Mealworms, scientifically known as Tenebrio molitor, are the larval stage of the darkling beetle, a common insect found globally. These creatures undergo a complete metamorphosis, transitioning through egg, larva, pupa, and adult stages. Many people encounter mealworms when looking for pet food, fishing bait, or even as a sustainable food source. Their size is not fixed but influenced by various factors throughout their development.
Typical Mealworm Size
Fully grown mealworms typically measure about 2.5 centimeters (1 inch) or more in length. Mature larvae can reach up to 3.2 centimeters (1.25 inches) before pupating. Under optimal conditions, they can extend to between 2.5 to 3.8 centimeters (1 to 1.5 inches). Commercial growers sometimes use juvenile hormones to prevent larvae from metamorphosing, allowing “giant” mealworms to grow larger, weighing over 300 milligrams. When mealworms first hatch from their eggs, they are minuscule, usually only 2 to 2.5 millimeters long.
What Influences Mealworm Size
Several environmental and biological factors influence mealworm growth. Diet plays a substantial role; a nutritious and abundant food source leads to larger, healthier mealworms. While wheat bran is a common base diet, supplementing it with fresh plant materials like carrots, oranges, or red cabbage improves growth rates, resulting in larvae that are 40% to 46% heavier. Protein content in their diet also affects their final weight.
Temperature is another primary factor affecting mealworm growth, survival, and developmental speed. The optimal temperature range for rapid growth is between 22 and 30 degrees Celsius (71.6 to 86 degrees Fahrenheit). Higher temperatures within this range, such as 25 to 30 degrees Celsius, typically result in faster growth rates. Temperatures outside this favorable range, like 10 or 35 degrees Celsius, can hinder growth and reduce survival rates.
Relative humidity also impacts the mealworm’s final size. Higher humidity levels lead to increased larval mass and length. For instance, larvae raised at 84% relative humidity were 1.96 times heavier and 1.31 times longer than those reared at 43% relative humidity. Maintaining humidity above 70% generally promotes better larval development.
Size Across Life Stages
The mealworm’s life cycle involves distinct stages, each with its own characteristic size. It begins as a tiny egg, measuring approximately 1.4 to 1.5 millimeters in length and 0.6 to 0.66 millimeters in width. After hatching, the organism enters the larval stage, which is the largest and most recognized form.
Following the larval stage, the mealworm transforms into an inactive, non-feeding pupa. These pupae are typically white, gradually turning yellowish as they mature, and range in length from 14 to 19 millimeters (0.5 to 0.75 inches). Their width usually measures between 5 and 7 millimeters. Finally, the pupa develops into an adult darkling beetle, generally smaller than the mature larva. Adult Tenebrio molitor beetles typically measure between 1.25 to 1.8 centimeters (0.49 to 0.71 inches) in length.