Big is different

We drag Physics into Halloween.

Last week we mentioned that skeletons are big this year as Halloween decorations.  Not only are there (plastic replicas of) human remains, but dogs, cats, birds and others make their appearance.  Now, no one expects decorations to be biologically correct.  One would not count ribs or vertebrae and write scathing letters to one’s neighbors if they didn’t come out to the right number.  We do think that having ears on dog and cat skeletons is pretty obviously wrong.  But for the purpose of the decorations, which is to induce fear by showing something familiar in a macabre way, ear-bones have their place.  A proper dog or cat skeleton is strange enough to most people that it might be hard to recognize.  There is an actual cat skeleton in the nearby Alexandria Archaeological Museum, and the curators have to assure some visitors that it’s not a dinosaur.

A spider skeleton is different.  Spiders do not have skeletons.  Spiders do not have bones.  Spiders have an external hard covering, made of a material called chitin, and if they die and their soft parts decay they look very much like they do alive.  So the apparent reasoning here (skeletons are scary, spiders are scary, big spiders are scarier, skeletons of big spiders are scariest) is fatally flawed.  A deceased spider would not have a skull-bone attached to a backbone with ribs.

Well, there are no four-foot spiders.  The reason is not historical or biological so much as a matter of Physics, or perhaps Engineering.  The strength of a long shaft, say a spider leg, increases proportionately to its area; but its weight increases proportionately to its volume.  If you make the spider-leg bigger by a straightforward magnification, at some point the chitin is not longer strong enough to bear its own weight, and it breaks.  In a similar way, if you were to simply make spider lungs the same shape but larger size, oxygen would not diffuse as effectively and it would have trouble breathing (if it managed it at all).  To the relief of many people, spiders the size of our decorations don’t exist.

But if they did?  Suppose some mad scientist wanted to make a four-foot spider.  Just changing the scale on the blueprints, as it were, wouldn’t work.  A chitin exoskeleton and scaled-up organs would simply fail.  So it might make sense to go to an internal framework of bones to handle the structural load, with ribs to form a flexible protection for the more elaborate internal organs.  A four-foot spider might indeed leave a vertebrate skeleton.

And that would indeed be scary.