Compartmental Osmotic Pressures in the Rectal Complex of Tenebrio Larvae: Evidence For A Single Tubular Pumping Site

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Abstract
The distribution of osmotic pressures derived from melting points of frozen sections of mealworm rectal complex is described. Animals rapidly frozen during atmospheric absorption show radially increasing values from lumen to Malpighian tubules suggesting that water uptake is driven by a single pump located in the area of highest solute concentration. Volume-concentration relationships of isolated rectal cuticle and perirectal fluid are consistent with their transmitting water passively to an area of lower solvent activity. The rectum of animals frozen with some delay after atmospheric absorption show high osmotic pressures with no radial gradients, a condition which is thought to exist when the rectum is in its faecal dehydration mode. Rectal osmotic pressures in all absorbing animals increased towards the posterior end. Several, presumably extreme posterior tubular, osmotic pressures agree well with the 6.7 osmol.kg−1 value expected for an 88% R.H. threshold for atmospheric absorption.