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.