Two types of concretions in Drosophila Malpighian tubules as revealed by X-ray microanalysis: A study on urine formation

In the Malpighian tubules of Drosophila two types of luminal concretions are formed. Type-I concretions mainly accumulate high quantities of calcium and magnesium in a matrix of proteoglycans; type-II concretions accumulate potassium. Higher quantities of organic matrix are found in type-II concretions than in type I concretions. The latter are stored in the distal segment of the anterior Malpighian tubules while type-II concretions are formed between apical microvilli in the transitional and the middle segment of anterior tubules and in the distal and the middle segment of posterior tubules. In the tubules of the pupa and in the first meconium to be produced, the magnesium and calcium contents of excreted type-I concretions are similar. The outer envelope of the puparium exhibits only traces of calcium in contrast to the cuticle of the larva. Both types of concretions are products of excretion: they eliminate calcium and magnesium or potassium, respectively. During winter time and under normal photoperiods both types of concretions can join together to form large “biocrystals” inhibiting the flow of urine. Concretion formation is discussed as an economical method to excrete high amounts of cations in a condensed form with the loss of little water.