Adult mealworms are beetles. Both species are about ½- to 5/8-inch long and very similar in shape and color. However, the dark mealworm adult is a dull black, whereas the yellow mealworm adult is a polished and shiny dark brown or black.
The larvae are 11/4-inches long when fully developed. They are cylindrical, hard-bodies insects very similar to wireworms. The ninth abdominal segment has a pair of upturned, dark, hard spines on the upper surface. The yellow mealworm larva is bright yellow, while the dark mealworm is dark brown.
Meal worms over winter as larvae, emerging in the spring as adults that soon begin laying eggs, during their two to three month life, the adult female lay approximately 276 bean shaped, white, sticky eggs in food material. The larvae molt 14 to 15 times, over a period of four to 18 months, before pupating near the surface of the food upon which they are feeding. There is one generation a year.
Meal worms are seldom major stored product pests in the home, the larvae are found in dark undisturbed locations feeding on collections of grain, grain dust and cereal products. Poor sanitary conditions and high moisture content are conducive to infestation. Meal worms are medically important because the eggs and larvae are often eaten in cereals or other breakfast foods and can cause significant gastrointestinal irritation. The adults of both species have well-developed wings and are attracted to lights.