All living things produce waste chemicals that have to be removed from the body so that they do not build up and poison them. This waste is not to be confused with the undigested solid food material which passes out of the body in the droppings or dung.

So in humans the bloodstream picks up waste products from all over the body and delivers them to the Kidneys which filter the blood and send out the waste material in the urine. The Liver is also concerned with making safe poisonous chemicals in the blood before they are taken to the kidneys.

Insects do not have arteries and veins like us. Their body cavities are filled with a liquid like blood (though it is usually see-through, not red). The waste chemicals are filtered out of this body fluid by special organs near the tail and added to the droppings before they are passed out of the insect’s body. In crabs which are Crustaceans and closely related to insects, the ‘kidney’ organs are sometimes called ‘dead man’s fingers’ and have to be separated from the crab meat in cooking as they taste very bitter and can be poisonous.