Imagine the following scene: you go out to buy bread and when you cross the street you have to wait for a millipede of 2.5 meters to pass. Later, when you arrive at the bakery, you look up and see a dragonfly the size of a seagull flying. It looks like a scene from some low-budget fantasy movie, but that’s how arthropods were 300 million years ago.
This group, which in addition to insects also includes myriapods (centipedes, millipedes, millipedes, among others), arachnids and crustaceans, managed to evolve in an environment rich in oxygen, which was probably a decisive factor for the exaggerated growth.
According to a study carried out by scientists at the University of Plymouth, in the United Kingdom, it was the way in which oxygen affects the larvae of these animals that influenced their evolution. “We think it’s not just that oxygen affects adults, but that oxygen has a greater effect on larvae,” commented Wilco Verberk, one of the study’s authors.
The oxygen problem
Copy of the fossil of a dragonfly with a wingspan of 68 cm. (Source: Wikimedia Commons)
Have more oxygen available may seem like an advantage initially. The problem is that at high concentrations, it can also be a problem. In humans, for example, excess oxygen can cause cellular damage, leading to vision problems, difficulty breathing, nausea, and seizures.
Adult insects can regulate their oxygen intake opening or closing their spiracles—holes in their bodies that are like valves. As insect larvae normally absorb oxygen directly through their skin, they have little or no control over the amount of gas absorbed.
Nature’s solution to this is the proportion between surface area and volume. “If you grow, your surface area decreases relative to your volume,” explained Verberk. Therefore, conditions in the Earth’s atmosphere during the Carboniferous—between about 359 and 299 million years ago—were favorable for larger larvae. The rest is natural selection and evolution.
But if these arthropods were so successful over a long period, why don’t they exist today? The answer to that traces the opposite path of what made these animals grow. As oxygen levels decreased, animals that needed more of this gas to move around became slower, while smaller animals became more successful.
“Over time probably caused the performance of the larger insects to decline. Such reduced performance ended up allowing other species to surpass the giants”, concluded Verberk.
