In each definite habitat the environmental conditions may change to a degree exceeding the degree of ecological plasticity of the given species; in such cases the population either must become extinct or its representatives must migrate to some new habitats. The lower its ecological plasticity, the greater is the significance of dispersal for the species.Dispersal may be active or passive. In active dispersal the possibility and probability for representatives of population to encounter suitable conditions depend on the perfection of their locomotory organs, organs of orientation in space and on capacity for energy production. Active dispersal requires high differentiation of organs and is consequently performed by the definitive stage of development. In passive dispersal (anemochory, phoresy etc.) the possibility to meet with favourable conditions is ruled out by the laws of random distribution. Therefore, for passively transported species, highest numbers of disseminating individuals and their smallest mass are advantageous. Consequently passive dispersal is performed, 1s a rule, by juvenile stages, whereas for the adults of the passively dispersing species, high sexual production is characteristic, connected with general degeneration of the organs of active life.For Insecta‐Pterygota active dispersal by flight is typical. The appearance of adult Pterygota is that of the dispersing organism. The main functions of the adult stage in Pterygota are dispersal and reproduction, whereas those of the larva are feeding and growth.Only the function of active dispersal determines the progressive evolution of the imago in insects. Functions of dispersal and propagation approach each other in the time but the period of dispersal precedes that of reproduction. The wide separation of these functions in time leads to the regressive changes in adult females.Passing of the function of dispersal from the adult stage to passively disseminating larvae leads to regressive changes in the course of ontogenesis and to features of degeneration in adult females. In rather rare instances of passive dispersal of imagines additional means of propagation are developed (parthenogenesis, paedogenesis, polyembryony).Even in those groups of Pterygota where the females are highly degenerate the males do not exhibit any regression in locomotory and sense organs, still being able to fly.This is connected with the role of sexual reproduction for the persistence of characters, of genes. In the same manner as dispersal is a necessary condition for getting individuals of the species into appropriate environment, amphimixis is the premise for every gene, for each character, to meet with a favourable genetic environment. Sexual differences of adult insects reflect the phenotypic differences of gametes. Males ensure the encounter of sexes. Movements of pairing adults are of the same significance for the further persistence of genes, of characters, as the active dispersal flight of insect females is for the survival of individuals, for persistence of the species. Independent movements of spermatozoa in mating animals (especially in those with internal insemination) are of the same importance for genes dispersal as the crawling of first instar larvae (corrective movements) in actively (by flight) dispersing alate insects in the dispersal of a specie