Giant insects are stalwart monsters in science fiction, and million of years ago, arthropods grew to Brobdingnagian sizes. But why aren't ants the size of rats disrupting our picnics today?
Giant insects make great movie monsters. Everyone deals with bugs on a daily basis – usually by squishing or exterminating them. We all know what they look like and what they can do, and I think we're all a little bit terrified of what would happen if the insects of the world decided to take revenge on humanity. They outnumber us. They have built in pincers and fangs and wings and armor. They are proportionally stronger, faster, and tougher than humans. Really the only thing keeping them in check is their size. Take that limitation away, and they would overrun the world.
Insects haven't always been the tiny nuisances we're familiar with today, though. During the Carboniferous period, the Meganeura dragonfly had a wingspan of 28 inches. The Arthropleura, a relative of the millipede, could grow over 8 feet long. It was the largest known land invertebrate of all time. A majority of insect diversity has escaped the fossil record because the exoskeleton is made of chitin, a material that simply doesn't preserve well. Every insect during the Carboniferous period wasn't a giant, but just imagine what else could have been wriggling around in a world with dragonflies the size of hawks and millipedes the size of boa constrictors! So why aren't there any super bugs nowadays? Why did they get smaller over time?
To answer that you have to understand how insects breathe. Mammals absorb oxygen through the lungs, which is then transported throughout the body via the circulatory system. In insects, the circulatory and respiratory systems are separate. Oxygen is delivered directly to the cells via a complex series of tubes called the tracheal system. The tubes allow gas exchange to occur between the cells and the air. Oxygen diffuses in, and carbon dioxide diffuses out. It's not really very efficient. The larger an insect is, the larger the tracheal system has to be to support respiration. Eventually you run out of room for other organs. How big an insect can grow is limited by its ability to breathe.
Ok, so maybe giant insects don't make such great movie monsters when you know they could never actually get that big without suffocating. So how were they able to get so large during the Carboniferous period? Well, for one thing the Earth was covered in vast, swampy forests at the time. That's actually where the time period gets its name, since the plant matter deposited then forms the majority of the planets' modern coal deposits. All of this plant life absorbed carbon dioxide from the atmosphere and produced oxygen. It is estimated that atmospheric oxygen levels in the Carboniferous period were as high as 35%, compared with 21% today. The most widely accepted theory is that with a higher oxygen concentration, the tracheal system was able to function more efficiently and insects were able to grow much larger.
Knowing that, the Mad Scientist in me wants nothing more than to get some atmospherically controlled tanks and breed me some giant spiders. It's fool proof, don't you see? If they ever escaped, they'd suffocate in normal atmosphere! No need to fear them turning against their creator or running amok in the nearest city. Spider silk, with its incredible tensile strength and flexibility, has been the Holy Grail of the textile industry for decades. Obviously the solution is just to build a bigger spider. Also, giant spiders are cool.
Unfortunately, it's been a long time since the Carboniferous period. A little something called evolution has been going on since then, and modern insects are adapted to a modern atmosphere. Researchers have examined the tracheal systems in insects of various sizes, and they have found that as insects increase in size the tracheal system also increases at a disproportionately higher rate. Modern beetles, for example, can not grow larger than approximately six inches no matter how high the oxygen concentration is because the tracheal system still takes up too much room. In the Carboniferous period, the trachea were likely much narrower in diameter. With the higher oxygen concentration, the smaller tubes could have delivered enough oxygen to support larger insects.
Given enough time in a more oxygen-rich environment, you can bet that at least some insects would become super-sized. Unfortunately for me and my plans for world domination, it would happen on an evolutionary scale and not because some spiders were accidentally zapped with gamma radiation. Ah well, back to the drawing board.