Malpighian tubules in Insects

• Malpighian tubules:-
Malpighian tubules are numerous slender, thread like, yellow coloured structures. These are the principal excretory & osmoregulatory organs in insects.
• Position:-
 They are attached to the alimentary canal at the junction of midgut & hindgut.
• Origin:-
Malpighian tubules are ectodermal in origin.
•  Structure:-
The outer layer of the malpighian tubule which is in contact with the haemolymph is composed of thin, elastic, connective tissue & muscle fibres.
The malpighian tubule lumen is lined by cuboidal epithelial cells.
There are 2 distinct regions in each tubule –
A) Distal blind secretory region: - It hangs freely in the haemocoel. The inner cells lining the distal region have well developed brush border.
B) Proximal absorptive region:- It opens into the gut. The inner cells lining this region are less differentiated & have honey comb border.


•  Physiology of excretion:-
       Insects produce nitrogenous waste in the form of potassium urate which is liberated into the haemolymph.
 This along with water is taken up by the distal region of the malpighian tubule.
In the cells of the tubule potassium urate reacts with H2O & CO2 to form potassium bicarbonate & uric acid.
 Potassium bicarbonate is absorbed back into the haemolymph but uric acid is left out in the lumen.                              
 As the uric acid in dissolved condition moves into the proximal region of the malpighian tubule, the H2O is reabsorbed. [ Reabsorption of H2O occurs to such an extent that the basal part of the proximal region becomes filled with solid crystals of uric acid.] Resorption of water further takes place in the rectum.


• Importance:-
Thus the malpighian tubules function excretory as well as osmoregulatory as they not only helps by excreting nitrogenous wastes but also in conserving water in proper amount. This has helped insects in leading effective life activities in terrestrial environment.