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Why Were Prehistoric Insects So Large? Exploring the Giant Bugs of the Carboniferous Era

by Felix Omondi

Insects were significantly larger in the past, particularly during the Carboniferous and Permian periods (approximately 359 to 252 million years ago), due to a combination of environmental and physiological factors. Here are the main reasons:

  1. Higher Oxygen Levels:

During the Carboniferous period, atmospheric oxygen levels were much higher than they are today, reaching up to 35% compared to the current 21%. Higher oxygen levels would have facilitated more efficient respiration for insects, which rely on passive diffusion through their tracheal systems to distribute oxygen throughout their bodies. This would have allowed insects to grow larger because their respiratory systems could support greater body sizes.

  1. Respiratory System Limitations:

Insects’ respiratory systems are one of the limiting factors for their size. They rely on a network of tracheae (small tubes) to deliver oxygen directly to their tissues. In modern atmospheric conditions, the diffusion of oxygen through these tubes is sufficient only for smaller body sizes. In a high-oxygen environment, this limitation is less pronounced, allowing insects to achieve larger sizes.

  1. Predation and Competition:

The evolutionary pressure from predation and competition can also influence body size. During periods when larger predators were less prevalent or when competition for resources favored larger body sizes, insects could evolve to be bigger. Additionally, larger body sizes can offer advantages in terms of mating, territorial defense, and resource acquisition.

  1. Climate and Environment:

The Carboniferous period was characterized by warm and humid climates, with extensive swampy forests and abundant plant life. Such environments could support large herbivorous insects by providing ample food resources. Additionally, these environments might have favored larger body sizes for thermoregulation and other ecological interactions.

  1. Lack of Vertebrate Predators:

During the periods when giant insects thrived, vertebrate predators were less diverse and abundant than in later periods. This lack of significant predation pressure from vertebrates may have allowed insects to grow larger without the need for rapid mobility or other defenses that favor smaller sizes.

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Examples of Giant Insects:

Meganeura: A genus of giant dragonfly-like insects with wingspans up to 70 cm (28 inches).

Arthropleura: A genus of large millipede-like arthropods that could reach lengths of over 2 meters (6.5 feet).

Decline in Insect Size:

As oxygen levels decreased towards the end of the Paleozoic era and into the Mesozoic, the size of insects generally reduced. The evolution of more efficient respiratory systems in vertebrates, increased predation pressure, and changes in climate and vegetation further contributed to the decline in the size of insects.


The gigantic size of insects in the past can primarily be attributed to higher atmospheric oxygen levels, which facilitated more efficient respiration and allowed insects to overcome the limitations of their tracheal respiratory systems. Environmental factors, ecological interactions, and evolutionary pressures also played significant roles in shaping the size of insects over geological time scales.

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