One of the best things about evolution (besides the opposable finger) is that it didn’t create a tank-sized army of bugs. With the exception of a few past giants and some extremely long stick insects, insects tend to remain relatively small. How big can insects really grow?

Many insects and arthropods were larger in the ancient past. The largest insect of all time was Meganeuropsis Permianaa dragonfly that lived at the end of the Permian era, about 275 million years ago.

These dragonflies had a wingspan of about 75 centimeters and weighed over 450 grams. Insects such as the Atlas moth (Attacus atlas) reach impressive sizes, with a wingspan of 27 centimeters, but do not compare with their distant relatives, he points out IFL Science.

By understanding smaller modern species, we might be able to learn how big insects can grow.

Here’s what one theory says about how big insects can grow

One theory says that insect exoskeletons are not strong enough to support larger bodies, and as insects grow, their exoskeletons should become thicker than possible.

In support of this theory is the fact that arthropods in the sea are getting bigger. In the sea, their exoskeletons do not have to support their body weight in the same way as on land. However, as Dr. Jon Harrison, an entomologist at Arizona State University, in the US, explained, the data doesn’t really support this theory. Larger arthropods (on land) do not have thicker exoskeletons than smaller arthropods, which you would expect if the theory were correct.

The theory most likely to be true

Another theory is that the way insects breathe prevents them from growing really large.

“Insects breathe in a completely different way than humans. They have a series of holes along the body, and oxygen enters through these holes and passes into some tubes. These tubes branch, like a tree, and branch a lot, up to a micron in size. Thus, oxygen can reach almost every cell,” he said.

If insects got bigger, it might not be possible for them to get enough oxygen in these tubes, known as tracheoles, to keep them alive. What supports this theory is the fact that insects were larger millions of years ago.

“This idea has recently received evidence from geologists showing that at the end of the Paleozoic, atmospheric oxygen increased well above what it is today,” Harrison explained.

How can we get bigger insects?

“Oxygen currently makes up 21% of the atmosphere. At the end of the Paleozoic, we think it was about 32% oxygen. And this happens to coincide with the time when we had much larger insects than we have today. That led to the idea that the oxygen supply is what keeps the insects small, and that more oxygen in the atmosphere could allow them to get bigger,” says the researcher.

So all you need to do to get goat-sized insects might be to pump more oxygen into the atmosphere, add some stressors that would make the larger size advantageous, and wait for evolution to take its course .

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