Mealworms come from the beetle Tenbrio molitor, which is a species of darkling beetle. There is a common lifecycle, or series of four stages, through which mealworms go before entering adulthood. These are: 1) egg, 2) larva, 3) pupa, 4) adult.
Tenebrio molitor is a prolific breeder, and can lay up to 500 eggs at a time. This number is necessary due to external pressures from predators; mealworms are commons target for numerous species, including reptiles. Depending on the environment in which the larvae lives, it can take anywhere between three and thirty days for the mealworm to emerge as a beetle.