What does the first apparition in Act Four, scene one acutally signify in Shakespeare's Macbeth?

The witches' predictions play a large role in
Shakespeare's Macbeth. They are, of course, what lure Macbeth (and
Lady Macbeth) in murdering Duncan in the first place. However, as Macbeth becomes more
paranoid, he seeks out the witches again for a new set of
predictions.

There are several important elements here that
lead directly to Macbeth's downfall. Banquo has already
asked...

What,
can the devil speak true?
(I.iii.113)

The Elizabethans
thought it was impossible for the devil to tell the truth. What is Macbeth thinking in
believing anything that the servants of a darker power tell him? Secondly, he believes
he can order them about—what, a mere mortal, trying to give orders
to the devil's servants? Again, this is foolishness on Macbeth's part—he is partly
blinded by his ambition and partly by his own elevated sense of power. Finally, Hecate
has already made arrangements for this next set of predictions to
trick Macbeth into delivering himself into circumstances that will lead to his
destruction—by appealing to the false sense of security that the second set of
prophecies will create in Macbeth.

The first apparition
that accompanies the first prediction in Act Four is the hardest (I think) to
understand. I see this as a "throw away" line—something that really isn't that
important. With the first set of predictions (in Act One), the Weird Sisters greeted
Macbeth by his name: this was meaningless, except to perhaps give him the sense that
they had the power to magically know who he was. With this new set
of predictions, they do the same: Macbeth already knows he has to
be careful of Macduff.

The armed head refers to Macduff as
a soldier, a worthy opponent—the man who will end his life (though he has no idea of
this by seeing the apparition).

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This apparition foreshadows Macduff's leading
role in the attack on Macbeth, and perhaps also Macbeth's beheading by
Macduff.

Perhaps the
apparition is something that Macbeth ignores, as he seems not to understand or be too
worried about the second and third apparitions either. Macbeth hears and sees what he
wants to hear and see. This is where the witches begin to tighten the noose around
Macbeth's neck.