Observations on the osmotic pressure measurement in cryptonephric insects

Osmotic pressure of the haemolymph, perirectal fluid and of the rectal fluid of four "Species of cryptonephric insects (Tenebrio, Dermestes, Galleria and Pieris) has been measured. In all cases the osmotic pressure of the rectal fluid is much higher than that of the haemolymph and the osmotic pressure of the perirectal fluid is intermediate between these two. The cryptonephric Malpighian tubules increase the osmotic pressure in the perirectal fluid. Consequently between the blood and the rectum there is a chamber which has an osmotic pressure higher than that of the blood but lower than that of the rectum. This higher osmotic pressure in the perinephric chamber enables the modified epithelium to do less work in the absorption of water and nutrient salts from the faecal waste.