Given all the fascinating changes that evolution has brought – have you ever wondered why insects have gotten smaller rather than bigger?

And how large is it possible for an insect to get?

Most modern bugs are relatively small, but the largest insect of all time lived during the late Permian era, about 275 million years ago. The dragonfly named Meganeuropsis Permiana boasted a wingspan of approximately 2.5 feet and weighed more than 1 pound.

There are a few giant relics still around today, including some stick insects and the atlas moth with a 10.6-inch wingspan. But why have insects gotten smaller?

One theory is that insect exoskeletons aren’t equipped to support bigger bodies and would need to grow thicker to hold the added weight. While the existence of larger arthropods in the sea, where buoyancy helps offset the weight, seems to support this theory, it has been largely debunked.

The biggest reason is that larger land arthropods don’t have thicker exoskeletons than smaller arthropods.

Others hypothesize that insects can never become very large because of how they breathe. Instead of lungs like humans, insects take in air through their abdomens into a network of tiny tubes called tracheae.

Because the largest bugs have the longest tracheae, they would need the most oxygen to breathe, bolstering this theory because atmospheric oxygen levels in the Permian era were much higher than today. Arizona State University entomologist Dr. Jon Harrison gives more detail about the theories in a 2012 SciShow video.

If you were hoping for a future of giant cockroach armies, science fiction is still your best bet.