One of the treats along the stream was a thriving population of damselflies, including American Rubyspots. The photo below shows a rubyspot (right) fending off an aggressive California Dancer.
Notice how the rubyspot flashes the red patches on his wings, which are usually hidden as it rests with its wings closed. At the same time, it arches its abdomen and opens its appendages, which look like the pincers on an earwig. The display generally has the desired effect of warding off the dancer; even so, the dancer usually lands just a short distance away on the same twig to await another chance to get the rubyspot out of his territory.
Here's a closer look at the rubyspot's threat display. Can he actually do harm with those menacing appendages? I wonder.