Subclass Pterygota is further divided on basis of metamorphosis. Insects that have undergone incomplete metamorphosis are the Exopterygota. Insects that undergo complete metamorphosis are the Endopterygota. Scientists have described and named about 800,000 insects and believe there may be from 1 million to 10 million kinds still undiscovered.
Insects live almost everywhere on earth - from steamy tropical jungles to cold polar regions.
They live high on snow-capped mountains, and in deserts below sea level. They can be found in caves deep in the earth, or flying many thousands of feet high in the sky. Most insects are small - less than a fourth of an inch long. They have an amazing variety of shapes and special structures. Insects have an outer bilateral exoskeleton to which the muscles are attached and which provides protection for internal organs.