The rectal complex of the mealwormTenebrio molitor, L. (Coleoptera, Tenebrionidae)

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Abstract
The rectal complex of the mealworm is an example of the ‘cryptonephric’ condition, wherein the distal ends of the Malpighian tubules are closely applied to the rectum and enclosed with it in a special chamber, the perinephric space, which is separated from the rest of the body cavity by the perinephric membrane. Wigglesworth has suggested that this arrangement serves to assist in the removal of water from the faeces by the rectal epithelium, but its physiological mechanism has remained virtually unexplored. In the present work the ability of the rectal complex to remove water from the faeces has been quantified and it is shown that the faecal pellets are in equilibrium with an atmosphere of 90 % relative humidity (average) and sometimes of as low as 75 % relative humidity. The concentration of the perinephric fluid is greater than that of the haemolymph (first observed by Saini); when the insect is deprived of water the freezing-point depression of the perinephric fluid may reach 8 °C at the posterior end of the rectal complex, and is mainly due to some nonelectrolyte. The freezing-point depression of the tubular fluid from the perirectal tubules is always very close to that of the perinephric fluid and can be almost completely accounted for as potassium chloride which may reach a concentration of over 2 m . The perinephric membrane is relatively impermeable to water. Water injected into the rectal lumen is quickly absorbed into the perinephric space and eliminated via the tubules. It is suggested that the physiological mechanism of the rectal complex is as follows. In water-deprivation the concentration of the haemolymph rises and this activates the rectal complex. Activation involves the secretion of potassium chloride from the haemolymph into the perirectal tubules (either directly or indirectly) whereby water is passively drawn into the tubules from the perinephric fluid. The concentration of the perinephric fluid is thereby increased and the work required to be done by the rectal epithelium in removing water from the faeces is decreased.