Evil flies
That's what Met catalog says about the painting, very justly: The apples and fly are symbols of sin and evil and are opposed to the cucumber and goldfinch, symbols of redemption. http://www.metmuseum.org/toah/hd/sacr/hod_49.7.5.htm

I would add the crack at the right bottom to these symbols (for death). The fly completes the set of four symbolic objects contrasting the purity of baby Jesus and his Mother (the bird might be the Spirit rather than redemption). I am not sure about cucumber as a positive symbol, though. Quite often

...the cucumber is a symbol for the human sin that did not affect the Immaculate Virgin (based on an interpretation of Isaiah 1:8: "And the daughter of Zion is left...as a lodge in a garden of cucumbers, as a besieged city.") In De universe Hrabanus Maurus states the cucumber symbolized lust, because the Jews in the desert "preferred cucumbers to the manna sent from Heaven."

Alternatively, it contrasts the apple in the same way a Karpas does at the Passover (the fruit of the underground).The talisman and optical illusion theories are almost certainly incorrect.