This algae-covered beech with a narrow cavity in its trunk is one of my slug trees in the park near my house. The cavity, which always has some water in it, is a favorite refuge of the arboreal slug Megapallifera mutabilis. From early spring until the beginning of winter usually at least one and often several slugs can be seen inside the cavity.
Earlier today when I searched the cavity with my flash light, I thought I saw 2 slugs, one above the water across from one entrance and the other sequestered in the 2nd and narrower entrance. A closer look, however, revealed that the thing in the narrow entrance was not a slug but a little froggie partially hidden behind the dried-out corpse of what appears to be a cricket.
If I am not mistaken this is one of the 2 similar tree frogs of eastern North America: gray treefrog (Hyla versicolor) or Cope's gray treefrog (Hyla chrysoscelis). First, I thought it was a juvenile, because I estimated its length to be only about 3 cm long, but now I just learned that these frogs don't grow much larger than that; according to White & White (Amphibians and reptiles of Delmarva, 2002), the usual adult size of either species is 3.3 to 5.1 cm.
This is the 1st time I have seen a tree frog in this park and I hope it won't be the last one.