The mealworm (Tenebrio molitor) is not a true worm but the larval form of the Darkling Beetle, an insect that undergoes a complete metamorphosis. This process involves four distinct stages: egg, larva, pupa, and adult.
The Egg Stage
The egg stage is difficult to observe due to its tiny size and protective coloration. Female darkling beetles lay hundreds of eggs, depositing them individually or in small clusters within a food source or substrate. These eggs are oval or bean-shaped, measuring approximately 1.7 to 1.8 millimeters long, roughly the size of a pinhead.
When freshly laid, they possess a pale white or creamy color. A characteristic feature is the slightly sticky exterior of the eggshell. This coating causes fine particles of the surrounding substrate, such as flour or grain, to adhere to the surface, effectively camouflaging the egg.
The Larval Stage
The larval stage is the longest-lasting phase. A mature larva is elongated and cylindrical, typically reaching lengths of 2.5 to 3.2 centimeters. This stage is defined by its hard, chitinous exoskeleton, which is golden-yellow to golden-brown and provides a tough, polished look, distinguishing it from soft-bodied true worms.
The body is clearly segmented, consisting of 13 sections. Located near the head are three pairs of short, jointed, true legs, a feature that immediately identifies it as an insect larva. The head is relatively small and flat, and the mealworm uses its strong mandibles to consume organic materials. The larva must molt its rigid outer skin numerous times, sometimes up to 20 times, to accommodate its increasing size before transitioning to the next phase.
The Pupal Stage
The mealworm enters the pupal stage after the larval phase. The pupa has an exarate form, meaning its appendages, such as legs, antennae, and future wing pads, are visible and lie free against the body, rather than being encased in a cocoon. The insect remains largely immobile and does not feed during this time.
Initially, the pupa is a soft, creamy white color, but its exterior gradually hardens and darkens to a yellowish-brown shade. The body is typically around 1 to 1.5 centimeters long and has a conical shape that tapers toward the posterior end. Although the pupa appears motionless, it can wiggle its abdomen vigorously if disturbed.
The Adult Darkling Beetle
The final stage is the emergence of the adult Darkling Beetle, which signals the end of the metamorphic process. When first emerging, the newly formed beetle is soft, pale, and often a light reddish-brown or yellowish color. Over the next few days, its exterior hardens, and its color deepens significantly.
The mature Darkling Beetle is typically a uniform dark brown to black color and measures between 1.25 and 1.9 centimeters in length. The adult is characterized by its hard, protective wing covers, called elytra, which often have linear grooves running down the abdomen. The beetle also possesses segmented antennae and three distinct body parts: head, thorax, and abdomen.