It is believed that insects originated in the seas. The first group of arthropoda were believed to have come from the sea, and were originally descended from mollusks. The link people use to support this is the barnacle , which has arthropod and mollusk tendencies. Some of our modern insects have much more interesting extinct relatives. For instance, at one point there was a huge species of dragonfly that doesen't exist any longer.
Insect distribution is affected by climate changes of the past.
It is believed, that when ice ages began, some insects fled to the equator, while others, better adapted stayed behind in the cold. Then the earth warmed up again, became a mild place, and the cold weather insects retreated to the last truly cold places like mountain tops. A perfect example of this is that a rare cold weather butterfly occurs in only two places in the world, Pikes Peak Colorado, and Mount Washington, New Hampshire. Insects are the most populous group of animals on the Earth. Everyone is affected by their presence.