Movement of the prey triggers a feeding response. Frogs have been caught on fish hooks baited with red flannel and green frogs ("Rana clamitans") have been found with stomachs full of elm seeds that they had seen floating past. Toads, salamanders and caecilians also use smell to detect prey. This response is mostly secondary because salamanders have been observed to remain stationary near odoriferous prey but only feed if it moves. Cave-dwelling amphibians normally hunt by smell. Some salamanders seem to have learned to recognize immobile prey when it has no smell, even in complete darkness. Amphibians usually swallow food whole but may chew it lightly first to subdue it. They typically have small hinged pedicellate teeth, a feature unique to amphibians. The base and crown of these are composed of dentine separated by an uncalcified layer and they are replaced at intervals. Salamanders, caecilians and some frogs have one or two rows of teeth in both jaws, but some frogs ("Rana spp.") lack teeth in the lower jaw, and toads ("Bufo spp.") have no teeth. In many amphibians there are also vomerine teeth attached to a facial bone in the roof of the mouth. The tiger salamander ("Ambystoma tigrinum") is typical of the frogs and salamanders that hide under cover ready to ambush unwary invertebrates. Others amphibians, such as the "Bufo spp." toads, actively search for prey, while the Argentine horned frog ("Ceratophrys ornata") lures inquisitive prey closer by raising its hind feet over its back and vibrating its yellow toes. Among leaf litter frogs in Panama, frogs that actively hunt prey have narrow mouths and are slim, often brightly coloured and toxic, while ambushers have wide mouths and are broad and well-camouflaged. Caecilians do not flick their tongues, but catch their prey by grabbing it with their slightly backward-pointing teeth. The struggles of the prey and further jaw movements work it inwards and the caecilian usually retreats into its burrow. The subdued prey is gulped down whole. When they are newly hatched, frog larvae feed on the yolk of the egg. When this is exhausted some move on to feed on bacteria, algal crusts, detritus and raspings from submerged plants.