I found these brightly colored insects tucked under a log in Arizona, where they appeared to be feeding on a growth of fluffy white material–perhaps fungi or mold. These are immature planthoppers, members of the same group of tiny hopping green specks you sometimes run into in lawns or trees. This was, for me, a pretty surprising place to encounter a hopper, so I looked around to see what else I could find out about these little fellows.
The lovely community over at BugGuide has tentatively identified these as members of the family Derbidae, based at least partially on the context I discovered them in. While adult derbid hoppers, like most planthoppers, are sap suckers, the nymphs of some species feed on fungi, particularly in rotten logs.
Because these hoppers are immatures, their wings have not yet developed (you can see the developing wing buds on their backs). Adult derbids typically have long, delicate wings for hoppers. In fact, they are noted for gathering to perch under broad leaves, a behavior which may protect their fragile wings (U Del).