The newspaper OGlobo recently reported a discovery in the state of Espirito Santo, the aptly named Flea Frog ("Sapo pulga"), or Brachycephalus didactylus, the worlds smallest frog. It has actually been found before near Rio, but this is the first siting elsewhere in the Mata Atlantica. Of course, just because it isn't seen, that doesn't mean it isn't there. After all, Flea frogs are a) very, very, small, b) they live hidden in leaf litter on the forest floor, c) they are camouflaged, and d) they only move around at night.
Obviously baby Flea frogs are even smaller, but not that small, females only lay one egg at a time, rather than the 1,000+ the European Common frog for example. However, like Common frogs, Flea frogs eat more or less any invertebrates they can catch, which means very small ones, like mites or springtails.
If you would like to see the original report (in Portuguese) it can be found here...