Why There Are No Fifty Foot Cockroaches
Sorry B-Flick Movie Lovers, 50 Foot Cockroaches Can't Really Exist!
I absolutely adore good B-flick movies, especially those with giant lizards, killer tarantulas, man-eating mole rats, and destructive turtles. However, the biologist in me always has a running commentary in the back of my head when I watch those films, I can't help it. So, just because it's useless fun knowledge, here's why it is impossible for fifty foot cockroaches to exist:
1. Cockroaches are invertebrates, which means that they have an exoskeleton or hard outer skeleton, made out of keratin (similar to what your fingernails and hair are made out of). This type of external structure can only support smaller creatures, because once the creature gets up past about the size of a bread box the exoskeleton materials won't support it, the skeleton would be way too heavy! This is why some of the largest invertebrates only get up to about lobster size. A fifty foot cockroach would literally crush itself to death with its own weight, like a whale on land.
2. As invertebrates cockroaches don't have lungs, they have spiracles, that they breathe with. These are a series of tubes that feed into increasingly smaller channels that line the cockroach's abdomen and allow oxygen to reach their internal tissues and organs. Because of the size and dimensions of a fifty foot cockroach, approximately the width and thickness of a whale or bus, this type of breathing system wouldn't deliver enough oxygen to the roach's body and it would die.
3. Cockroaches don't have blood, they have hemolymph and an open circulatory system. Unlike humans, that have a closed circulatory system of blood flowing through veins and arteries, cockroaches have open channels of hemolymph in their body. Hemolymph is similar to blood in that it carries oxygen and nutrients to the organs and tissues of the roach's body; however, it flows through open straw like structures that lattice throughout the roach's internal body. This open system is very inefficient because oxygen rich and oxygen poor blood mix together. The open circulatory system acts much like a radiator. The hemolymph sloshes around inside the roach, coating the organs and tissues with nutrients. It's a very inefficient system. A fifty foot cockroach would never be able to get enough oxygen and nutrients to its vital organs because of its open circulatory system and inefficient hemolymph.
There are other more esoteric reasons for not having 50 foot cockroaches on Earth, but these are a good start. Hope you've enjoyed this bit of science with fiction, and perhaps now you can rest just a bit easier.