Animals that perspire (humans, apes, monkeys, horses, ---) and eat plants with high sulfur concentrations produce their own insect repellent. Horses are often fed sulfur for that purpose. All types of wild onions, including garlic, are very high in the element. For hundreds of years rural folks ate sulfur powder mixed with various liquids during the summer to offer themselves protection against "skeeters", ticks, chiggers and horse and deer flies. Maybe the typical sulfide smell associated with the reclusive forest primates is a result of a diet of high sulfur plants.