While most insects have eyes, they are often only fully developed in adults. Insects use their eyes just as we do, but their structure and mode of working are highly different.
Insects use their eyes, just as other animals do, to find their way about, recognize members of their species, and identify enemies and food sources. Insects have two distinct types of eyes, often found together in the same individual. The more straightforward eye is called an ocellus and has a single lens. In the second kind, called the compound or faceted eye, there are many lenses, sometimes thousands.
If you want to know more about insect vision, please read on.
An ocellus consists of a convex lens with a transparent layer underneath it. Below this are light-sensitive cells grouped in little bundles called retinulae.
Simple eyes are developed in two distinct situations. These can be found on the sides of the head of some larvae of more highly evolved insects. The caterpillars of butterflies and moths usually have groups of six ocelli, while sawfly larvae have just one on each side of the head. The eyes of larvae can detect movement and light but not much more.
In most adult insects, there is another group of three ocelli on top of the head. These stimulatory organs help increase the speed of the insects’ reaction to light.
Most adult insects have compound eyes. Many butterflies have good color vision, but it is unknown whether all insects have the same. Some species, like the elephant hawkmoth, have pigmented eyes to give them the appearance of having a pupil.
This is thought to serve as a defense mechanism to help frighten away predators. Large eyes, such as the horseflies, are efficient at locating movement.
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Nocturnal vs Diurnal Sight
Some insects have an interesting adaptation affecting the dark pigment layer surrounding the cones and retinulae. In some nocturnal insects, this pigment is missing, and light leaks from one ommatidium to the next.
The presence of the pigment, which isolates each ommatidium, gives better image formation. However, much of the available light is absorbed by the pigment and wasted.
Where there is no pigment, the insect sees images less clearly but has better night vision since light is not absorbed. Nocturnal moths have adapted to see clearly and let in light.
In the dark, their ommatidia allow the passage of light, but daylight causes the pigment to move up between the ommatidia to form sheaths that block the light, typical in diurnal insects.
Do Insects Have Good Sight?
Insects that have large eyes with multiple ommatidia give the best images. The largest and most highly developed insect eyes are found in dragonflies, needed because they hunt at high speeds. Each dragonfly eye can contain as many as 28,000 ommatidia.
Compound eyes are at their best when detecting movement. Horseflies have large eyes and use them to locate animals on whose blood they feed. Hoverflies are, as their name suggests, able to hover in one place in moving air.
Unlike hummingbirds that truly hover, hoverflies will lock themselves onto nearby objects that shift with every movement of the fly, and the fly corrects its position when blown out of position.
Insects that live mainly underground often have poorly developed compound eyes. Many worker ants have less than a dozen ommatidia in each eye, and their eyes hardly function.
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Do Insects Have Colour Vision?
Some insects are known to have color vision. Bees, for example, have been shown to recognize color. In an experiment, transparent dishes were placed on colored squares, one on each. One dish was filled with a sugary water solution, and all the others with water.
It should be no surprise that the bees went for the sugar water. However, once they were allowed to feed for a while, the saucers were moved around. The bees returned to the same colored square and continued to feed, even though it had been swapped to water.
Butterflies are also known to have good color vision. Their eyes are sensitive to blue, and they have been seen to fly down to investigate blue artificial flowers. Peacock butterflies, bred in captivity, were trained to feed from test tubes filled with a honey and water solution.
If the tubes have a flower-shaped entrance and are blue or purple, the butterflies find them more often and quicker. Butterflies also recognize their mates by their coloration.
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Most experiments have found that most insects do not see as much of the red end of the spectrum as we do. Many don’t see red but make up for this in other ways. Many can see ultraviolet, the radiation beyond the violet colors in sunlight. UV light is invisible to humans.
Many flowers look yellow to us, including evening primrose, silverweed, and ragwort. If shown in UV light, however, they have a pattern that directs the insect to the nectar at the center of the flower. The pattern is often a black bullseye pattern with a blue surround. This directing pattern is solely for insects to be able to pollinate the flowers.
Many butterflies, especially the yellow species of the family Pieridae, have ultraviolet patterns on their wings. In sunlight, butterflies can see them just as well as we see the colors that are within our spectrum.
True Or False Eyes
A caterpillar often looks like it has two big staring eyes on its head, with the elephant hawkmoth caterpillar a great example. These are false eyes, however, and are special body markings designed to frighten away predators. The eyes a caterpillar has are several ocelli or simple eyes which can detect movement and light intensity but not much more.
Insects Can See Better Than We Thought
A study at the University of Sheffield’s Department of Biomedical Science has found that compound eyes can generate higher-resolution images than previously thought. This is due to how the photoreceptor cells deal with movement.
While we can move our eyes, insects can’t. However, the study found that they can compensate for this. The photoreceptor cells move in and out of focus as they take in the world around them. The movement is so fast that we can’t see it, so a special microscope was used.
Once they have sensed changes in the light, electrical signals are passed on to the brain. While not as good as ours, insect eyes are not as bad as we first thought.