The sexes are distinct in insects, and there is often a marked sex dimorphism; in numerous species the males are winged while the females are wingless, and in a few cases this condition is reversed. Where there is a difference in size between male and female, the females are usually the larger. Fertilization of the egg takes place in the body of the female and, strangely, this fertilization is effected after the eggshell has been formed. In all insect eggs there is a minute opening in one pole of the eggshell called the micropyle through which the sperm-cells enter. In a few cases the young are born alive, but such a viviparous condition is exceptional. In a few species, too, young are produced parthenogenetically, that is, are produced from unfertilized eggs. And in the case of a few insect species male individuals are not known. [Illustration: FIG. 52.--The young (at left) and adult (at right) of the bed-bug, _Acanthia lectularia_, a wingless insect with incomplete metamorphosis. (After Riley.)] =Development and life-history.=--The young insect when just hatched from the egg either resembles, except for the absence of wings, its parent in general appearance as in the case of the locust, or it may, as in the butterfly, emerge in a form very unlike the parent. In the first case the young has simply to grow, that is, to increase in size, to develop wings, and to make some other not very obvious developmental changes in order to become fully grown. But in the case of the butterfly, and similarly in the case of all other insects as the flies, beetles, bees _et al._, whose young hatch in a larval condition differing markedly from the adult, some radical and striking developmental changes occur before maturity is reached. Such insects are said to undergo complete metamorphosis in their development, while those insects like the locusts, the sucking-bugs, white ants, and others, the just hatched young of which resemble their parents, are said to have an incomplete metamorphosis (fig. 52). [Illustration: FIG. 53.--The larva of the violet tip butterfly, _Polygonia interragationis_, making its last molt, i.e. pupating. (Photograph from life.)]