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Concealing Coloration
Concealing coloration means that an animal's or insect's body color evolved to exactly match a leaf or grass blade. This makes it more likely that predators like birds and monkeys will ignore the katydid, since they rely on their eyes in order to hunt prey. For example, male true katydids, or Pterophylla camellifolia, spend most of their lives on one tree, usually an oak or a hickory. Their bodies are the same shade of green as the leaves on those trees. This makes them incredibly hard to spot, even when the males call for a mate.
Disguise
Not only are their bodies colored like the leaves in the bushes or trees where they live, but katydids are shaped like these leaves -- they even grow line patterns that identically resemble leaf veins. Bruce Purser's book "Jungle Bugs" features katydids from Papua New Guinea that mimic curled dead leaves. Using this disguise, the katydids can live on any tree because they perfectly blend in with the tree.
Trade Off
In order for some katydid species, like the true katydid, to look exactly like leaves or other plant parts, many cannot fly and walk slowly and awkwardly. But since they spend most of their lives staying completely still in order to resemble a plant, they usually do not need to fly or run. Some species still can fly, according to "Jungle Bugs." Katydids that live on grass blades can hop thanks to their long hind legs.
Fun Facts
Despite all of the remarkable adaptations that katydids evolved to hide from predators, bats can still zero in on them by listening to the mating calls. Recorded katydid calls are used by scientists to attract bats and thus determine the local bat population. But if katydids detect the wing beats of flying bats, then they immediately stop singing until the bat leaves. Katydids hear ultrasonic frequencies, including the ultrasonic noises produced by bats when they use echolocation in darkness to navigate, according to Hannah ter Hofstede, a researcher at the University of Bristol.