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Buffalo-bur flowers are buzz pollinated.  Beyond that, they have developed two sizes of anthers.  This is a strategy to defeat pollen theives that do not also fertilize the flowers.  It results in a complex interaction between anther length and insect size that insures the pollinators get some of the pollen.  One author even found cause to wax poetic (http://thepoetryofscience.scienceblog.com/163/the-pollen-thieves/). In the image, you can see four shorter anthers, and one longer.  I initially mistook the longer one for a pistil.

Buffalo bur is pretty but nasty.  The spines are very painful, leaving behind a couple of poisonous chemicals.  The entire plant is poisonous and is listed as a nasty weed.