3.7.2. Nitrogen excretion
Many predatory, blood-feeding and even plant-feeding insects ingest nitrogen, particularly certain amino acids, far in excess of requirements. Most insects excrete nitrogenous metabolic wastes at some or all stages of their life, although some nitrogen is stored in the fat body or as proteins in the hemolymph in some insects. Many aquatic insects and some flesh-feeding flies excrete large amounts of ammonia, whereas in terrestrial insects wastes generally consist of uric acid and/or certain of its salts (urates), often in combination with urea, pteridines, certain amino acids, and/or relatives of uric acid, such as hypoxanthine, allantoin, and allantoic acid. Amongst these waste compounds, ammonia is relatively toxic and usually must be excreted as a dilute solution, or else rapidly volatilized from the cuticle or feces (as in cockroaches). Urea is less toxic but more soluble, requiring much water for its elimination. Uric acid and urates require less water for synthesis than either ammonia or urea (Fig. 3.19), are non-toxic and, having low solubility in water (at least in acidic conditions), can be excreted essentially dry, without causing osmotic problems. Waste dilution can be achieved easily by aquatic insects, but water conservation is essential for terrestrial insects and uric acid excretion (uricotelism) is highly advantageous.
Deposition of urates in specific cells of the fat body (section 3.6.4) was viewed as “excretion” by storage of uric acid. However, it might constitute a metabolic store for recycling by the insect, perhaps with the assistance of symbiotic microorganisms, as in cockroaches that house bacteria in their fat body. These cockroaches, including P. americana, do not excrete uric acid in the feces even if fed a high-nitrogen diet but do produce large quantities of internally stored urates.
By-products of feeding and metabolism need not be excreted as waste — for example, the antifeedant defensive compounds of plants may be sequestered directly or may form the biochemical base for synthesis of chemicals used in communication (Chapter 4) including warning and defense. White-pigmented uric acid derivatives color the epidermis of some insects and provide the white in the wing scales of certain butterflies (Lepidoptera: Pieridae).
The high N : H ratio of uric acid relative to both ammonia and urea means that less water is used for uric acid synthesis (as hydrogen atoms are derived ultimately from water).