Major factor of size growth: the oxygen supply
The reason for this is her so-called Trache system. The network of fine trachea traverses the entire body of the animals and supplies all parts directly with air-oxygen. The larger an insect, the more space these tracheae need: The diameter of the trachea does not increase linearly, but must grow disproportionately strong in order to provide the body with sufficient oxygen. Especially in the filigree legs of the insects, the trachea can not grow indefinitely. In some periods of the earth's history, however, giant insects could develop because at that time the earth's atmosphere had up to 30 percent oxygen. At today's 21 percent, such giants would simply fall asleep, according to current doctrine.
The evaluations of the researchers clearly confirmed this theory: In the long evolutionary history of the insects, the species always grew when the oxygen content of the atmosphere was high. If he was low, they shrank.
But there was a relevant exception to this rule, Clapham and Karr report: At the end of the Jurassic era, about 150 million years ago, oxygen levels rose, but insects became smaller. This unusual trend was exactly parallel to one of evolution's biggest success stories? the birds. The first species developed at the end of the Jurassic and then fanned out in the Cretaceous to ever more skillful insect hunters, the researchers explain. According to them, this has become a strong readout factor: Small insects have a better chance of escaping the birds' beaks. display