There is a growing consensus that most 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”. Insects must navigate by 2-Demensional patterns, but the information is updated more frequently, as much as 200 time per second versus 12 per second in humans.
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. 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 its 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. Insects adjust their flight until the optic flow below them is moving from front to back at a comfortable rate. When flying close to the ground, insects reduce their flight speed which helps avoid collisions.