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Why do crabs have a pair of handle eyes?
Most kinds of crabs walk sideways. After the eye stalk is erected, the two eyeballs can extend to the left and right front of its body, thus avoiding the obstruction of its head and chest to the line of sight. By giving full play to the broad advantages of compound eye imaging, it not only takes into account the field of view in front, but also makes it move to the left and right direction or even to the rear side without any blind spots in the line of sight.

In the south of the Yangtze River, hairy crabs are a delicious food that people have loved since ancient times. The official name of hairy crab is Eriocheir sinensis, because its two pincers have soft fluff. If you look closely at the living hairy crab, you will find that its eyes are sometimes upright and sometimes flat, just like a pair of folding binoculars. It turns out that hairy crabs have very special eyes. The eye of the handle, as the name implies, is that the eyes grow on the handle. Not only hairy crabs have handle eyes, but all kinds of crabs also have handle eyes, and the base of the handle has movable joints, so that the handle can be erected and fallen.

then why do crabs have a pair of handle eyes?

First of all, it has something to do with the way crabs walk. Except for a few crabs (such as monk crabs) who can walk straight, most kinds of crabs walk sideways. At this time, if the enemy appears in the left and right direction, I am afraid it will already become someone else's food when it turns around. But with the handle, the eyes are different. After the handle is erected, the two eyeballs can extend to the left and right front of its body, thus avoiding the obstruction of its head and chest to the line of sight. By giving full play to the broad advantages of compound eye imaging, it not only takes into account the field of view in front, but also makes it move to the left and right direction or even to the rear side without any blind spots in the line of sight. Therefore, the crab's eyes are really wonderful. When you pretend to attack its front, side or even back, its eyes will stand up straight, reaching out to the sides of its body, locking the target tightly and waiting for a fatal blow to the intruder.

Secondly, it has something to do with their latent life. For example, the sand crab perched on the seashore beach has two long eyestalks, which stand up like two matchsticks. The sand crab often buries its whole body in the sand, and only exposes its two handle eyes, like a periscope, vigilantly observing the movements around it.

Like insects, crab's eyes are all compound-eye structures. Each eye is composed of many small eyes, which are respectively imaged on the retina, and finally transmitted to the brain through the optic nerve and finally spliced to form a complete image. Compound eye is a convex spherical body, and its field of vision is wider than that of human monocular eye. Coupled with the help of the eyestalk, it can be said that it is even more powerful. When the eye handle is erected, the crab seems to have installed two lookouts, so it can see six ways; When you fall down, you can even hide the handle in the eye socket. If it is damaged by carelessness or accident, it can grow new eyes. If its eye stalk is cut off, a very useful antenna will grow in the eye socket to make up for the lack of eyes.