Apparent Transport of Water by Insect Excretory Systems

Abstract

Insects are capable of producing strongly hyperosmotic urine but most species do not possess the anatomical equivalent of the mammalian kidney's couiitercurrent system. Concentration of the excreta occurs in the rectum where water is absorbed against increasing osmotic gradients without strict dependence on simultaneous absorption of solute. Properties of this process are reviewed. It is currently postulated that this apparent transport of water is driven by local transport and recycling of solute within the lateral intercellular spaces of the epithelium of the rectal pad. The most concentrated excreta so far reported are those of the mealworm, Tenebrio molitor. This species possesses a cryptonephridial complex in which the posterior end of the malpighian tubules is closely applied to the rectum and both are enclosed within a complex membranous sheath. Active transport of potassium chloride by the malpighian tubules into the complex creates a local high osmotic pressure within the complex which is responsible, in part if not completely, for removal of water from the rectal lumen. This system bears some resemblance to the countercurrent system of the mammalian kidney.