Where Have all the Giant Insects Gone?
In the early days of planet Earth, enormous insects ruled over the skies. The abundant plant life of the prehistoric era had caused atmospheric levels of oxygen much higher than we have today. This oxygen abundance allowed the incredible growth of flying insects, including horrific predatory dragonflies over two feet wide. A new study from the University of California (UC), Santa Cruz, was conducted to examine the relationship between insect size and oxygen levels. They found that for about 200 million years, insect size mimicked oxygen levels. Then at 150 million years ago, oxygen went up but insect size went down.
Oxygen concentrations used to be over 30 percent. Today, it is only 21 percent. The higher concentration allowed giant insects to get enough oxygen through their tiny breathing tubes which insects use instead of lungs.
The trend was altered about 150 million years ago. This point in time coincided with the start of the Cretaceous Period on Earth, the third and final era for the dinosaurs. It also coincides strikingly with the evolution of birds.
Birds were powerful, predatory creatures and made life hell for flying insects. The larger bugs were huge targets and easy to catch. So even though oxygen levels went up, allowing for larger sizes, evolutionary forces acted on flying insects to make them smaller and more maneuverable.
Obtaining information to reach this conclusion was no easy task. Assistant Professor, Matthew Clapham and graduate student, Jered Karr compiled the dataset of over 10,00 fossil insect wing lengths from an extensive review of publications on the subject.
Other transitions in insect size occurred at the end of the Cretaceaus period which could be attributed to the mass extinction event, the further specialization of birds, and the evolution of bats.
"I suspect it's from the continuing specialization of birds," Clapham said. "The early birds were not very good at flying. But by the end of the Cretaceous, birds did look quite a lot like modern birds."
The study has been published in the journal Proceedings of the National Academy of Sciences (PNAS)
Dragonfly image via Shutterstock