Life cycle Darkling beetles follow a life history known as complete metamorphosis. Like butterflies and moths, they go through four distinct stages during their life cycle. A female beetle lays eggs, as many as 500 in her brief lifetime of a month or two. The eggs are about the size of the period at the end of this sentence. After a couple of weeks the equally tiny larvae emerge from the eggs. The larvae are known as mealworms, but of course they are not true worms. The larvae are golden yellow and have 12 body segments. They are the counterpart of the familiar caterpillar in the butterfly story. Mealworms pull themselves around on six stubby legs that are all crowded at the front.
The larvae seem to have two purposes in life: eat and grow. Beetles are arthropods, and like all members of their phylum they wear their skeleton on the outside like a suit of armor. This is very practical when they are being attacked, but very inconvenient when they are trying to grow. The arthropods have solved this problem by shedding (molting) their shell periodically. Immediately following the molt the soft, white larvae expand before the new larger shell hardens. This process may repeat half a dozen or more times over a 3-month period, after which time the larvae are about 2 cm (3/4″) long. The final larval molt reveals the next stage, the pupa.
The pupae don’t eat and they don’t move except for a twitch or two when disturbed. Inside, however, the mealworm is turning into a beetle, much the same as a caterpillar turns into a butterfly while sequestered inside the chrysalis. In 2 or 3 weeks the pupa splits open and out walks a beetle, white at first, but soon turning to brown and finally black after a day. The beetles mate and lay eggs, and the cycle repeats.
Information supplied by: http://www.mealworms.co.za/mealworms/