Crowdin’ up the place

Everyone’s heard of roly poly’s before, haven’t they? They’re those little bug-like critters that, depending on the species, can roll up into little balls when threatened.  Scientifically known as isopods, and also commonly known as sow bugs and pillbugs, these critters can often be found bunched together in groups known as aggregations.


Common Woodlice (Oniscus asellus) congregating under tree bark

Woodlice aggregating on moist wood. Photo credits:


So why do these guys tend to be found in groups? Some studies have suggested that aggregating is a way of preventing drying up. Terrestrial isopods breath using modified gills on their legs, and without water, they won’t function properly. By clumping together, they can increase the group’s overall volume, without increasing the surface area available for water loss as much. This reduces their chances of drying up!

However, what I wanted to find out was, would isopods still prefer to bunch up if drying up wasn’t as big as a problem? Will they stay alone if their environment was moist and dark (their preferred habitat conditions)? To test this, established two tests. In one of them, I basically gave a bunch of isopods the choice between aggregation in a dry and bright area or solitude in an alternative area. This alternative area alternated between being dry and bright as well, just wet, or just dark. And what do ya know, when the other choice was also dry and bright, most individuals chose to group up, and when the other choice wasn’t dry and bright, most chose to stay alone in that other area. Though not “proof”, this suggests that if the conditions are good enough and the risks of drying up are low, they don’t need to use  anti-drying up methods.


8 choice experimental set up, including tea balls used to contain aggregations, and wet paper towel, and make-shift darkness lids.


The other test involved giving isopods 8 different choices, with each choice being a combination of the three factors of brightness, moisture, and aggregation. For example, one of the choice areas was dark, wet, and had aggregations; another was dark, dry, and had no aggregations; and so on. Surprise, surprise (not really), most isopods chose dark and wet areas. Within these dark and wet areas, they didn’t particularly prefer grouping up. This supports what I found in the previous set up. Hurraay! (??!) Given “ideal” conditions, these critters don’t necessarily need to group up to prevent drying up.


Ok end of boring experimental stuff. Now, what was really cool though, was the other critters that I found while out hunting my test isopods in Pacific Spirit Regional Park.




These larval forms were found on and within rotting logs, sharing their homes with isopods, springtails, and many other invertebrates. Unfortunately, I was not able to identify their species.

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