The contention of Maddrell (1981) that the rate of fluid secretion by insect Malpighian tubules is much slower than the rate of primary urine production (glomerular filtration rate) of vertebrates is valid only when the comparison is made with mammals and birds. Fishes and reptiles have filtration rates similar to or even slower than the rates observed in insects.

Calculations of the concentration or dilution work involved in primary urine production show that the (weight-specific) energy requirement of the insect excretory system is similar to or surpasses that reported for reptilian, and even mammalian, kidneys.

Because the energy demand of any excretory system is small when compared to the resting metabolism of the entire organism, it is likely that selection pressures acted less on a reduction of energy expenditure for excretion and ionic regulation than on the adaptations to the demands of specific environments and life styles, leading to diversified specializations of excretory (and regulatory) mechanisms.

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