The Mystery of the Big Bugs

By Jack Myers, Senior Science Editor

What made the world’s “little creatures” so huge?

The animal in the photograph on the left is a Hercules beetle, named after a macho hero of Roman mythology. Though it is one of the largest insects and looks scary, actually it is only about six inches long.

Of all animal groups, insects are the most successful. No other group has so many individuals and so many different kinds. But insects are always small, at least the ones we have today.

There is a reason for insects to be small. An insect can’t breathe the way we do, but it still needs to get oxygen to every cell of its body. It has a simple and effective way of doing this. Every insect has a network of hollow tubes leading from its outside to its inside. These tubes have no pump to move air through them. They just provide an air space where bouncy air molecules are always moving and mixing together.

Oxygen molecules from outside air are always working their way through the tubes, trading places with other air molecules. So the insect gets its oxygen by the mixing-up process of gas molecules, which is called diffusion.

Since insects are small, usually smaller around than your little finger, their breathing system just fits their needs. But if they were to become much larger, they would not be able to get oxygen fast enough—especially insects that fly. Wing muscles need lots of oxygen to get the energy for flight.

An Old Mystery
You can see why it is part of their nature for insects to be small—the little bugs of our world. This brings us to an old scientific mystery, the mystery of the big bugs. For there are fossils proving that Earth once had some giant insects.

These insects are not found in all the fossil record, only in rocks formed during a particular period, about 350 to 250 million years ago.

The fossils show that during that time, there were cockroaches big enough to scare a modern-day cat, and dragonflies as big as small hawks. There’s the mystery: How could such large insects possibly have gotten oxygen into their bodies fast enough?

Carbon in the Rocks
Something else of interest happened during that ancient age of giant insects. Geologists can tell a lot about Earth’s history by studying rocks. They have a big catalog of different kinds of rocks telling what they are made of and when they were formed. The catalog shows that in a special period between about 350 and 250 million years ago an extra amount of carbon was buried in rocks.

That extra buried carbon also tells us something about oxygen. Carbon and oxygen are tied together in Earth’s recycling system. During daylight hours, green plants are working hard, using light to drive the chemical reaction of photosynthesis. That process makes gigantic quantities of oxygen as it changes carbon dioxide into the carbon compounds that make up plants. It is the big supplier of oxygen to our atmosphere.

There also are chemical reactions that do just the opposite of photosynthesis. These are oxidation reactions that use oxygen to burn the carbon stuff
of plants and animals back to carbon dioxide. In holding Earth’s big supply of oxygen, the atmosphere really acts like a leaky bucket. Some oxygen is always leaking away because of the oxidation reactions.

During our lifetimes the oxygen in the atmosphere has been very steady at 20.9 percent. So there must be a neat balance between the rate of photosynthesis, which makes oxygen, and the rate of oxidation reactions, which cause oxygen to leak away.

The balance was upset about 350 million years ago. That was a time when land plants were becoming successful, creating great swampy forests. Then massive amounts of carbon temporarily escaped recycling. This carbon was trapped in plants that were buried in rocks, where the carbon could not react with oxygen.

The Mystery Solved?
Recently geologists have realized that all the extra buried carbon must have been matched by a great surge of oxygen into the atmosphere, almost enough to double the present level. That high level of oxygen lasted about 100 million years, until movements in the Earth’s crust slowly brought the high-carbon rocks back to the surface. Then burning of that extra carbon used up all the extra oxygen.

Just recently those long-ago changes in oxygen and changes in insects have been making science news. A group of scientists has written an article to propose a hypothesis, a big idea: A period of high oxygen in the atmosphere must have had important effects on living things, especially on insects. Higher oxygen would have made the insect breathing system work better. That would have made possible the evolution of giant insects.