Insects are a very successful group of animals, with a remarkable ability to conserve water so that they can be found in the hottest and driest places on earth. Their ability to conserve water can be attributed to several factors:
• their outer surface is waterproof because of a layer of wax;
• the spiracles prevent water loss from the gaseous exchange surface which is inside the body; and
• they have an extremely efficient excretory system.
The excretory organs are the malpighian tubules. They are found between the midgut and the rectum. One end of each tubules opens into the gut, while the other free end floats in the haemocoel.
Nitrogenous waste products and water which are librated into the haemocoel are absorbed at the distal end of the malpighian tubule. The nitrogenous waste is converted to uric acid as it passes along the malpighian tubule towards the gut. A lot of water Is also reabsorbed so that by the time the uric acid reaches the proximal end of the malpighian tubule it is changed to solid crystals. In the hindgut, more water is reabsorbed by the rectal gland. Thus, the urine which eventually leaves the body is very concentrated, almost a dry solid.
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