For millions of years, Earth was home to insects far larger than any seen today. Fossil records reveal a past where dragonflies soared with wingspans comparable to large birds and millipedes stretched to the length of small cars. This raises a compelling question about the conditions that once allowed such enormous arthropods to thrive and what factors led to their eventual reduction in size.
Giants of the Paleozoic
The Carboniferous and Permian periods, roughly 360 to 250 million years ago, represent a remarkable era for insect gigantism. During this time, the planet hosted immense invertebrates. Among the most famous examples is Meganeura, a griffinfly resembling a modern dragonfly, which boasted a wingspan reaching up to 75 centimeters (2.5 feet). Another colossal creature was Arthropleura, a millipede-like arthropod that could grow to an astonishing length of over 2 meters (6.5 feet).
The Oxygen Hypothesis
A primary explanation for the immense size of ancient insects centers on higher atmospheric oxygen levels during the Carboniferous and Permian periods. Earth’s atmosphere during this time contained up to 35% oxygen, compared to today’s 21%. Insects possess a unique respiratory system known as the tracheal system, which differs from vertebrate lung-based systems. This system uses a network of tubes (tracheae) branching throughout the body, delivering oxygen directly to tissues and cells.
Oxygen enters these tubes through small openings (spiracles) and then diffuses passively through finer tubes (tracheoles) directly to individual cells. This diffusion-dependent method becomes less efficient over longer distances, placing a physical limit on body size. In an atmosphere with higher oxygen concentrations, the partial pressure of oxygen is greater, allowing for more efficient diffusion over larger distances. This enhanced oxygen delivery would have facilitated the growth of much larger insect bodies, as their internal tissues could receive sufficient oxygen without relying on a circulatory system to transport it. Conversely, in lower oxygen environments, the efficiency of this passive diffusion drops, making it unsustainable for larger body masses.
Other Environmental Influences
While atmospheric oxygen is considered the most significant factor, other environmental conditions during the Paleozoic era contributed to insect gigantism. The absence of major aerial vertebrate predators during these early periods allowed insects to evolve larger sizes without facing predatory pressure. Larger insects might have been less vulnerable in an environment lacking agile, flying hunters.
The warm, humid, and stable climate of the Carboniferous and Permian periods also played a role. Vast swamp forests provided abundant plant life. This extensive vegetation offered an ample food source, supporting the metabolic demands of large-bodied insects and further promoting their growth.
The Shift Towards Smaller Sizes
The era of giant insects began to wane as atmospheric conditions changed. Following the Permian period, oxygen levels in the atmosphere started to decline, eventually reaching modern concentrations. As oxygen became less abundant, the passive diffusion system of insect respiration became a limiting factor for larger body sizes, making it difficult for giant insects to sustain metabolic needs. This environmental shift favored smaller forms that could more efficiently acquire oxygen.
The evolution of new predators also contributed to the reduction in insect size. Birds began to evolve around 160-165 million years ago, with bats appearing approximately 50-52 million years ago. These agile, flying vertebrates became effective aerial predators, hunting insects. Larger, slower-moving insects would have been more susceptible to these new threats, creating evolutionary pressure that favored smaller, more maneuverable, and harder-to-catch forms.