Why can't you always catch flies?

Today, we are going to talk about a little creature that is very annoying, that is, flies. They are the most unwelcome uninvited guests on the summer table. They cannot be driven away or killed. Wherever they settle, they make the plates of food disliked for hygiene reasons.

In the competitive sport of chasing flies with a fly swatter, humans are more likely to fail than to succeed. Why can flies beat humans who claim to be the most intelligent creatures? The reason is that flies have a pair of compound eyes with six directions and a pair of balance bars that can be called "natural navigators."

1. Wide field of view of compound eyes

Our eyes are located in the front of the head, and each eye has only one pair of retinal imaging systems, which is called a simple eye. However, many insects, including flies, have a pair of huge compound eyes that occupy a large area of ​​the head.

Each compound eye is made up of several ommatidia. The number, size and shape of ommatidia vary greatly among insect species. The compound eyes of houseflies are made up of 4,000 ommatidia, while the compound eyes of butterflies and moths have 28,000 ommatidia. The ommatidia of insects are generally hexagonal, while the compound eyes of male scale insects have countless round ommatidia.

Each ommatidium is an independent imaging system, consisting of a cornea, a lens, and photoreceptor cells. The cornea is a hexagonal convex lens on the surface of the ommatidium that can gather light. Below the cornea is a conical lens, which together with the cornea forms a light collector. Below the light collector is connected to the optic nerve.

Compared with human eyes, the imaging system of each ommatidium in a compound eye is relatively simple. After each ommatidium senses the stimulation of light, the image formed by the light spots can only distinguish between light and dark and color. The insect brain is not powerful enough to process the images from the ommatidium in the compound eye. The compound eye vision formed by the superposition of ommatidium is more like a mosaic collage picture with low pixels and poor picture resolution.

With such obvious disadvantages, why do insects insist on evolving compound eyes? This is related to insect flight.

After weak insects mastered the skill of flying, their foraging space was greatly expanded, and the space for escaping from natural enemies was also greatly increased. Some animals on the ground like to eat insects, and many birds in the sky even list insects as the main course in their diet. When birds prey on insects, they are accustomed to using the gravity of the earth to accelerate their dive. When a bird dives down, it blocks out the sun for insects, which directly causes the light and shadow above the insect's body to change.

Although compound eyes have poor imaging capabilities, they are extremely sensitive to changes in light and shade. Each eye can respond to changes in light and shadow at a very high frequency in a very short period of time. A normal person's eyes can distinguish 24 pictures per second (which is also the lowest speed of cartoons), while insect compound eyes can distinguish about 240 pictures per second.

Therefore, compound eyes can perceive changes in light and shadow at high frequencies and effectively calculate the relative position and distance between themselves and the observed object. Therefore, insects react very sensitively to moving objects approaching them. For example, when an object suddenly appears, the reaction time of bees is only 0.01 seconds, and predatory insects are even more sensitive to moving objects.

In addition, the field of vision of the compound eyes is relatively wide. The compound eyes of flies can see objects within a range of almost 360 degrees. No matter which direction you approach with a fly swatter, the fly can detect you sneaking up on you with its compound eyes!

2. Fast take-off speed The body of an insect is divided into three parts: head, thorax and abdomen. The thorax has three pairs of legs and two pairs of wings. However, insects of the order Diptera have degenerated a pair of hind wings into a dumbbell-shaped balance bar, leaving only a pair of wings for flying.

When a fly is flying, its balance bar vibrates at a frequency of 300 times per second. If the body tilts or deviates from the course, or the vibration plane changes, the receptors in the stem of the balance bar will sense it and transmit it to the brain nerve center, so that the body can be quickly balanced and the course corrected.

The balance bar is a good navigation tool for flies when flying. When the fly is stationary, it will also vibrate the balance bar regularly. This regular vibration is like a warm-up exercise, allowing the fly to take off at any time. Just like a car waiting for the green light at a traffic light intersection, the engine is not turned off, and as soon as the green light comes on, it can start driving quickly.

Why can't you hit a fly? Because flies are always preparing to take off. Common flies belong to the petal fly family, which takes off faster than other flies, taking off in an average of 7.34 milliseconds. It takes 300 milliseconds for a human to blink, so you can imagine how fast a fly takes off.

If the balance bar is damaged, the fly's reaction, take-off speed and flight stability will be greatly reduced.

Based on the principle of the fly's balance bar, scientists have developed new navigation instruments such as vibrating gyroscopes, which are used in airplanes, rockets and other spacecraft. This can not only prevent dangerous spiral flight and ensure flight stability, but also realize automatic driving.