Goliath beetles and sphinx moths would be described as large by just about anyone living today, but some prehistoric insects would dwarf these evolutionary descendants. During the Paleozoic era, the Earth teemed with giant insects, from dragonflies with wing spans measured in feet, to mayflies nearly 18 inches in breadth. While over a million insect species live today, truly giant insects no longer exist. Why did giant insects live in prehistoric times, but disappear from the Earth over time?
During the Carboniferous and Permian periods, atmospheric oxygen concentrations were significantly higher than they are today. Prehistoric insects breathed air that was 31-35% oxygen, as compared to just 21% oxygen in the air you're breathing as you read this. Atmospheric oxygen is the single most limiting factor on insect size.
The cells in your body get the oxygen they need to survive via your circulatory system. Oxygen is carried by the blood, through your arteries and capillaries, to each and every cell in your body. In insects, respiration occurs by simple diffusion through the cell walls.
Insects take in atmospheric oxygen through spiracles, openings in the cuticle through which gases enter and exit the body. Oxygen molecules travel via the tracheal system. Each tracheal tube ends with a tracheole, where the oxygen dissolves into the tracheole fluid. The O2 then diffuses into the cells.
When oxygen levels were higher, as in the prehistoric era of giant insects, this diffusion-limited respiratory system could supply sufficient oxygen to meet the metabolic needs of a larger insect. Oxygen could reach cells deep within the insect's body, even when that insect measured several feet long. As atmospheric oxygen decreased over evolutionary time, these innermost cells could not be adequately supplied with oxygen. Smaller insects were better equipped to function in a hypoxic environment. And so, insects evolved into smaller versions of their prehistoric ancestors.
- Dudley, Robert. (1998). Atmospheric Oxygen, Giant Paleozoic Insects and the Evolution of Aerial Locomotor Performance. The Journal of Experimental Biology 201, 1043–1050.
- Dudley, Robert. (2000). The Evolutionary Physiology of Animal Flight: Paleobiological and Present Perspectives. Annual Review of Physiology, 62, 135–55.
- Encyclopedia of Insects, edited by Vincent H. Resh and Ring T. Carde