A whole bunch of these were spotted at the Claremont Colleges' Bernard Field Station in an artificial vernal pool constructed to facilitate reproduction of Western Toads (Anaxyrus boreas). The critters were preying on the toad tadpoles, and, in fact, they ate almost all of them this year. I think they are dragonfly nymphs. Can someone confirm? Is it possible to come up with a more specific ID? A list of dragonflies we've observed at the field station can be seen here.