The back of the leaves of a volunteer sunflower is filled with these things. They move about, so I assume they are some kind of insect.
Anybody have any idea what they might be. I noticed ants all around the leaves as well, but this doesn't look look like an aphid.
These are treehoppers (family Membracidae). They are sap feeders, they do not appear to be serious pests - http://digital.library.okstate.edu/oas/oas_htm_files/v65/p65frames.html
