Deep within the humid, emerald canopy of the Central and South American rainforests, a biological magic trick unfolds every morning. As the sun rises, the Fleischmann’s glass frog ( Hyalinobatrachium fleischmanni ) retreats to the underside of a broad leaf to sleep. To survive the daylight hours when predators are active, this tiny amphibian does something extraordinary: it turns itself almost completely transparent. By fading into the green hue of the foliage, the frog becomes virtually invisible to the birds, snakes, and spiders hunting below. For decades, the exact mechanism behind this vanishing act remained one of biology’s most tantalizing mysteries. While many aquatic creatures use transparency for camouflage, terrestrial animals rarely do, because their bodies are packed with light-absorbing tissues and pigments. Most notably, the red blood cells that carry oxygen throughout a vertebrate’s body are highly visible, absorbing blue and green light and reflecting a stark...