Mealworms, despite their common name, are not actually worms. They are the larval stage of the yellow mealworm beetle, Tenebrio molitor, a species belonging to the darkling beetle family (Tenebrionidae). These larvae are used as food for pets like reptiles, birds, and fish, and as fishing bait. Mealworms undergo complete metamorphosis, transforming through distinct life stages before reaching their adult form.
The Mealworm Larva
The mealworm larva is the initial visible stage after hatching from an egg. It has a segmented body, yellowish-brown, and is covered by a hard outer shell called an exoskeleton. This exoskeleton does not grow, so as the mealworm increases in size, it must shed its old skin in a process called molting. A mealworm can molt between 10 and 20 times during its larval stage, appearing soft and white immediately after molting before its new exoskeleton hardens and darkens.
During this phase, the mealworm primarily eats and grows to accumulate energy for its upcoming transformations. They are scavengers, consuming grains, decaying leaves, sticks, and even other dead insects. Mealworms inhabit dark, warm, and damp environments, found under logs, decaying leaves, or in food storage areas like grain bins.
The Pupa Stage
After the larval stage, the mealworm transitions into the pupa, a seemingly dormant but internally active phase. This transformation involves the larva shedding its skin one final time to reveal the pupa underneath. The mealworm pupa is creamy white or yellowish-white, stout, and often C-shaped or curved. While appearing inactive, it may wiggle if disturbed, but it does not eat during this period.
Within the pupal casing, the mealworm undergoes internal reorganization, breaking down larval tissues and rebuilding them into adult structures. Rudimentary legs and wing buds become visible, hinting at the future adult beetle. The pupa gradually darkens in color as it matures, signaling its progression towards the adult stage. The duration of the pupal stage can vary, lasting one to three weeks, extending to several weeks depending on temperature.
The Darkling Beetle
The culmination of the mealworm’s transformation is the emergence of the adult darkling beetle, Tenebrio molitor. Initially, the newly emerged beetle may appear light-colored, but its exoskeleton quickly hardens and darkens to dark brown or black. These beetles possess a hard shell, known as elytra, which covers their wings. While they have wings, many darkling beetles, including this species, are flightless or poor flyers.
Adult darkling beetles are nocturnal scavengers, feeding on organic materials such as decaying plant matter, grains, and sometimes fungi or dead insects. They live for several months, with a lifespan ranging from three to twelve months. During this adult stage, their role is reproduction.
Completing the Life Cycle
The adult darkling beetle plays a role in continuing the life cycle. After emerging from the pupa, female beetles mate, often within one to two weeks. Males attract females, and mating involves males mounting females to transfer sperm. Following successful mating, the female beetle burrows into soft substrate, such as soil or grain, to lay her eggs.
A female darkling beetle can lay hundreds of tiny eggs throughout her adult life, up to 500 eggs. These eggs are sticky and often become coated with substrate, making them difficult to spot. After an incubation period, ranging from one to four weeks depending on conditions, these eggs hatch into new mealworm larvae, restarting the cycle. This process ensures the presence of mealworms and darkling beetles in their ecosystems.