Washington, Oct 21 (ANI): When news of Ant-Man - a comic superhero who can shrink to the size of an ant - being cast in a movie emerged, scientists wondered why ants couldn't balloon to man size.
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, according to the news site Grantland.
If it's anything like the comic, it will also feature ants as big as humans, which got researchers thinking: Could ants be as big as people? And why aren't insects bigger than they are?
Researchers don't know exactly, although there are several hypotheses as to why insects and other arthropods don't get bigger, insect physiologist Jon Harrison, at Arizona State University in Tempe said.
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 added.
Because exoskeletons are rigid, insects need to molt as they grow, shedding the old skin and growing a new one.
Scientists have suggested that 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," 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 is that 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.
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. (ANI)