Jimscott: Maybe your cats have springtails
kentosaurs: sounds like you have springtails
Of course you can always try to make a flea trap the either Peter or Barry talk about. At night put Pinguicula and Drosera on a tray under a very low wattage incandescent light. A night light perhaps 5-10 watts. The fleas are attracted to the light and heat of the bulb and get trapped.