As we press deeper into the rainforest, further down an undulating path, under an ever-denser, darker canopy, the creepy-cool creatures reveal themselves, one at a time. First, a giant morpho butterfly, its wings flashing bright purple, skimming just above the path, and our heads, fluttering in a rush, like it was late for an appointment. Then, off to the right, a black, spiny-tailed iguana, just hanging out on a tree limb.  The last two are deep in the green, and I start to think we’re in the middle of “Pan’s Labyrinth.” Up high, spotted by our guide, we crane our necks to see a three-toed sloth, lolling on his back, his huge, curved, almost-comical claws above him, gripping the tree. And then—almost too close for comfort, right next to the path—a big boa constrictor, just his head and neck visible, lurking a few feet away, mostly camouflaged by the lush …