advantage. Crickets and flies, for instance, use these patterns to stick to a straight line when moving. Even though the dorsal rim area is comprised of relatively few ommatidia (usually only about 100 compared to the thousands that compose the core eye), its function helps a multitude of insects go from day to day.
Surprisingly, compound eyes have remained relatively unchanged over the past several million years. Very little has been altered about their overall structure, as evidenced by a half-a-billion-year-old trilobite eye, which had ommatidia that lacked lenses and were much more spaced out than modern eyes but were structurally the same. An excellent ability given to both these ancient arthropods and modern-day insects is the ability to quickly discern motion. Something called the ‘flicker effect’ occurs in compound eyes as something moves across the field of ommatidia. As structures designed to pick up light in one specific direction, they are turned on and off as something moves by, flickering and alerting an insect to react. It’s used both for avoiding predators and for finding food. If your hand is coming down to swat a fly, it’ll zip away as its ommatidia detects your obvious threat. If a bee sees a swaying flower, it’ll be drawn to it much more than a stationary flower, its presence highlighted against the rest of the still world.
Even though most insects have ommatidia that work independently, gathering light and forming ‘pixels’ of images that later fit into the collective puzzle of every sensory organ, some insects have taken it a step further. Light-collecting pigments in the ommatidia of some insects are shifted further down than normal in their pigment cells, so light captured in a single ommatidium stimulates the pigments in adjacent ommatidia. Insects that have this unique feature, like praying mantises, are much more sensitive to light and can move during evenings and nights.
Insects are all around us, whether you like their company or not. They come in a variety of colors, shapes, and sizes, but their eyes have distinguished and connected them for millions of years. Hardly-changed and always intriguing, compound eyes let insects navigate along invisible patterns of light and detect motion on almost every side of their body. It is a unique eye type that has survived hundreds of millions of years, continuing to be of service to help the insects of today find their way around the world. Consider that when you move in an insect’s field of view, you are a flickering of light on an insect’s mosaic, a glaring image formed from ancient structures and primitive pigments against an invisible grid of polarized light. No matter your intent when approaching an insect, it can see much more than you think.
Works Cited
Image credit:
No changes were made, https://en.wikipedia.org/wiki/File:Marco_Hebing_-_Eyes_of_a_dragonfly_(by).jpg, License: Creative Commons Legal Code
No changes were made, https://upload.wikimedia.org/wikipedia/commons/9/96/Bee_eyes.jpg, License: Creative Commons Legal Code
Educational Questions:
Q. What basic structures form the bulk of a compound eye?
A compound eye is composed of thousands of ommatidia, tubes shaped like hexagons that direct light to photoreceptors. They contain light-collecting pigments that help to produce individual frames for the overall view of an insect, and are responsible for detecting movement within eyeshot.
Q. What’s the dorsal rim area of a compound eye?
The dorsal rim area is a small part of a compound eye, usually only composed of about 100 ommatidia, but it is the part of the eye used by insects to detect polarization patterns. These patterns help insects locate water, move in straight lines, and orient themselves—it’s like having a mini, invisible compass.
Q. How come certain insects are much more sensitive to light than others?
There are some insects with compound eyes that have a unique way of stimulating more than one ommatidium. Because of a shift in pigments in the ommatidia, oncoming light stimulates multiple at once rather than just one at a time, leading to higher light sensitivity. This is usually found in insects that are very active later in the day or throughout the night.