Before vertebrates moved out of the ocean, insects ruled the land and air. Rhyniognatha, the earliest known insect, arrived on land nearly 400 million years ago, roughly 40 million years before the first tetrapods arrived. Monster creatures like Meganeura, one of the so-called griffinflies (the larger cousin to dragonflies), ruled the air with two foot long wingspans. And even after amphibians, reptiles, and dinosaurs spread and diversified the insects stayed just as massive and just as dominant.
But today, insects are tiny. Even the largest ones (such as the atlas moth and the goliath beetle) are barely larger than a person’s hand. The disappearance of the mega-insects has been an evolutionary puzzle for years. The prevailing theory is that it all has to do with oxygen. During the Permian period oxygen levels were about 50% higher than they are today. This abundance of respiratory energy could have fueled super-sized metabolisms and allowed the insects to grow larger than they could today. And this theory seems good. The size of insect fossils seems to track right along oxygen levels. When the atmosphere was rich, they were big; when the atmosphere was oxygen-poor, the insects shrank. But this trend suddenly stops about 140 million years ago. Without warning, the size of insects shrank dramatically, even though oxygen levels were starting to rise. Why?
The answer is that the giant bugs had finally met their match. Pterosaurs had been to clumsy to be a real threat, but this era saw the rise of a new aerial creature, the bird. These ponderous mega-insects made a ready and easy food source. The smaller, more maneuverable bugs could still outrun or out-fly the proto-birds, but the larger ones succumbed to predatory pressure and began to disappear. Slowly, larger size lost its advantages and became nothing more than a liability.