excretion
The Excretory System
The excretory system maintains a constant internal environment by
- Regulating water and ionic balance
- Eliminating nitrogenous wastes
Nitrogenous wastes are eliminated either as
- Ammonia - as in aquatic insects, meat-eating maggots and aphids
- Urea - as in clothes moths (and humans)
- Uric acid - as in most insects
The choice of nitrogenous excretory product is dependent upon the need to conserve water.
Ammonia, NH3, is simple, easy to make but quite toxic. It needs to be dissolved in large quantities of water so is suitable for insects in moist environments.
H : N = 3 : 1 so it requires a lot of water to make.
Urea is moderately toxic and also needs to be eliminated in water, but H : N = 2 : 1 so less water is needed for its manufacture than ammonia.
Uric acid is fairly harmless and insoluble. It crystallizes out of solution and can be excreted as a solid or retained in special body cells in the insect. H : N = 1 : 1 so it requires the least amount of water for its manufacture.
Malapighian Tubules
These are the main excretory organs of the insect body.
They are fine, (one cell thick), lying free within the body cavity or attached to the outside of the gut. They absorb wastes from the haemocoel either by diffusion across a concentration gradient or by active transport.
The number of Malpighian tubules varies and they enter the alimentary canal at the junction of the mid and hind guts.
Faeces
Reabsorption of salts and water occurs in the rectum. The faeces, (or urine if liquid), contains wastes from both the alimentary canal and the Malpighian tubules. The texture is variable depending upon the diet and ranges from a clear liquid in aphids to hard pellets or a dry, powdery material in borers.
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