Investigating chemotaxis by Drosophila larvae. (a) Louis et al.  generated a well-structured airborne odor concentration gradient by suspending droplets of odorant at different concentrations from the ceiling of their test chamber (yellow denotes the highest concentration; black the lowest). The arrangement of droplets generated a spatial concentration distribution that varies from one end of the chamber to the other and from the middle of the chamber (high) to the sides. (b) Both unilateral and bilateral transgenic larvae navigate odor fields by detecting local concentration gradients. By moving along the direction of the steepest intensity variation, the larvae reliably locate the source of the odor.