In summer an aphid can be born pregnant. The females of that season reproduce without males and give birth to live young, and the embryo inside a mother can already have an embryo of its own started inside it, so that for a while a grandmother is carrying her granddaughter. Biologists call this telescoping generations, and it is part of why aphids arrive in such numbers. Only when the days shorten does a single generation of males and females appear, to lay eggs that can last the winter. We usually measure a generation as a length of time. In an aphid in June it is closer to a depth.