Insects try to protect themselves from their enemies in many different ways. Some insects like wasps and ants have powerful stings or shower poisonous fluid on the enemies.
The housefly doesn’t sting, but since its colour resembles a wasp or a bee, its enemies are wary of it. Stick insects use camouflage. They look like the leaves and twigs among which they feed. The bright colours of some insects warn their enemies that they may be poisonous.
Below Stick Insects info from Wikipedia (Read More):-
The Phasmatodea (sometimes called Phasmida) are an order of insects, whose members are variously known as stick insects (in Europe and Australasia), walking sticks or stick-bugs (in the United States), phasmids, ghost insects and leaf insects (generally the family Phylliidae). The ordinal name is derived from the Greek "phasma" meaning an apparition or phantom, and refers to the resemblance of many species to sticks or leaves. Their natural camouflage can make them extremely difficult to spot.
http://en.wikipedia.org/wiki/Stick_insects
