Insect conundrum

Now and then, an ant will appear on my computer table. Not "ants," but "an" ant—single, solitary, and quite large—and which seems to have materialized out of thin air. I never see one anyplace else, just on my computer table. I'll be working away, typing an e-mail or reading someting on the Web, and there it will be, zigzagging slowly and apparently aimlessly around. This morning, when I went over to the computer to check my e-mail, I found another one, this time on the keyboard . . .
As you can see, this is a pretty large ant. I have no idea where this one, or her (his?) comrades, come from. The only ants I ever see outside are the small, Argentine ants, never any the size of this lummox. They move slowly until they are cornered, then they hoof it—this particular one nearly made it into the keyboard before I managed to capture it and squash it (and they don't go quietly, let me tell you).

It's a bit disconcerting when one shows up (and why just one at a time?), as I can't imagine how it gets here or why only here on the computer table (though I'd rather have 'em confined than roaming free-range throughout the house). But with no other information and no working knowledge of ant behavior, it's liable to remain a mystery.