Where did all the giant bugs go?

By Jasmine McBain-Miller, Class of 2016

Insects are big enough as it is. But what’s preventing them from being bigger? – Image Isolino Ferreira via Flickr

 We’ve all experienced the unpleasantness of having an insect fly into our mouths. If we’re lucky and fast enough, we might manage to spit them out. If we’re unlucky well… at least they’re nutritious! In the end we should just be thankful that swallowed bug wasn’t any larger.

Most insects are quite small, although stick-insects can be relatively large, with an Australian species growing to 50cm in length. Fortunately, the stick insect is somewhat of an exception, most bugs never reach anywhere near that size.

But why not? What is stopping them from growing to the size of a household pet?

Well there are a couple of theories floating around, but the most popular one relates to how bugs take up oxygen.


Too small to breathe

Insects don’t exactly “breathe”. At least, not in the way other animals do. Sure they still need to take in oxygen and remove carbon dioxide to survive, but they do so in an entirely different way.

When you breathe in, the diaphragm (muscle beneath the lung), tightens and moves downward, allowing your lungs to inflate and bring in oxygen. The oxygen goes to tiny air sacks in the lungs (known as alveoli). Here it can easily pass through the walls of the alveoli and attach to blood cells on the other side. Via the bloodstream, oxygen is spread to all cells throughout our body.

Insects, on the other hand, fail to see the need for lungs, or even a proper heart for that matter. Instead, they have small holes that line the sides of their bodies, known as spiracles. Oxygen enters their bodies through these holes, and travels down tubes (known as tracheae), which extend throughout their body. Because their bodies are so tiny, the oxygen can easily get to all cells. Some insects also have air sacs and use muscles to move oxygen around the body faster, but none require lungs. For those of you who were particularly sadistic children: this is why you couldn’t drown that fly by dipping its head underwater.



Too large to exist

“But how does the absence of lungs prevent insects from growing, taking over the world and enslaving humanity?” I hear you ask.

It’s really quite simple. Using spiracles and tracheae for oxygen transportation isn’t as efficient a set of lungs. If the animal is too large, the oxygen probably wouldn’t be able to reach all the cells in time. The insect wouldn’t survive.

If oxygen levels increased in our atmosphere, there might be the potential for bigger bugs. In fact, hundreds of millions of years ago, when atmospheric oxygen levels were higher, dragonflies the size of birds did exist. Scientists have experimented to see whether the atmospheric oxygen levels could really be responsible for hawk-sized dragonflies. To do this, they raised dragonflies in an environment with an increased oxygen concentration, and saw that the dragonflies grew larger than the ones we see today.

Of course, there are a few other theories for why insects are so small. Such as that the insect wouldn’t be able to support their exoskeleton if they grew too large, or that the evolution of birds around 150 million years led to the shrinkage of insects.

It seems that the oxygen issue is at least partially responsible for the small bugs we have today. Let’s just take a moment to be thankful for the decrease in oxygen atmospheric levels.