I was used to: 1- place a beaker filled with distilled water in the incubator and re-fill it daily/every 2 days, 2- add a drop of distilled sterile water on the dish cover, facing the inside of the plate. It worked well in forbidding medium to dry out in prolonged microbial growth (5-10 days), maybe it will work also for your purposes! good luck!!

Hi Caroline

Try putting the dishes into a snap-top sandwich box that you can then humidify by some appropriate means - damp paper kitchen towelling, beakers of salt solutions, etc.  We use this approach for various kinds of arthropods just changing the means of humidification according to their specific requirements.

I agree with Ian > damp paper kitchen towelling or even toilet paper will work well. However, it depends on the analyzed species: whereas taxa as Ligidium rely on high humidity, others do not (e.g. Porcellio scaber).

It is P. scaber, we need to keep the leaf litter from going dry over the month. Opening the dish and spraying with water may result in contamination of sterilised leaf letter plates. 

Thanks for the suggestions, I will try some of the sugggestions

How about wet cotton wool?

Humidity can be maintained by putting a lump of wet cotton-wool in one corner of the Petri dish to create a humidity gradient across the floor. However, care should be taken that water should not overflow from wet cotton wool lump.

If I put wet cotton wool in the Petri dish, I would surely have to re wet it every few days, exposing the dish to contamination?

Put piece of filter paper in petridish as per requirement and add one or two drop of water every day and first try this as SOP method and put the experiment.

I think Ian's spot on. I used ziplock containers lined with non-fenestrated sterile medical towels and moistened them with sterile water. When growing P. destructans humidity needs to be at close to 100%. Thanks to the ziploc, I may have had to re moisten only once or twice over 50 days.

You also can measure the relative humidity with a miniature r.H. ; temp. -Sensor. (4 x 4 mm)

So you can see or logg the humidity in your dishes. Also with a salt - water solution in a second vessel you can make a constant r.h ambient in range 11%r.H. to 100%r.H.

If you want I can give you more informations.

Putting a water soaked sponge in the Petri dish  and soak it with water as per need is the best, easy and non laborous method to maintain humidity for your research work.

That would be dependant on if the woodlice ate it as I'm extracting the GI tract and looking at the communities of bacteria and fungi and that would mean taking the lids off the Petri dishes to re wet them and risk contamination, but I'll keep it in mind thank you

The easiest way is to seal outside the Petri dish with a tape of Parafilm (streching it). We use commonly this system in isolation of fastidious bacteria, after micromanipulation of few cells: some took months to grow and the agar remained wet. If you need to open the Petri dish, you can seal again it with same tape of parafilm. For observing  modifications or growth inside the dish, if the content if semisolid  (as agar) and transparent, you can see directly under microscope (10 or 20X) putting the dish upside down. This is the system we do to obsrve colonies growtrh in agar: in this case you do not even need to open the Petri dish.

Thank you but there is no agar involved, this is a micro environment for woodlice, so I need to keep the leaf litter from drying out and don't want to put parafilm around to seal the dishes due to oxygen requirement