The flaming fall displays of sugar maples and other deciduous trees may have a function beyond luring tourists. Two British scientists believe that the spectacular red and yellow leaves are actually a warning signal to insect pests that typically attack the trees in autumn. The trees are essentially alerting the pests that they face a variety of countermeasures--from thickening leaves to chemical defenses--if they attack. The scientists base their hypothesis on their finding that the trees with the brightest displays are the most likely targets of sap-eating aphids. "In short," says Sam Brown, one of the experts, "they are pick on someone else' signals to specialist autumn-flying insects."