Blowflies shed mercury at maturity.

If an old lady swallows a blowfly, she may be better off ingesting an adult insect than an immature one. That's one implication of the discovery that blowflies, which can absorb mercury from fish carcasses that they feed on as larvae, rid themselves of much of the toxic metal when they develop into adults.

Canadian and French researchers raised blowflies on four mercury-containing trout carcasses and collected some flies for testing at each developmental stage, from egg to adult. Average mercury concentrations in the flies rose steadily as development progressed, reaching a maximum value of about 160 nanograms per gram in the pupal stage. It then dropped to about 50 ng/g in adults. Nearly all methylmercury, the chemical form most toxic to people, disappeared from the flies in that transition.

The flies seem to readily excrete the metal once they reach maturity, Marc Amyot of the University of Montreal in Quebec and his colleagues say in the March Environmental Toxicology and Chemistry.

Children's songs aside, the varying concentrations of mercury in the flies suggest that animals that feed on adult flies are at less risk of poisoning than are, for example, juvenile salmon and certain birds that often eat larvae.--B. H.