"The Siphonaptera" is a nursery rhyme, sometimes referred to as Fleas.
Big fleas have little fleas,
Upon their backs to bite 'em,
And little fleas have lesser fleas,
and so, ad infinitum.
Sometimes a second verse appears, with lines such as
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 rhyme is seemingly related to lines by Jonathan Swift from his long satirical poem "On Poetry: a Rhapsody" (1733):
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.
Lewis F. Richardson adapted the poem to meteorology, specifically discussing fractal wind patterns:
Big whorls have little whorls
That feed on their velocity;
And little whorls have lesser whorls
And so on to viscosity.
Gene V. Glass quoted an anonymous adaptation of the poem to educational measurement:
Big criteria have little criteria,
Upon their backs to bite ‘em;
And little criteria, have littler still,
And so on ad infinitum.