Explanation for why we don't see two-foot-long dragonflies anymore fails

Giant dragonflies once ruled the skies, but their massive size is no longer possible due to a fundamental limitation in their breathing system. The loss of these giant insects is a significant aspect of the Earth's evolutionary history. Three-hundred million years ago, during the late Palaeozoic era, the skies were teeming with massive insects, including the 70-centimeter wingspan Meganeuropsis permiana. This ancient dragonfly weighed 100 grams and was a formidable predator. Biologists have long been puzzled by the disappearance of these giant insects, and a widely accepted explanation was the "oxygen constraint hypothesis." However, recent research has challenged this theory, suggesting that it is incorrect.

The oxygen constraint hypothesis posited that giant insects required highly oxygenated air to survive due to their inefficient breathing system. Insects breathe through a network of internalized tubes called the tracheal system, which is less efficient than the closed circulatory system found in mammals, birds, and reptiles. Air enters the insect's body through specialized portholes called spiracles and travels down larger tubes, the tracheae, which branch into microscopically thin tubes called tracheoles. These tracheoles are embedded deep within the insect's tissues and rely on passive diffusion to deliver oxygen to the muscles.

The tracheoles are the limiting factor in an insect's size, as they are responsible for delivering oxygen to the muscles. As an insect grows, the tracheoles must increase in number or size to maintain oxygen delivery. However, this increase in tracheoles would lead to a significant decrease in muscle mass, making flight performance severely impaired. This structural limitation is the reason why giant dragonflies are no longer possible.