Compound eye is an organ composed of an indefinite number of small eyes. It mainly appears in arthropods such as insects and crustaceans, and organs with the same structure also appear in bivalves. The compound eye is generally hexagonal, and the number, size and shape of the compound eye vary greatly among various insects. The compound eye of male scale insects is only composed of round eyes.

Compared with monocular eye, compound eye is the main visual organ of insects, and usually occupies a prominent position in the head of insects. The compound eyes of most insects are round, oval or kidney-shaped. It consists of many small eyes.

Each eye has cornea, lens vertebra, pigment cells, retinal cells, rods and other structures, and is an independent photosensitive unit. Axons extend backward from retinal cells and cross the basement membrane to synthesize optic nerve. Some arthropods have pigment cells in their compound eyes. When the light is strong, the pigment cells extend. Only the direct light can reach the rod and be felt by the optic nerve. The oblique light is absorbed by the pigment cells and cannot be felt by the optic nerve. In this way, each small eye can only form one image point, and the image points formed by many small eyes are combined into an image. When the light is weak, the pigment cells contract, so that the light that enters through each small eye can enter other small eyes through refraction except the direct light, so that the rod in each nearby small eye can feel the light refracted by several adjacent small eyes. In this way, when the light is weak, the object can also be imaged.