Hello Ton;

It might be helpful to reframe the question with a couple of anecdotal observations.

1. Many ants are highly sensitive to humidity gradients.

2. Sand-dwelling roaches in the sand dunes of southern California are also very sensitive to humidity gradients.

Demonstrating these sensitivities are straightforward in the lab. I'm not aware of any papers that describe a mechanism. There's a project!

Best regards, Jim Des Lauriers

It is an interesting question. Humans have water 'taste buds' near the back of their tongue, so it makes sense there may be a similar receptor for insects (and other invertebrates). The ability of insects to follow a pheromone gradient while flying would be similar to following a humidity gradient, and I think James suggestion that a gradient would function as a means of smelling water makes sense. Spiders are also sensitive to humidity gradients.

I've wondered this quite a bit as well. It would seem to be relatively easy to test. Certainly, this would be a much more useful cue than light for nocturnal insects than would light for locating water bodies. And since many insects move between water bodies, and so must relocate them, moving toward gradients of water concentrations would be more feasible than than using light reflections which would be largely unavailable except at close proximity

I agree that 'smelling' water via humidity gradients would be very useful for nocturnal insects. The question of light might be more complicated. Sea Turtle hatchlings are believed to be able to detect and orient to the lighter sky over the ocean, so vision might play a role in insects finding lakes, and possibly even rivers as the light from the sky would reflect more from a stream surface than from the soil/litter.

We have here a plethora of possible cues and evidently no water-detecting sensor among arthropods on offer. Is there a paper that demonstrated in the lab that an arthropod can orient in a humidity gradient? If not, there is the first simple study to do. Ants would provide an excellent taxon to study...or, perhaps, Hydrophilid beetles. Answering that question at least would reduce the number of candidate cues...Ton's initial question was about a water sensor.

Cheers, Jim Des Lauriers

Thank you all for your reply. I was just curious. I work in a lab were we do not perform such studies. In The Netherlands that would be a typical study for a PhD or so, not in any private institute, such as ours. I do thank that it may not be the water itself, for pure H2O is without smell but perhaps the arthropods do smell anything else dissolved in water, such as sulfur or any other chemical component that are released during degradation processes. Maybe they can even smell fungi feeding on decomposing organic material, but that is just a thought. Hydrophilidae beetles would also be a good study case because some species are able to find the smallest of puddles. My discussion with a colleague was on Trichoptera. If any of you do ever find such a paper on this subject please let me know. You're welcome to do such a study yourself of course

best wishes from Amsterdam

I agree that both Hydrophilidae and Dytiscidae would be good for a case study because of their ability to find very small water bodies and aquatic habitats without open water at the surface (bogs with a high cover of mosses, marshes etc.) and easily locate small temporary puddles in densly forested areas. Many species disperse at night, also when heavily clouded. I cannot imagine that there are no published studies on this, but a quick search on google only reveals studies where they focus on sight rather then any other senses. A recent study on mushroom bodies, complex nervous centers of the insect brain, in Dytiscidae ( DOI: 10.1134/S0013873814050029) could maybe provide a different angle...

I think amphibians are another taxa that almost certainly can smell water. They must navigate often long distances from one water body to another across dry areas. They die if they don't reach another moist habitat, and this is a big problem for species in dry habitats in like in the western united states like the Columbia Spotted Frog which will travel very far across cliffs and dry desert.

Hi Ton;

I found a paper that addresses your quest ion pretty directly. Here is the citation.

Anders, E, EE Zaharieva, et al. 2016. Humidity sensing in Drosophila. Current Biology 26(10): 1352-1358.

I found it in Google Scholar with this query... humidity sensors in animals.

Enjoy, Jim Des Lauriers

https://theconversation.com/a-sixth-sense-for-humidity-helps-insects-stay-out-of-climatic-trouble-59039

https://www.researchgate.net/publication/319436067_Humidity_sensing_in_insects_-_from_ecology_to_neural_processing

Awesome Ton, Thanks for the citation

Not that "smell" is less important, but there are other ways that dispersing insects can find water. See attached for more info.