There’s really no other way to put this, and I want to share it with you. It’s long and half-boring, but reassuring, that’s for sure.
Dear Straight Dope [Edith note: yup, this is a drugs website]:
After watching numerous films like Eight-Legged Freaks and Starship Troopers, I started wondering if giant bugs (and by giant I mean larger than humans) can exist in this world. Do the laws of physics limit the size of these creatures that have hard exoskeletons and move via hydraulics? (Come to think of it, I haven’t heard of giant prawns, either.) What if they had a constant supply of food, enough reproductive control that they didn’t destroy the ecosystem, and no animals (e.g., us) to hunt them to extinction? If they can carry things many times their size when they’re small, could they haul food sources like commercial airplanes once they become huge? More importantly, could giant bugs fly and terrorize our skies? �ngangoteer, Quezon City, Philippines
SDSTAFF Doug replies:
Two factors impose an upper limit on the size of insects or other arthropods and prevent them from becoming giants. The most restrictive is the way they breathe. Arthropods don’t have a closed circulatory system, meaning their “blood” (called hemolymph) doesn’t transport oxygen the way ours does. Oxygen reaches their tissues primarily by diffusing in through tubes called tracheae, which connect to the outside via small pores called spiracles. There’s a physical limit on how long these tubes can be before the tissue at the inner end can no longer effectively exchange gases with the outside air. If arthropods got much bigger than they are, the deepest tissues in their bodies would suffer from oxygen starvation and build up too much carbon dioxide. We’re fairly certain this is the chief limiting factor because prehistoric insects lived in an atmosphere with more oxygen and attained sizes as much as three times that of the largest insects living today. Even so, these ancient bugs topped out at maybe two feet long – formidable if encountered at a picnic [Edith note: YOU SAID IT!!! Hahaha!!], I suppose, but hardly enough to send the citizens of Tokyo fleeing in panic.
Another reason arthropods can’t get any larger, which would be an insurmountable obstacle even if they could get around the oxygen delivery thing, is that their muscles are inside their skeletons. The strength (”power,” in technical lingo) of a muscle fiber or bundle is a function of its cross-section (how thick it is). Suppose you had a fast-growing radioactive mutant arthropod – as it got bigger in three dimensions, its muscles would become more powerful in only two dimensions. To retain equivalent strength, the critter’s muscles — which are inside its limbs, remember — would have to get bigger at a faster rate than the limbs themselves. Beyond a certain point, either the insect’s muscles would burst its exoskeleton or it would be too weak to move. That’s why the largest arthropods that ever lived (the Eurypterids, or giant sea scorpions) were aquatic, and why the largest living arthropods (giant crabs) are also aquatic — it doesn’t take as much muscle power to lift and move a limb underwater. But even there, the limit seems to be around nine feet long. Unless an arthropod were to shed the qualities that make it an arthropod, notably an external skeleton, a bug bigger than that isn’t physically possible.
Given all this, you can appreciate that the whole “lifting X times its own body weight” business is totally dependent on the critter being tiny. Generally, the smaller the body size, the bigger the value of X. Once an arthropod gets to anything near a foot long, it’s lucky if it can lift its own body weight, let alone carry any excess baggage. As for giant flying bugs, the largest flying insects ever found (giant dragonflies named Meganeura) had a wingspan around 75 cm (30 inches). Given that they had no predators — they were at the top of the Carboniferous food chain in their day, about 300 million years back – it seems fair to conclude they’d reached the limit for flying arthropods even in that super-oxygenated atmosphere. If we had things like that in our skies, the phrase “Look! A Meganeura!” would be uttered in awe and amazement rather than stark terror, though people might get uncomfortable when they saw them eating sparrows. Also, they would make a huge mess on one’s windshield.
So thanks to my friend Bob for this heads up. It is nice to cross one thing off my list.
A very short story.
