If you have ever stood at the edge of a mountain lake to enjoy a summer sunset, chances are that a bloodthirsty mosquito has enjoyed dining on you, even if you were covered from head to foot in commercially prepared repellents.   Researchers have long known that a person's individual chemical makeup determines their appeal to mosquitoes, but until recently research had been stymied by chemical-identification problems. Since many of the chemicals on our skin evaporate quickly, it's difficult to catch them before they disappear. However, Ulrich Bernier and his colleagues at the United States Department of Agriculture's Mosquito and Fly Research Unit have gained new insights.   The breakthrough came when Bernier and the team came up with a plan to transfer substances found on human skin onto tiny glass beads in order to more easily identify the chemicals. They found hundreds of such substances―some known but many new. After a long series of experiments to identify which combinations of substances attracted or repelled mosquitoes, they determined that a combination of three naturally occurring ingredients―lactic acid, acetone, and dimethyl disulfide―apparently formed a “super attractor” for mosquitoes. None of these substances in isolation attracted mosquitoes nearly as much as the three did in combination. Bernier realized that there was a chance he could create a substance that would prove even more attractive to mosquitoes than human flesh, and which could be used in a mosquito trap.   Equally importantly, researchers were closing on identifying a combination that could turn the normally attractive mixture of substances found on the skin into something that would actually mislead the insect. Painstaking analysis eventually led to a compound that reduced mosquito interest in the once-tempting chemical blend by 96%. This compound was less a repellent than a mask. Bernier smeared his arm with the newly developed compound and found that mosquitoes could no longer sense their target. “It's like their receptors are jammed,” said Bernier. These initial findings are exciting, and will soon lead to the development of commercial products that mask the presence or humans among mosquitoes.   What has researchers even more excited, though, is the prospect that they will someday discover a way to manipulate the body's ability to produce its own chemicals to make the human body effectively invisible to mosquitoes. With this target in mind, researchers are looking into diet and exercise as they relate to the production of chemicals on the skin, genetic determinants such as the density of skin pores, and the bacteria and other microbes commonly found on the skin, since they, too, play a role in attracting or masking.