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.