The book Insect Metamorphosis explains what is happening to these insects as it explains how insects go through stages. Unlike people, insects don't grow gradually into adults but go through a series of different stages.
Some insects look completely different at each stage of development.
The beautiful multi-colored butterfly we see fluttering through the flowers was once a tiny brown caterpillar crawling along a leaf. This method of growth is called complete metamorphosis. The book explains the four stages of growth in complete metamorphosis by showing us how several different insects look and act at each stage. Other insects do not change the way they look from one stage to another. The young insects look much like adults, only smaller.