Sci/Tech: The humidity jar

Most small animals die if they dry out. To keep them alive, you need a container with a supply of hidden water that the animals can’t drown in, but which still keeps them moist. You could put a piece of flywire on a jar with a wet tissue on top of that, before you screw the lid down, but a jar with a plaster base is easy to see things in.

These humidity jars are good for keeping snails, spiders, slugs, springtails and slaters alive, because they stop the animals drying out without making them wet. You can carry live animals in these jars and a large one can be used to ‘relax’ dead spiders or insects so you can photograph them:

Dead huntsmen occasionally show up in my garage, and if they are fairly fresh, they can be placed in a humidity jar, which softens them so they can be posed and held in place with pins as they dry again. Here, you see the pinning and the result. When I am feeling naughty, I claim that tyhe huntsman gets

its name because it hurls its spears around.

You need a place where you can safely make a bit of mess, and you need to get some old newspaper, Plaster of Paris, a spoon, water, and some wide-mouthed screw-top jars. I mainly use glass 400-gram Vegemite jars, but 400-gram plastic peanut butter jars are safer. You can buy plaster of Paris at a hardware store. Once you open the plastic packet, put the rest in sealed and labelled jars.

Materials and the first step: adding plaster to water in the jar. Notice the spillage.

Spread the newspaper to catch any spillage. Then put one centimetre of water in a jar, and add several spoons of plaster powder. Let the wet plaster settle to a flat surface, with some extra water on top. Tap the jar to make the plaster spread out, and to get rid of air bubbles, then leave it. The plaster sets in about 20 minutes, but wait an hour to be on the safe side, before you pour off the extra water, and wipe any splashes of plaster from inside the glass with a tissue.

Finishing off your humidity jar.

Always dampen the plaster with ‘aged’ water, tap-water that has been left in an open container for a few days to get rid of any chlorine. Leave it for a few minutes, pour off the water, and wash it out with more aged water, wipe the jar and plaster dry with a tissue or paper towel. People who keep very small orb-weaver (web-making) spiders often poke a branched twig into the wet plaster to make a place for the spider to hang a web.

Humidity jars are good for many animals. Keep the plaster moist by removing the animal to a new jar each week, washing out the old one and mopping it almost dry with a tissue, a paper towel or newspaper.

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