Most insects receive insufficient proteins and amino acids in their diet. Blood feeding insects have the opposite problem: too much protein. Terrestrial insects typically process excess amino acid nitrogen into uric acid, which can be excreted with little loss of water. Uric acid has low water solubility and is less toxic to insects than other nitrogen containing molecules such as ammonia.
The female tsetse fly, Glossina morsitans, the vector of African sleeping sickness feeds on the blood of livestock. It excretes up to 50% of the weight of its blood meal as uric acid and other waste products. The female tsetse fly has an overabundance of amino acids and is deficient in other dietary components. Sufficient blood is ingested to meet the other dietary requiements and the diet balanced by excreting uric acid. Although the tsetse fly is wasteful of protein and amino acids, those are easily obtained. Eggs can be produced more efficiently by an overeating, wasteful lifestyle.