1. The nitrogenous components of the urine are: allantoin, urea, amino acids, kynurenine, and a mucopolysaccharide. The main nitrogenous excretory product is not uric acid, as in other terrestrial insects, but allantoin.
2. The excretion of total nitrogen, allantoin nitrogen, urea nitrogen and amino acid nitrogen have been followed throughout the excretory phase. The concentration of allantoin in the urine is always less than that in the haemolymph, and is a result of the low permeability of the Malpighian tubules to this substance. When isolated Malpighian tubules were used the average U/P ratio was 0.45.
3. Waste nitrogen is also excreted by storage excretion of uric acid in the epidermal cells; the rate of deposition is most rapid during the excretory phase.
4. Low allantoin excretion and a low rate of uric acid deposition in the epidermis during the post-excretory phase suggest that uric acid synthesis is markedly reduced during this phase.
5. Uricase is concentrated in the Malpighian tubules, where it accounts not only for the absence of uric acid in the urine but also for the high concentration of allantoin in the haemolymph.
6. Calculations based on allantoin excretion show that only 28% of the primary tubule fluid is reabsorbed from the rectum. Since allantoin is more soluble than uric acid, it is better suited as a nitrogenous waste substance in such an excretory system, which recycles little water.
7. The method of excreting waste nitrogen is compared with that of other insects.