In all animals that are produced by the sexual method the beginning stage in the life cycle is a mature egg, either fertilized or unfertilized according to the species. Animals which develop asexually, on the other hand, begin their cycle with the first recognizable evidence of budding or fission.
As a rule budding or fission are sooner or later interrupted by the formation of sex cells, hence the life cycle of such animals may be considered to extend from the mature egg to that stage in the life history of the species when mature eggs are again produced. Such a life cycle consists really of two or more simple life cycles represented by individuals differing from one another in both structure and method of reproduction. As examples of some of the principal types of life cycles we may select certain insects and ccelenterates.
A very simple life cycle is that of the wingless insects of the order APTERA. The young, when they hatch from the egg, are similar in form, structure, and habits to the fully grown individual and undergo no perceptible changes, except increase in size, until they become sexually mature adults. In certain other groups of insects, such as the grass hoppers, the newly hatched young resemble the adult in many ways, differing principally in the absence of wings. The young Rocky Mountain locust (Melanoplus spretus), for example, changes its exoskeleton (molts) five times before the adult condition is attained. After each molt there are slight changes in color, structure, and size, the most notable difference being the gradual acquirement of wings. In still other orders of insects a larva hatches from the egg ; this larva, on reaching its full growth, changes in shape and structure, becoming a quiescent pupa, from which after a rather definite interval an adult emerges.
A combination of two simple life cycles to form one complex cycle occurs in certain hydroids. The eggs of these species produce free-swimming embryos which become fixed to some object and de velop into polyps. These polyps form other polyps like themselves by budding, but finally give rise to buds which become jelly-fishes or medusae. In stead of remaining attached to the parent colony the medusas, as a rule, separate from it and swim about in the water ; they later give rise to eggs which, after being fertilized, develop as before into polyps. There are thus in this species two life cycles com bined, that extending from the egg to the time when the colony forms medusa-buds, and that beginning with the medusa-bud and ending with the mature egg. Such an alternation of an asexual and a sexual generation is known as metagenesis.
There is another sort of alternation which normally occurs in many species, and that is the alternation of individuals developing from parthenogenetic eggs with those from fertilized eggs. In the aphids, or plant lice, for example, the race in the northern part of the United States passes the winter in the shape of fertilized eggs. All of the individuals which hatch from these eggs in the spring are females called stem-mothers. The stem-mothers produce broods of females from parthenogenetic eggs, and these in turn give rise to other broods of females in the same manner. Thus throughout the summer, generation after generation of parthenogenetic females appear ; but as autumn approaches females develop whose eggs must be fertilized, and males are also produced. The eggs of these females are fertilized by spermatozoa from the males, and the zygotes thus formed survive the winter, producing stem-mothers the following spring.