Stable carbon isotope ratios in Periplaneta americana L., the American cockroach

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Abstract
The distribution of stable carbon isotopes in carbohydrate, lipid, and protein of Periplaneta americana was studied in insects raised on two different diets. Cockroaches that received a sucrose supplement in their diet had chitin and lipids enriched in 13C compared with those insects not given sucrose. Sucrose has a high 13C content relative to the other dietary constituents and enrichment was due to the incorporation of sucrose carbon into these body fractions. The isotope ratios of protein and glycogen from insects raised on a low carbohydrate diet differed significantly from those fed sucrose. Respiratory CO2 collected from insects receiving sucrose was more enriched in 13C than that from insects on the other diet, reflecting the source of metabolic fuel. Starved insects showed a general decline in the heavier isotope in respired carbon dioxide, as lipids were used as an energy source. Cockroaches that were injected with corpus cardiacum extract responded by respiring CO2 depleted in 13C, a result similar to those induced by starvation. This result is consistent with experiments demonstrating a switch from carbohydrate to lipid oxidation caused by corpus cardiacum extract.

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