Most Animals With Eyes Have More Than Two. The Extra Ones Cannot See

In addition to the large compound eyes on the sides of the head, most flying insects carry two or three small simple eyes arranged in a triangle on the top of the skull. Each has a single lens over a patch of light sensitive cells. The lens is deliberately unfocused, so light from a wide area falls onto every cell rather than forming a picture, and the output is a single measurement of how much light is arriving from a broad region of sky. That is all the information they provide.

The design trades detail for speed and sensitivity:

• •Collecting light from a wide area makes them very sensitive
• •Fewer cells means the signal is processed almost instantly
• •They connect to flight muscles through very short nerve pathways
• •So the response latency is a fraction of the compound eye's
• •They point upward, where the sky is reliably brighter than the ground
• •Tilting the insect changes the reading immediately

The accepted explanation is stabilisation of flight, and the evidence for it is direct. Covering these eyes in a flying insect produces immediate and severe deterioration in its ability to hold level flight, particularly in poor light, while leaving its ability to navigate and to identify objects intact. Because the sky is brighter than the ground, the difference in brightness across the set of small eyes indicates which way is up, and a change in that difference indicates a roll or a pitch that needs correcting. The pathway to the wing muscles is short enough to make the correction faster than vision could.

Distinguishing the various kinds of eye an arthropod may carry avoids a common muddle. The large compound eye is built from hundreds or thousands of separate units each with its own lens, producing a coarse mosaic image with excellent motion detection. The small eyes described here have one lens each and produce no image. Larval insects and some adults carry a third type on the sides of the head, with a single lens over a retina, which does form a crude image and is the larva's only eye until metamorphosis. All three can be present at different stages of one life cycle.

The distribution across species is itself informative, since the organs are present where they would be useful and absent where they would not. Fast flying insects including dragonflies, bees, wasps and locusts have them prominently, and dragonflies have unusually large ones. Insects that do not fly, or that fly slowly, frequently lack them entirely, and many that live in darkness have lost them. Spiders have a different arrangement of several simple eyes that do form images, which is a separate solution to a different problem and should not be confused with these.

Small simple eyes on top of an insect's head carry a deliberately unfocused lens, so they measure overall brightness across a wide patch of sky rather than forming an image. That makes them extremely fast and sensitive, and the brightness difference between them indicates which way is up. Covering them ruins level flight while leaving navigation intact.