This is another maple flower, like the one shown HERE and (much closer) HERE.
What's different about the current flower is that the anther is starting to spill its pollen.
I'm unclear about exactly how this tree is pollinated. At http://www.aboutmapletrees.com/, I read that
The large stigmas of these flowers seems consistent with wind pollination, but somehow this pollen doesn't seem like the sort of stuff that's going to blow around very easily.The trees are self-pollinating, and some of the clusters of flowers are even able to pollinate themselves. The most common form of pollination comes from the wind, but flies and other insects will occasionally cause cross-pollination.
Maybe I shouldn't be too surprised, though, that things seem a bit confused. If I recall correctly, one of the features that made this particular tree attractive as a yard planting was that it does not produce large numbers of fertile seeds. Where the infertility comes from, I don't know. Perhaps another member does?
Hope you enjoy the pictures. These flowers look so "blah" from a distance, you'd never guess that they look like this up close!
Technical: Canon 300D with Nikon CF N Plan Achro 10X NA 0.30 objective at 10X. Electronic flash, 105 frames in 0.025" (0.00025" average step). Subject size as shown, about 1.5 mm square.