Commenter Marella asked a question based on the post about scorpions just below:

What I’d like explained is why arachnids don’t have necks. Everyone else has one and finds it useful, but arachnids have to move their entire bodies to look around.

While having my walk, the answer suddenly came to me—in the form of a limerick. Or rather, the answer came to me as it would be given by a molecular biologist who denies the ubiquity of natural selection (e.g., two posts down). So:

An anti-selectionist lout
Was asked why all spiders are stout:
“Why, the gene for the neck
Is deleted, by heck,
And that’s why they can’t look about!”

And a related one just came to me in the shower (I swear: soon I’ll start dreaming about monkeys holding each other’s tails):

“The giraffe,” said this colleague last fall,
“Causes me no amazement at all;
“Why the gene for the neck
“Is repeated, by heck:
“And that’s why the damn thing’s so tall!”