life cycle

The life cycle of the dragonfly. Eggs are usually laid under water and the larvae hatch as free-moving, water-dwelling nymphs. The carnivorous larva lives in the water for several weeks (or even years in some species) and undergoes a series of moults as it grows. It leaves the water when it is ready to undergo its final moult, during which it sheds its ‘skin’ to reveal the winged adult.

The life cycle of a butterfly (painted lady Vanessa cardui). Eggs are laid on plants (usually the underside of leaves) and the larvae (caterpillars) feed on the plant, growing in size until ready to pupate. When the caterpillar is full-sized it forms a chrysalis in which it undergoes transformation from caterpillar to adult butterfly. The life cycle of moths is very similar.

The life cycle of a frog. Frogs lay their eggs (spawn) in fresh, still water. The tadpoles hatch within about two weeks and then over the course of the next three months they grow larger and slowly develop legs and internal gills. By the time they lose their tails they resemble tiny adult frogs and can leave the water.

In biology, the sequence of developmental stages through which members of a given species pass. Most vertebrates have a simple life cycle consisting of fertilization of sex cells or gametes, a period of development as an embryo, a period of juvenile growth after hatching or birth, an adulthood including sexual reproduction, and finally death.

Invertebrate life cycles are generally more complex and may involve major reconstitution of the individual's appearance (metamorphosis) and completely different styles of life. Plants have a special type of life cycle with two distinct phases, known as alternation of generations. Many insects such as cicadas, dragonflies, and mayflies have a long larvae or pupae phase and a short adult phase. Dragonflies live an aquatic life as larvae and an aerial life during the adult phase. In many invertebrates and protozoa there is a sequence of stages in the life cycle, and in parasites different stages often occur in different host organisms.