Have you ever wondered why we don’t see a tank-sized army of cockroaches around us or in the wild? Insects have tended to remain relatively small throughout history, with the exception of some oddly long stick insects of the past. But what could be the reason for this? How big can an insect be able to grow?
In the ancient past, many insects and arthropods were larger than what we know today. The largest insect of all time was Meganeuropsis Permiana, a dragonfly that lived in the late Permian period about 275 million years ago. These dragonflies had a wingspan of around 75 centimeters and weighed more than 450 grams. Although insects such as the atlas moth (Attacus atlas) reach impressive sizes with a wingspan of 27 centimeters, they are certainly incomparable to these distant relatives.
Finding out why modern insects are shrinking instead of getting bigger may also help us understand why insects cannot grow above a certain size.
Why are insects small?
One theory is that insect exoskeletons are not strong enough to support larger bodies, and as insects grow, their exoskeletons must become thicker than possible. The larger marine arthropods are one example to support this theory. Sea creatures do not have to support their exoskeletons to support their body’s weight as they do on land. But Arizona State University entomologist Dr. As Jon Harrison explained to SciShow in 2012, the data doesn’t really support this theory. Larger arthropods (on land) do not have any thicker exoskeletons than smaller arthropods, and if the theory was correct, that should have been expected.
Another theory is that the way insects breathe prevents them from growing really big.
This theory is based on the fact that insects breathe in a completely different way than humans. They have a series of holes in the sides of their bodies, and then oxygen enters through these holes and goes as a gas into air-filled tubes. And these tubes branch out like a branching tree and become very small; down to a micron size. I mean, they’re really small and can get close to every cell.
With larger insects, it may not be possible to get enough oxygen through these tubes, known as tracheoles, to keep the animals alive. “This idea has recently received support from geologists, who have shown that atmospheric oxygen rose well above today in the late Paleozoic. We currently think it was 21 percent oxygen, in the Late Paleozoic, about 32 percent oxygen. And that’s a lot more than we have today,” Harrison said. “It’s around the time we have bigger insects. And that kind of strengthens the idea that it’s oxygen delivery that keeps insects small, and that higher oxygen in the atmosphere can make them grow.”
In short, it seems that the answer to why insects stay small and do not grow very large may be hidden in one of two different theories. But it still seems unlikely to give a definite answer?
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