Blowing up a beetle 2: the bones

We’re back with giant bugs and how we can get them. Last time we talked about the biggest problem with giant arthropods: their circulatory system. Today, it’s on to the bones.

First of all, forget that old saw about exoskeletons being inherently unworkable at large sizes due to the square-cube-law. Yes, the square-cube law is a problem when dealing with insects’ open circulatory systems, but we solved that already.

In terms of strength, chitin sheaths around legs (what bugs already have) works fine.  Do the math, and you’ll find that a beetle-leg scaled up to the dimensions of one of my legs ( 100cm long by 20 cm in diameter) will have a exoskeleton about 0.6cm thick, which about the same mass and a quarter of the thickness of the bones in my leg. That’s not bad, especially considering the fun you could have with air pockets, different materials, and the exact shape of the bone in question. I’m confident exoskeletal legs will work, at least for an animal of my size.

The real problem is that an exoskeleton must be shed as the animal inside it grows. Imagine a lion-sized arthropod molting and going from armored battle-demon to squishy pink lump. It might not be able to support the weight of its own organs, let alone run and pursue prey.

There are ways to solve the problem. Dig a hole and hide in it while soft. Immerse yourself in supportive water. Build a “mobile cocoon” out of the old cast-off exoskeleton and silk. Or just have the skeleton grow with you.

Sea-urchins have exoskeletons too, but theirs are made of hexagonal plates that can be separated and the interstices filled with an intermediary material (in this case collagen) that later toughens into the necessary hardness and rigidity (in this case calcium carbonate). The bones of our skull (which are exoskeletons, in a way) work the same way. The difference is that we also have specialized cells (osteoclasts) than can destroy old bone as well as create it (osteoblasts), so even once the plates have met to form a skull, the whole thing can continue to grow as old bone is subtracted from the inside and added to the outside.

Don’t like that idea? You can break the exoskeleton up into scales, which lock edge-to-edge like puzzle-pieces, and can be lost and regrown one-at-a-time like shark teeth without sacrificing structural integrity (bonus: video-game-boss weak spots!). Muscles that are anchored to areas with no shell-scale won’t have any leverage and will be useless until the new shell hardens.  The animal will have to change its behavior, either getting help from its conspecifics or building a temporary crutch for itself out of found materials (wood? old scales spun into silk?). Either that, or muscle-anchoring scales remain un-shed, built into large, dead structures as the animal grows, like the rattle of a rattle-snake.

There is an additional problem, raised by Sam Brougher here, that the joints on an exoskeletal limb can only be pin-joints, like door hinges, because any other kind of joint (for example the ball-and-socket joint in your thumb) would require the hard surface on the inside, which is sort of the opposite of an exoskeleton. And he’s right. I don’t see how to get other kinds of joints (unless the lower part is composed of nonliving, shed shell?). So that is a real limitation on the kinds of things your giant bugs can do.

But if we got rid of all their limitations, they wouldn’t be bugs any more, now would they?

 

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