Urate Management

Nitrogen excretion in cockroaches is a complex phenomenon that differs from the expected terrestrial insect pattern of producing and voiding uric acid. Several different patterns are apparent. The majority of species studied (thus far > 80) do not void uric acid to the exterior even though they may produce it in abundance (Cochran, 1985). When cockroaches are placed on a diet high in nitrogen, urates accumulate in their fat body (Mullins and Cochran, 1975a); they are typically deposited in concentric rings around a central matrix in storage cells (uro-cytes) adjacent to bacteriocytes (Cochran, 1985) (Fig. 5.6B). When the diet is deficient in nitrogen or individual nitrogen requirements increase, bacteroids mobilize the urate stores for reuse by the host, and the fat body deposits become depleted. Uric acid storage thus varies directly with the level of dietary nitrogen and is not excreted under any conditions. Even when fed extremely high lev els of dietary nitrogen, American and German cockroaches continue to produce and store uric acid in the fat body and other tissues, ultimately leading to their death (Haydak, 1953; Mullins and Cochran, 1975a). At least three other patterns of urate excretion are found in the family Blattellidae. In the Pseudophyllodromiinae, the genera Euphyllodromia, Nahublattella, Imblattella, and probably Riatia sparingly void urate-containing pellets, with urates constituting 0.5-3.0% of total excreta by weight (Cochran, 1981). Feeding experiments showed that high-nitrogen diets did not change urate output in Nahublattella nahua, but did increase it in N. fraterna in a dose-dependent manner. In both cases diets high in nitrogen content led to high mortality. The genus Isch-noptera (Blattellinae) excretes a small amount of urates (2% by weight) mixed with fecal material; this pattern is similar to that of other generalized orthopteroid insects, except for the very small amount of urates voided (Cochran and Mullins, 1982; Cochran, 1985).

The most sophisticated pattern of nitrogen excretion occurs in at least nine species in the Blattellinae (Par-coblatta, Symploce, Paratemnopteryx), which void discrete, formed pellets high in urate content. These pellets are distinct from fecal waste (Fig. 5.8), suggesting that the packaging does not occur by chance. The cockroaches store urates internally as well (Cochran, 1979a). The level of dietary nitrogen in relation to metabolic demand for nitrogen is the controlling factor in whether uric acid is voided (Cochran, 1981; Cochran and Mullins, 1982; Lembke and Cochran, 1990). This is nicely illustrated in Fig. 5.9, which shows urate pellet excretion in female Par-coblatta fulvescens on different diets over the course of a reproductive cycle. Excreted urate pellets serve as a type of external nitrogen storage system, which may be accessed either by the excretor or by other members of the social group in these gregarious species. Reproducing females have been observed consuming the urate pellets, and they do so primarily when they are on a low-nitrogen, high-carbohydrate diet. A female carrying an egg case was even observed eating one, although they do not normally feed at this time. This system allows the cockroaches to deal very efficiently with foods that vary widely in nitrogen content. High nitrogen levels? The cockroaches store urates up to a certain level, and beyond that they excrete it in the form of pellets. Nitrogen limited? They mobilize and use their urate fat body reserves. Nitrogen depleted? They scavenge for high-nitrogen foods, including bird droppings and the urate pellets of con-specifics. Nitrogen unavailable? They slow or stop reproduction or development until it can be found (Cochran, 1986b; Lembke and Cochran, 1990).

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