We received a short message from a reader who found a mysterious worm in the terrarium she built. A terrarium is a collection of small plants growing in a transparent, sealed container. She is curious about what the specimen is, and if it will harm her plants. In the photograph she provided the specimen blends in fairly well with the background. You can see it among the rocks towards the bottom. It is shiny, dark brown, and a few inches long:

Unfortunately, we aren’t able to identify this creature based solely on the photograph. We just don’t have a clear enough view of it to be confident about what exactly we are looking at. There are numerous organisms that fit the general description of this creature, but without more information we aren’t able to pinpoint which organism has invaded our reader’s terrarium.

Based on what we can see of the specimen, we think it might be a flatworm or a millipede. We know these two creatures actually have quite different appearances up close, but they can look similar from far away. Flatworms have the same shiny appearance as the mysterious specimen, and terrestrial flatworms typically live in the soil of forest floors among leaf litter and other debris.  They eat invertebrates, so if this is a flatworm then we don’t think our reader’s plants are in danger of being consumed! Millipedes, on the other hand, have shiny segmented bodies lined by lots of minuscule legs (they have two pairs of jointed legs per body segment!) Millipedes usually eat dead or decaying wood particles, but they can also survive on dead and decaying plants and leaf litter. So, if this specimen is a millipede, our reader might need to worry a little about her plants. Of course, this creature might not be a flatworm or a millipede but rather something else entirely!

Although we don’t know exactly what specimen our reader is dealing with, we don’t think she should spend too much time stressing about its presence. In general, worms in terrariums end up being beneficial or innocuous since they often break down organic waste and eat other bugs. Of course, we can’t guarantee this will be true for our reader, but we think she should see what happens over the next couple of days. We wish our reader the best of luck with her terrarium and we invite any of our other readers who have experience with terrariums to share their stories on this article!

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