Terrestrial mammals and birds that live in the polar regions are usually white in color

Terrestrial mammals and birds that live in the polar regions are usually white in color, while insects in the same places are dark in color. Why is fitness so different?

The former have a coloration similar to the main color of the environment. It allows them to disguise themselves and be invisible while remaining in open areas. Since they are warm-blooded animals, they do not need to attract the maximum amount of sunlight to heat their bodies due to their dark coloration. But insects do not have warm fur and plumage, and the corresponding circulatory system. Therefore, evolutionarily, they have adapted to survive in such an environment by maintaining their body temperature due to their dark coloration.