Insect Monsters? Be Glad They Aren't Here... Yet
As time goes on, many species, including humans, have all had the same tendency; getting bigger. In the 18th and 19th centuries the average height of a human was about 5' 5", while today the average is about 6' 1". Based on fossils, it has also been discovered that many other animals are getting bigger as time goes on, with an exception to all types of bugs. So while we are all getting bigger, why aren't bugs? Well, there are a couple of theories on that.
An insect physiologist at Arizona State University named Jon Harrison has one hypothesis; that their exoskeletons may not be strong enough to allow them to get much bigger at all. This was just one theory he was taught, but there is very little evidence to actually back this up. There has only been one study done on this theory, and in that study they found that even in the largest of insects, even their exoskeletons weren't thicker, so this is most likely not the case.
One leading theory is because they are just too delicious to other creatures! As insects get older and become larger from the time they are born, they have to molt. Meaning, they have to remove their current exoskeleton in order to grow a new one that will be big enough for their new size. During this process of molting, the poor insects have no protection. If an ant was the size of a dog and molted, thus having zero protection, it would likely find itself as the main course for another creatures meal that night. However, being as small as ants are, during the molting stage most animals wouldn't bother with eating them, as they are going to get very little nutrition from something that small.
One other theory is that they do not get enough oxygen. This is a theory that Jon Harrison has studied quite extensively. The idea is that once the insects reach a certain size, the insect will require more oxygen. The tracheas, the tubes that transport oxygen, would be too small to transport the amount of oxygen needed for them to grow. The reason for this theory comes from the fact that insects a long time ago were much bigger than they are today. In fact, we have a blog post all about the largest insect ever, which can be read here! When insects were this big, the amount of oxygen in the air was much more than it is today. Back then, air was about 35% oxygen, where as today it's 21% oxygen. According to an experiment done by Harrison, insects get smaller when they are raised in low oxygen conditions. To contrast, many of them would get bigger when they were raised in a higher oxygen environment.
So while these are just theories, they give us some ideas of why insects do not continue to get bigger, and what could one day cause them to get bigger. Hopefully ants and other bugs continue to stay the size that they are. If they get any bigger, we may run into some problems controlling them. Let's all hope that either the oxygen content in the air doesn't rise dramatically, or that other animals decide they wouldn't want to eat big bugs, or else we may end up in a situation with ants as pets instead of dogs!
Main, Douglas. “Why Aren't Insects Human-Size?” Live Science, 19 Oct. 2012, 10:04 AM, www.livescience.com/24122-why-insects-are-not-bigger.html