Only 3 extant groups of animals fly, birds, bats and insects. Insects were the first group of animals to fly, a key to their success as a group. Not only were insects the first group of animals to fly, they were the only fliers for well over 100 million years.
Walking takes place in two dimensional space Flying takes place in three dimensions and requires a well honed skills and behaviors to avoid collisions. Flying insects collect visual information and process that information with a brain that only has a fraction of the number of brain cells that humans use. The insect can avoid collisions using much less processing power and process the information faster than the human brain. Scientists and investors are developing flying machines that can self-pilot and avoid collisions. The insect brain is a simpler model to study. Insects have had over 400 million years of flight experience, so it makes sense to pick their brains.
There is a growing consensus that insect brains do not process 3 dimensional objects the way the human brain does. Producing 3 dimensional objects requires many more brain cells than an insect has. Instead, insects use the two dimensional patterns moving across their visual system or “optic flow”.
Humans commonly use optic flow information to judge distances and in common tasks such as driving at night. Objects that are far away (such as a mountain) expand slowly across the visual field as we approach. People who drive west across the plains of Colorado have experienced this. The vision of the mountains in the distance does not change much over dozens of miles. Objects that are very close expand rapidly across the visual field. Hold a piece of paper at arms length and bring it up to your eyes. The paper will expand across the visual field as it moves closer to the eyes. This system is useful for avoiding collisions. As a basketball travels toward your head, it expands rapidly across your visual field just before it hits you. The natural response to a rapidly expanding object is to duck to avoid the collision.
Insects use their optic flow system to avoid collisions. When objects are expanding across the visual field, insects turn to avoid them. If the optic flow from an object to the right of an insect is too rapid, the insect will move to the left until the optic flow drops to a more comfortable range. If the insect is flying through a narrow passage, it will slow the flight speed. (We use the same speed reduction when driving a car into a tight parking space.) Insects use optic flow relative to the ground to gage their forward flight speed. Insect adjust their flight until the optic flow below them is moving from front to back at a comfortable rate.
Optic flow information can also be used to adjust the height of the flight. If optic flow below the insect increases too rapidly, it is warning that a collision with the ground is imminent. Optic flow information could be used to set the height above the ground that an insect flies. A recent report in Current Biology by Andrew Straw and colleagues at CalTech tests that hypothesis and finds that fruit flies are not using optic flow to set the height of their flight. Instead the fruit flies prefer to set the height of their flight relative to horizontal lines.
This finding fits very well in the context of fruit fly behavior. Fruit flies mate and lay eggs on rotting fruit. Fruit flies must fly at the level of the fruit, not far above it. Optic flow information is a combination of forward flight speed and distance above the ground. Depending on the rate of forward movement, dependence on optic flow of the ground alone might cause the fruit fly to locate above or below the optimal height range. Flying relative to horizontal lines would locate the fruit fly at a height relative to the canopy that is more likely to lead it to rotting fruit.
Hawk moth hovering while collecting nectar. During hovering optic flow in all directions is zero.