Water is essential to life, yet water sensing remains poorly understood. Surprisingly, we discovered that C. elegans in a sealed dish can robustly localize toward external water sources at least 8 cm away. With 302 neurons and no visual system, C. elegans were long assumed to lack any complex sensory representation of their broader environment. Remote water sensing is influenced by the evaporation rate of the water source, which affects its temperature, and it also requires the thermo-sensory neuron AFD. AFD possesses a unique “antenna-like” sensory ending that is required for remote water sensing. Treating this structure as a putative antenna and applying structure/function principles from electronics predicts sensitivity to ∼10–16 μm or ∼20–30 μm radiation, in the far-infrared/low terahertz region of the electromagnetic spectrum. We used a series of optical tools to manipulate these wavelengths and found that worms seek lower ∼10 μm infrared and higher ∼24–30 μm infrared, suggesting a preferred ratio of signals. Infrared radiation comes from thermal motion of charges and is closely tied to temperature. However, real-world materials (not theoretical blackbodies) have different IR spectra at the same temperature due to differences in their internal charge structure (the basis of IR spectroscopy). Based on published data, the ∼10/∼20 μm ratio could be particularly useful for distinguishing water vapor from liquid. Finally, we find that animals only attend to infrared signals when evaporation in their immediate environment is changing. Evaporation information is encoded by a set of low-frequency voltage and magnetic field changes measurable at the agar surface, presumably due to evaporation-driven ion flow. Together, these findings describe a logically coherent sensory system that promotes water seeking when animals are most at risk of desiccation.
Persson et al. (Sun,) studied this question.