Insects are the most diverse class of animals on Earth, characterized by a three-part body, six jointed legs, and a rigid exoskeleton. This vast group includes over one million described species, ranging from microscopic parasites to giants of the arthropod world. Determining which species achieves the greatest body mass requires examining the largest adult specimens, the mass of their developmental stages, and the biological limits that cap their growth.
The Heaviest Adult Insect Species
The title for the heaviest adult insect is a closely contested record, primarily fought between certain large beetles and a flightless orthopteran. The Goliath beetles, belonging to the genus Goliathus native to the tropical forests of Africa, are generally considered the most massive beetles. These impressive scarabs can reach a length of over four inches as adults, with males commonly weighing between 40 and 60 grams.
While the Goliath beetle often claims the title in popular culture, the verified record for the heaviest single adult specimen belongs to the Little Barrier Island giant weta, Deinacrida heteracantha. A female of this species, found off New Zealand, was officially recorded at 71 grams. This weight, comparable to a small bird, was achieved because the female was gravid, heavily distended with a large clutch of eggs.
Other substantial contenders include the Elephant beetles (Megasoma species), which inhabit the rainforests of Central and South America. Males of this genus, such as Megasoma elephas and Megasoma actaeon, can routinely exceed 50 grams, sometimes reaching up to 70 grams. These massive beetles owe much of their bulk to their robust bodies and the large horns used by males in combat for mates.
Mass Measurement Challenges and Record Holders
Determining the heaviest insect is complicated by the vast difference in mass between adult and larval stages. For many large beetle species, the larval stage, or grub, holds the maximum weight record. Larvae are specialized eating machines, designed to accumulate energy and mass before their non-feeding pupal and adult phases.
Goliath beetle larvae often weigh significantly more than their adult counterparts, reaching masses between 80 and 100 grams. Some reports suggest a maximum larval mass of up to 115 grams for Goliathus goliatus. This weight fuels the dramatic transformation during metamorphosis, resulting in an adult insect that may only weigh half as much as the fully-grown larva.
The largest verified weight for an insect specimen overall belongs to a larva of the Actaeon beetle, Megasoma actaeon. A South American specimen bred in Japan was recorded to weigh an exceptional 228 grams (8 ounces) just before pupation. Setting these records is challenging due to the difficulty of accurately weighing living insects in the field and the variability between wild and captive-reared specimens.
Why Insects Cannot Grow Larger
Insect size is constrained by physics and biology, primarily concerning their method of respiration and external skeleton. Unlike vertebrates, which actively pump oxygen using lungs and a closed circulatory system, insects rely on a passive system. Their respiratory apparatus, the tracheal system, is a network of tubes that delivers air directly to the tissues through openings called spiracles.
Oxygen moves through this system primarily by diffusion, a process that becomes inefficient over long distances. As an insect’s body mass increases, the length of the tracheae must also increase, making it harder for oxygen to reach the innermost cells efficiently. This passive delivery mechanism effectively sets a ceiling on how large an insect can grow in the current atmospheric oxygen levels.
The physical limitations are explained by the square-cube law: as an object increases in size, its volume (and mass) increases much faster than its surface area. The exoskeleton provides structural support, but if an insect were to scale up, its weight would increase disproportionately to the strength of its supportive structures.
For a giant insect, the exoskeleton would need to be impossibly thick to support the body weight, making movement nearly impossible. To grow, an insect must molt, shedding its rigid exoskeleton and leaving it soft and vulnerable until the new shell hardens. A human-sized insect would be crushed under its own weight during this molting process, reinforcing why the insect body plan is best suited for small to medium scale.