A Little Bit More
Although scientists can’t know exactly how much or how well insects see with their compound eyes, they’re quite sure that many insects have good eyesight. Although the compound eye has some big advantages, it does not have the same detail resolution found in vertebrate eyes. In most cases, it is believed that insect eyes see only form and movement but insects such as the dragonfly are so adept at catching insects in flight that it is believed their eyesight is much better.
Compound eyes are generally made up of a few hundred to thousands of facets, each of which faces a slightly different direction from its neighbor. So, each facet records a slightly different portion of its surroundings (each records color and light intensity but not an entire image) and the brain puts them all together to form a picture. At the base of each ommatidium is an optic nerve (nerve that relays messages from the eye to the brain) that transfers these messages to the brain for interpretation. Although most insects can see fairly well within a few feet, they don’t see clearly much beyond that distance. The farther away an object is located, the fewer the number of facets that can see it and the image resolution therefore suffers.
Probably, the biggest advantage to compound eyes is the ability for insects to see the tiniest movement much more quickly than a human eye can. The slightest movement of an object is immediately apparent as an image (or part of an image) moves from one of its ommatidia to another. This allows insects like the dragonflies to zoom at great speed through dense areas without crashing into objects. Furthermore “pixel eyes” provides the predatory dragonfly with vision that can detect any movement in its prey and react accordingly.
Activities
Blow It Up
Objectives: Provide a clearer understanding of compound eyes
Materials: Newspaper, hand lens, Adobe Photoshop (or comparable photo program)
It’s always easier to learn a new concept if it can be incorporated into what one already knows – children AND adults learn better in this manner. With this in mind, the compound eye with its many tiny facets has been compared to pixels on a computer screen. Almost every child in school has seen a computer screen and has some concept, although maybe rather vague, of the pixels that are used to create images/letters on the screen. Here are a couple of quick activities that may help students better understand how a compound eye works – at least the basics!
A. The newspaper is a reasonable example of the concept we’re trying to convey. The photographs in a newspaper are merely an assortment of dots on a page that are meaningless until “assembled”.
Have your students take a hand lens or magnifying glass to look at the newspaper images closely. They should be able to see the thousands of dots that are assembled to create a photograph. Be sure to point out that it is only when the dots are “assembled” that the picture emerges. No individual dot is helpful but all together, (an insect’s brain puts each ommatidia’s image together) the dots make an image that can be interpreted.
B. Now have the students look at an image on the computer. If you have Adobe Photoshop this exercise is easily accomplished. Any other program that will allow you to magnify an image, however, should provide you with this same information.
Have the students increase the size of an image on their computer screen multiple times until they can see the actual individual pixels that are assembled to create that image. Students can imagine that each pixel represents an image from one of a dragonfly’s ommatidia. No one image is worth much but put together (again, an insect’s brain puts these together) as a unit, creates an image that can be interpreted.
Key Concepts
Structures and Functions
Questions
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