The fossil record reveals a period when insects and other arthropods reached immense sizes, dwarfing their modern relatives. This raises questions about how these creatures became so large and what factors allowed such gigantism to flourish.
Unveiling Prehistoric Insect Giants
Among the most well-known examples is Meganeura, a giant dragonfly-like insect from the Late Carboniferous period, around 300 million years ago. Meganeura monyi had a wingspan that could reach approximately 65–75 centimeters (2.13–2.46 feet), making it one of the largest flying insect species known. Its wingspan was comparable to that of a hawk and significantly larger than any modern dragonfly.
Another giant was Arthropleura, a millipede-like arthropod that lived from the Carboniferous to the early Permian period. This terrestrial invertebrate could grow to at least 2 meters (6 feet 7 inches) long. Fossilized trackways of Arthropleura have been found measuring up to 50 centimeters (20 inches) wide, providing evidence of its colossal size. These fossil discoveries, often of shed exoskeletons or wing impressions, allow scientists to reconstruct the scale of these ancient inhabitants.
The Role of Earth’s Ancient Atmosphere
The immense size of prehistoric insects is attributed to the higher oxygen levels present in Earth’s atmosphere during the Carboniferous and Permian periods. During the late Carboniferous, oxygen levels are estimated to have reached as high as 35%, greater than the current atmospheric oxygen concentration of about 21%. This elevated oxygen content directly influenced insect respiration.
Insects breathe through a system of tubes called tracheae, which branch throughout their bodies and open to the outside via small pores called spiracles. Unlike vertebrates, insects do not use a circulatory system to transport oxygen; instead, oxygen diffuses directly from the tracheae into their tissues. In an atmosphere with higher oxygen concentration, this diffusion process becomes more efficient, allowing oxygen to penetrate deeper into an insect’s body. This efficiency relaxed the physiological constraint on body size, enabling insects to grow much larger than they can today.
Environmental Influences on Size
While oxygen levels were a factor, other environmental conditions also supported the gigantism of prehistoric insects. The absence of aerial predators, such as birds, during these early periods reduced the selective pressure for insects to remain small for swift evasion. Birds evolved much later, approximately 150 million years ago. This meant that early flying insects faced fewer threats from above, allowing larger, slower-moving forms to thrive.
The lush, swampy forests characteristic of the Carboniferous period also provided abundant food sources and habitat. Plant matter accumulated, leading to the formation of extensive coal deposits. Warmer global temperatures during parts of these periods may have influenced metabolic rates, contributing to conditions favorable for larger body sizes. This combination of factors created an environment where large insects could flourish.
The End of the Age of Giants
The era of insect gigantism eventually came to an end due to changes in Earth’s environment. A factor was the decline in atmospheric oxygen levels after the Permian period. Oxygen concentrations, which peaked around 35%, gradually decreased, falling to levels as low as 12% by the beginning of the Triassic period. This reduction in available oxygen made the tracheal respiratory system less efficient for large insects, imposing a physiological limit on their maximum size.
The evolution of new predators also played a role in reducing insect size. The appearance of pterosaurs and later, birds, introduced agile aerial hunters. Smaller, more maneuverable insects had an advantage in escaping these predators, leading to selective pressure for reduced body size. While modern insects like the Atlas moth or the Giant Stick Insect can still be quite large, with wingspans up to 30 cm or lengths over 50 cm, they are dwarfed by their prehistoric counterparts.