The decorations of the spider crab are a kind of camouflage. Camouflaging anything means making it look so much like its surroundings that it is hard to see.
There are many examples of camouflage in nature. From all these examples people have learned a great deal about camouflage.
One of Shakespeare's plays, Macbeth, tells a story of camouflage. Macbeth thought that he could not be killed in battle because an old prophecy said that he could not be killed "Tul Birnam forest come to Dunsinane." Macbeth, knowing that trees cannot walk, felt safe. But he was not. An enemy army marched through Birnam wood. To camouflage himself each soldier cut down a small tree and held it in front of him. The trees of Birnam wood really seemed to march to Dunsinane, and Macbeth was killed.
Camouflage is especially important in wartime. Most modern armies have camouflage experts.
In World War I many objects were painted to camouflage them. Artists worked out colors and patterns that could be used to make an army truck or a boat or a gun seem to melt into its background. But in World War II camouflage was harder. Special filters were made for cameras so that an enemy, by taking pictures, could pick out objects painted to match the surroundings. Camouflage experts had to go back to the plan of the spider crab. They had to use real plants and sometimes even real animals rather than paint.
