Drosophila melanogaster are sensitive to humidity conditions, and they don’t like it when it’s too dry since desiccation is very stressful to flies and has profound impacts on survival (Parsons, 1970). Flies also don’t like conditions that are too wet (Le Bourg 2001). We have certainly observed that humidity drastically affects olfactory preference and can affect locomotion. There’s evidence suggesting that environmental humidity affects progeny number, oviposition preference, foraging, pupation and affects how flies smell, which can affect adult fly behavior (Castrodenza 1973, Hoffman et al 2001; Hoffman 2010; Parkash et al, 2012; Stinzanio et al, 2015).
Humidity preference and hygrotactic behavior in the lab is often measured by water depriving flies (or larvae) and measuring proximity to higher humidity (Ji & Zhu 2015; Sun et al 2019). Drosophila have humidity sensors and the circuits for the ability to sense humidity (i.e. hygrosensation) are beginning to be characterized (Liu et al 2007; Enjin et al 2016, Knecht et al 2016, Knecht et al 2017, Sun et al 2018; Marin et al 2020). Like temperature, hygrosensation was one of the first behaviors Benzer studied (Sayeed and Benzer, 1996). They used a T-maze test to look at humidity preference and found that the same mutant, bizarre, that affects temperature also affects humidity preference. The effects of humidity on behavior are likely dependent on temperature since temperature and humidity are integrated in the fly brain (Frank et al, 2017) and humidity affects the genetic architecture of heat resistance (Bubliy et al, 2012).
So if humidity affects behavior, how can we control it in the lab? First, you need to know if your lab environment fluctuates in humidity. If you live in a place that has seasons (like New England) it definitely fluctuates. You should keep a humidity sensor somewhere in the lab so you have an idea of what the seasonal humidity conditions are and keep track of this in your lab notebook.
I’d also recommend use of humidity controlled incubators for rearing flies for behavior. The problem with these incubators is that you need to have a water source for the incubator, and the parts of the incubator that control humidity wear out every few years and you need to replace them. An inexpensive alternative is to put open bins of water in the bottom of the incubator. If you do this, you’ll have to figure out how much water works best for optimal humidity (I’d recommend ~60%, we find 70% which is what a lot of people use can make our food a bit soupy). You can do this by trial-and-error recording humidity with different sizes of water bins (the amount of surface area of the water is what is important here). Also remember to keep filling your water bins - make sure to have bins big enough that you don’t have to fill them every day so you can take weekends off if you want.
How do you control humidity when you are performing behavior experiments? There are expensive ways to do this, like performing your experiment in a humidity-controlled behavior room. We prefer to perform our humidity experiments between 50-70% and have found (via trial and error) that we only need to control humidity within a +/- 10% humidity range. Because of this, we prefer a more inexpensive method that involves placing a small drugstore humidifier plugged into a humidity sensor that controls the on/off of the humidifier. You can see some pics of different boxes and our humidifiers on our Virtual Tour.
Note - if you keep your behavior chambers humid, you can’t perform experiments in and/or near cardboard chambers because they get soggy and grow mold. This is why we like to use the corrugated plastic behavior chambers mentioned in the temperature blog post. Speaking of mold, I highly recommend wiping down environments that are kept humid fairly frequently. We find that green or black mold can grow pretty rapidly and you don’t want this to affect your health or the health of your flies.
Do you have questions, helpful advice or anecdotes? Please post in the comments below!