Why Aren't Insects Human-Size?

News that Ant-Man, a comic superhero who can shrink to the size of an ant, may be in line for a movie, LiveScience wondered why ants couldn't balloon to man size. Turns out, scientists are still pondering this question. CREDIT: Marvel Comics View full size image

The Ant-Man cometh.  

Director Edgar Wright, known for movies such as "Shaun of the Dead," announced earlier this week he would be making a movie about Ant-Man, a comic superhero who can shrink to the size of an ant and communicate with his formic brethren, according to the news site Grantland. If it's anything like the comic, it will also feature ants as big as humans, which got us thinking: Could ants be as big as people? And why aren't insects bigger than they are?

The short answer is, researchers don't know exactly, although there are several hypotheses as to why insects and other arthropods don't get bigger, said insect physiologist Jon Harrison, at Arizona State University in Tempe.

The first hypothesis is that insects' exoskeletons may not be strong enough to allow them to get much bigger — that they'd have to become impossibly thick. Harrison learned this theory as an established fact during his training, but little experimental evidence to support the idea exists, he said. The only study to look at this question found that larger arthropods don't have thicker exoskeletons, he said. "So there's no direct evidence for this," he said.

Too tasty

Because exoskeletons are rigid, insects need to molt as they grow, shedding the old skin and growing a new one. Scientists have suggested this vulnerable time puts a ceiling on size: Larger animals, particularly those without protective skeletons, would make for more attractive meals to a predator. "The bigger you get, the more of a tasty vulnerable package you are," goes the thinking, Harrison said.

A related theory suggests being larger makes you a more attractive meal, whether molting or not. One study found that the size of ancient flies declined as birds evolved, suggesting smaller creatures were better able to avoid hungry raptors and pass on their genes.

Another possibility: Insects have open circulatory systems, where blood and bodily fluids aren't bound up in vessels, as is the case with most vertebrates. This makes it more difficult to move blood throughout a large body, as circulation would be hampered by gravity, which pulls blood downward.

Not enough oxgyen

Perhaps the most plausible hypothesis, and one that Harrison has studied extensively, is the role played by oxygen. Insects "breathe" via tiny tubes called trachea, which passively transport oxygen from the atmosphere to bodily cells. Once insects reach a certain size, the theory goes, the insect will require more oxygen than can be shuttled through its trachea.

Support for this theory comes from the fact that about 300 million years ago, many insects were much larger than they are today. There were, for example, dragonflies the size of hawks, with wingspans of about 6 feet (1.8 meters), and ants the size of hummingbirds.  At this time, the oxygen content in the atmosphere was about 35 percent, versus 21 percent today.

Harrison's work has shown that almost all insects get smaller if you rear them in low oxygen conditions; many of them get bigger when you give them more oxygen. Certain species can get about 20 percent bigger in a single generation when given more oxygen, he said.

Bulky insects also seem to need more trachea. "If you extrapolate that out with a much bigger insect, perhaps there'd be nothing left but trachea," he said. And there's only so much room — an animal needs room for other organs, muscles and the like.

But that hasn't been proven, and scientists don't understand exactly why insects aren't bigger, or more broadly, the biological basis for controlling body size. There are many more questions than answers, he said.

Okay, but ants as big as humans? "I'm not willing to say it couldn't happen," he said.

Reach Douglas Main at dmain@techmedianetwork.com. Follow him on Twitter @Douglas_Main. Follow LiveScience on Twitter @livescience. We're also on Facebook& Google+.