Why were prehistoric insects so huge?

By EarthSky in Earth | December 5, 2016

Before the dinosaurs, giant insects ruled the world more than 300 million years ago.

Poster for the film The Deadly Mantis (1957) by artist Reynold Brown, via Wikimedia Commons. Okay, prehistoric insects weren’t this big, but many were much larger than insects today.

Do you complain about dead bugs on your windshield? Just be thankful that insects today are smaller than their prehistoric ancestors.

Hundreds of millions of years ago, giant insects were common on Earth. Consider Meganeura, a genus of extinct insects from approximately 300 million years ago, related to modern-day dragonflies. One member of this group – M. permiana – was first described by researchers in Kansas in 1937 as having a wingspan of over 2 feet (0.6 meters). It’s still considered one of the largest known insects that ever lived.

While over a million insect species live today, truly giant insects no longer exist. Why did they disappear?

There are two main reasons. The most important is that our atmosphere has changed. Millions of years go, the air surrounding our planet was warmer, moister and contained more oxygen. During the Carboniferous and Permian periods, Earth’s air contained 31-35% oxygen, as compared to just 21% oxygen in the air today.

Oxygen levels are especially important for insects because they don’t have lungs. Instead, they rely on air flowing through a series of opening across their bodies called spiracles, which connect directly to the tissues that need oxygen.

M. monyi - whose fossilized remains are shown here - is one of the largest known flying insect species; the Permian Meganeuropsis permiana is another. This specimen is housed at the Fossil at the Museum of Natural History in Toulouse. Image via Wikimedia Commons.

Fossil remains of M. Monyi, a member of the extinct insect genus Meganeura. Their wingspans could reach 2 feet (0.6 meters). This specimen is housed at the Fossil at the Museum of Natural History in Toulouse. Image via Wikimedia Commons.

Model of ancient M. Monyi made for the Denver Museum of Natural History.

Model of M. Monyi made for the Denver Museum of Natural History.

But there’s another reason giant insects disappeared. As ancient dinosaurs evolved evolved the ability to fly, eventually becoming modern birds, they put a cap on insect size through predation and competition.

The earliest known bird – Archaeopteryx – appeared about 150 million years ago. Birds proved to be faster and more agile than the giant insects. In an article in LiveScience in 2012, paleobiologist Matthew Clapham of the UC Santa Cruz commented:

Where giant insects fit in the history of life on Earth.

Bottom line: Hundreds of millions of years ago, giant insects were common on Earth. The decline in atmospheric oxygen – and the rise of birds – both contributed to the demise of these prehistoric insects.