The Siphonaptera

https://en.wikipedia.org/wiki/The_SiphonapteraBig fleas have little fleas,
Upon their backs to bite 'em,
And little fleas have lesser fleas,
and so, ad infinitum.
And the great fleas, themselves, in turn
Have greater fleas to go on;
While these again have greater still,
And greater still, and so on.
The vermin only teaze and pinch
Their foes superior by an inch.
So, naturalists observe, a flea
Has smaller fleas that on him prey;
And these have smaller still to bite 'em,
And so proceed ad infinitum.:
Big whorls have little whorls
That feed on their velocity;
And little whorls have lesser whorls
And so on to viscosity.
Big criteria have little criteria,
Upon their backs to bite ‘em;
And little criteria, have littler still,
And so on ad infinitum.