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Greenfly eye

Kevin Mackenzie, University of Aberdeen

Download this image from Wellcome Collection.

Scanning electron micrograph of a greenfly eye. Many species of aphids, also commonly known as greenfly, are major agricultural pests that feed on plant sap. Aphids have a pair of curved compound eyes that bulge out of the head and have a wide angle of view. Each eye is made up of thousands of repeating units known as ‘ommatidia’, each with a tiny lens on the front surface. Each lens faces a slightly different direction, and together they produce a mosaic image. This allows the fly to see very quick movements but not fine details or objects that are far away. The small circular structure (ocular tubercle; top left) may help insects see polarised light. The width of the image is 280 micrometres (0.28 mm).

How do insect and human eyes compare?

Insects have compound eyes with a curved, hemispherical shape; they bulge out from the head, giving a wide angle of view with wraparound vision. Compound eyes are fixed and cannot move but as they are made of thousands of lenses each, insects can look in many different directions at once. Humans, by contrast, have eyes that swivel but can only look in one direction at any given time. Humans have eyes with one lens, but that lens can be focused to see detailed images both close up and in the distance. Compound eyes have thousands of lenses but as these can’t change focus, insects have a short range of vision and can only see objects close by. This does, however, allow them to detect fast movement.

The more lenses present in a compound eye, the better the image resolution. Dragonflies have one of the largest compound eyes with 30,000 lenses per eye. Some insects are able to distinguish colour and – unlike humans – some can also detect ultraviolet light and polarised light, which they use for navigation. Researchers have made a camera with 180 lenses that mimics the behaviour of a compound eye.