The word "migration" is considered, for the purposes of the present article, to refer to movements of animals in a direction and for a distance over which they have control, and which result in a temporary or permanent change of habitat. In insects such movements may cover a thousand miles or more, but in other cases the distances are shorter and the lower limit of a true migration is difficult to determine. The numbers of individuals in a single movement may range from a few hundred up to thousands of millions.