Alternative chambers are used in laboratory studies of insect behaviour to determine responses to divergent pairs of temperatures, humidities, or light intensities. Theoretically, they provide an efficient means of presenting two different uniform intensities of a physical factor in equal areas that are separated only by a narrow, central boundary zone. In practice. however, only dark-light chambers approach this theoretical ideal. Jn temperature or humidity chambers, the boundary gradient often varies in width and steepness at different points, and the two halves may acquire unpredictably vagrant gradients which change in slope and extent with passing time.