Compound eye

A compound eye is a specialized visual organ found in arthropods. This group includes many insects and crustaceans, such as shrimp.

Unlike the single-aperture eyes of humans, a compound eye is made of many small parts. These parts are called ommatidia. Each ommatidium is an independent photoreception unit. Every unit contains a cornea, a lens, and photoreceptor cells. These cells allow the animal to distinguish between different colors and levels of brightness. While compound eyes have poor image resolution, they offer a very large view angle. They are also excellent at detecting fast movement and, in some cases, the polarization of light.

To understand how these eyes work, we must look at the individual ommatidia. Because the eye is a collection of these units, light enters each ommatidium through its own separate lens. This is different from a simple eye that uses one entrance point. As an object moves, the light intensity changes. This causes the individual light receptors behind each lens to turn on and off. This rapid turning on and off creates a flicker-effect. Scientists call the rate of this effect the flicker frequency. This mechanism allows for extremely fast reactions to movement. For instance, a honey bee can respond in 0.01 seconds. In comparison, a human takes about 0.05 seconds to react.

Biologists classify compound eyes into two main types: apposition eyes and superposition eyes. Apposition eyes form multiple inverted, or upside-down, images. In a typical apposition eye, the lens focuses light from one direction onto the rhabdom. The rhabdom is the light-sensitive part of the unit. Light from other directions is absorbed by the dark walls of the ommatidium. The mantis shrimp has a very advanced version of this eye. There is also a group called the schizochroal compound eye. This is a form of apposition eye found in Strepsiptera. In this type, each lens forms an image, and the brain combines them.

Superposition eyes work differently by forming a single erect, or right-side-up, image. There are three specific subtypes of superposition eyes. The refracting superposition eye has a gap between the lens and the rhabdom. It also lacks side walls. Each lens takes in light at an angle and reflects it to the other side. This type is mostly used by nocturnal insects that need to see in the dark. The parabolic superposition eye is found in arthropods like mayflies. In these eyes, parabolic surfaces inside each facet focus light from a reflector to a sensor array. Finally, reflecting superposition eyes are found only in long-bodied decapod crustaceans. This group includes shrimp, prawns, crayfish, and lobsters. These eyes use corner mirrors instead of lenses to manage light.

Some insects have developed highly specialized zones within their eyes to improve vision. Good fliers, such as flies or honey bees, often have a fovea area. This is a specialized zone of ommatidia that provides acute vision. In this fovea, the eye is flattened and the facets are larger. The flattening allows more ommatidia to receive light from a single spot. This increases the resolution of the image.

Another unique example is the shrimp Dioptromysis paucispinosa. It has a refracting superposition eye, but it also features one massive facet in the rear. This facet is three times the diameter of the other facets. Behind it, an enlarged crystalline cone projects an upright image onto a specialized retina. This creates a mixture of a simple eye and a compound eye.

There are also unusual variations of eye structures in the animal kingdom. Some insects possess a single lens compound eye. This is considered a transitional type between a multi-lens superposition eye and a simple eye. Another version is the pseudofaceted eye found in Scutigera. This eye consists of a cluster of many ocelli, which are simple eyes, organized to look like a compound eye. Interestingly, asymmetries in compound eyes can relate to behavior. For example, the ant Temnothorax albipennis shows a bias toward making left turns. This behavior is correlated with slight asymmetries in their compound eyes, specifically a difference in the ommatidia count.

The concept of the compound eye has even appeared in human history and culture. Between 2000 and 2500 years ago, people in Western and Eastern Asia made glass beads called "dragonfly eyes." These knobbly, multi-colored beads are a famous cultural reference. In modern times, the compound eye continues to inspire art and literature. You can find references to these eyes in the 2011 novel "The Man with the Compound Eyes" by Wu Ming-yi. There is also the 2017 movie "Dragonfly Eyes" by Xu Bing. These works often explore the unique way these creatures perceive the world around them.