The compound eyes of butterflies are made up of repeating units that have their own light-detecting cells.

Butterfly Vision

Excerpted from: Butterfly Anatomy

One well-known phenomenon is the relationship between butterflies and nectar-producing flowers. Their huge compound eyes can detect ultraviolet light. Many flowers have evolved to display ultraviolet patterning that helps lead the butterflies directly to nectar.

  The Butterflies' Eyes

Butterflies have a pair of spherical compound eyes, each comprising of up to 17000 ommatidia, individual light receptors with their own microscopic lenses. These work in unison to produce a mosaic view of the scene around them.

Each ommatidium consists of a cornea and cone, which together function as a lens. Emerging from the back of each cone is a rod down which light travels to reach a cluster of 2-6 sensory cells, each of which is sensitive to a particular part of the visual spectrum.

What Are Compound Eyes

Compound eyes are made up of repeating units, the ommatidia, each of which functions as a separate visual receptor.

Each ommatidium consists of:

  A lens (the front surface of which makes up a single facet)
  A transparent crystalline cone
  Light-sensitive visual cells arranged in a radial pattern like the sections of an orange
  Pigment cells which separate the ommatidium from its neighbors.

The pigment cells ensure that only light entering the ommatidium parallel to its long axis reaches the visual cells and triggers nerve impulses. Each ommatidium is pointed at just a single area in space and contributes information about only one small area in the field of view.

The composite of all their responses is a mosaic image — a pattern of light and dark dots. The finer the pattern of dots, the better the quality of the image.

  What Does A Butterfly See?

The compound eyes of a butterfly.

Butterflies can see everything from about one centimeter to 650 feet will be rendered in sharp focus, as their ommatidia are of very short focal length.

The butterfly's brain can instantly detect whether the image formed by each ommatidium is dark or light. If a predator approaches or if the butterfly moves its head a tiny fraction, the amount of light hitting each receptor changes instantly because of its very narrow angle of view.

This sensitivity to changes in its surroundings means that a butterfly is extremely efficient at detecting movement and at gauging the distance of an approaching predator, enabling it to take immediate evasive action.

The sensitivity to changes in their visual field, combined with a high flicker-vision frequency of about 150 images per second, may also help butterflies to piece together the thousands of elements of the mosaic image produced by the compound eye.

It is not known whether butterflies are able to merge these mosaic elements into a single image. If are able to do so, it would render them capable of distinguishing patterns at close distances.

Vertebrates need to move their eyes and heads to scan their surroundings, but the compound eyes of butterflies provide them with almost 360 degree vision. They can see everything at the same time, so they can accurately probe into flowers in front of them, and at the same time devote equal concentration to detecting threats from behind.

Butterflies can see polarized light, enabling them to determine the position of the sun, even when it is partly hidden by cloud. This lets them relate their position to the sun and use it as a compass when moving around their habitats.

  Color Perception

The electromagnetic light spectrum.

Humans and birds perceive colors in a different way to butterflies, as the latter are ultra-sensitive to Ultra-Violet light as well as visible radiation. Flowers have ultra-violet patterns that are invisible to humans but which can be recognized by butterflies.

These UV patterns guide butterflies to the source of nectar in much the same way that runway lights guide an aircraft in to land.

Butterflies can communicate with each-other visually, but they use a private channel of ultraviolet patterns which are overlaid on the visible patterns, and cannot be seen by vertebrates. They enable butterflies to recognize conspecifics during the initial approach phase of mate location.

As well as being sensitive to UV patterns, butterflies are also alert to the iridescent colors produced when sunlight refracts from the wings of other butterflies. Many species have also evolved selective color response - that is they are tuned to react to colors that are dominant in the wing patterns of their own species.

  Shape Perception

Male butterflies will intercept and chase any insect of approximately the same size and color as the female of their own species during the approach phase of mate-location. Females of some species however seem capable of recognizing plants purely on the basis of leaf-shape and color. This ability varies from one species to another.

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