I've been composting for about 4 months, and the worms have been doing remarkably well. The population has boomed, we've taken a batch of finished compost to the garden, and the bin can now process all of the compostable food scraps that we typically generate in our house.
But there has been an interesting development. It turns out that I've got a mixed species worm bin. When I purchased my worms, I thought I was buying a bunch of these guys:
Eisenia fetida (Red Wriggler)
They have a striped appearance and the band around their bodies (the clitellum) is very pronounced. It actually looks like it is swollen. They move around relatively slowly and they are ideal for composting bins because they tolerate temperatures between 40 to 90 degrees F, they prefer about 70 to 80 degrees F, they do well in confined spaces and they eat approximately half their body weight per day.
I've definitely got Red Wrigglers in there but, after composting for a while, I started to notice these guys:
Perionyx excavatus (Blue Worms)
These guys don't have the striped apprearance, their clitellum is flush with their bodies, and they have an iridescent blue/purple shine in the light. They tend to be longer and skinnier than my Red Wrigglers. The biggest difference that I've noticed is that these guys are fast! They also like to be between 70 to 80 degrees F, but they can't tolerate temperatures below 50 degrees F. They multiply very quickly, which is probably why I didn't notice them at first. I probably started with just a few, but now I've got a bunch. They're also good composters, but they are supposedly more prone to wandering. I haven't had this problem yet, so they must be reasonably happy staying inside the bin.
So where did they come from? Apparently worm farmers typically have their worm beds "contaminated" with these little guys. I use the word "contaminated" because there is some debate in the worm composting community (yep, it exists) as to whether or not these guys are good for a worm bin. People who keep outdoor bins don't like them, because they die off below 50 degrees F. Other people have trouble with them escaping the worm bin due to their wandering nature. A few people are just "creeped out" by how fast they crawl around.
So these guys must have tagged along with my worm order. As far as I'm concerned, no big deal. My bin won't drop below 50 degrees F, my blue worms have been staying inside the bin, and people who are easily "creeped out" by some worms probably shouldn't keep a box of them anyway. As long as they're keeping some of my garbage out of the landfill, and giving me free fertilizer for my garden, a little biodiversity is fine with me.