Background Story
I have a small, old cactus on my sunny, south-facing window sill in southern Germany. Since a few years, small black dots gathered around its pot. At first, I thought the dots were dust or sand and wiped them away. But the dots appeared again and again, so I had a closer look at them and saw that they were tiny insects or arachnids, probably lice or mites. We just call them arthropods from now on.
More information
The following picture shows my cactus with some of the arthropods around the pot. Click here for higher resolution.
Here are some close-ups.
The arthropods seem rather dead. They do not move and most of them lie on their back. However, they are not dead. Once they are picked up and set on a paper towel they start to move. If they get on their legs (with a little help) they walk around, which can also been observed by the naked eye.
I inspected some individuals using a detached camera objective as a magnifying glass. The arthropods are brown, shiny, somewhat translucent, and have 8 legs. I could recognize a small head without a neck (like most bugs have) but couldn't recognize a mouth or eyes – probably because of the size. I couldn't take a picture, but tried to draw what I saw:
Later I got hold of a cheap microscope and took a few videos. The video quality is miserable. In reality, the arthropods are not that dark, see my drawing and the photographs. Here's on individual laying on its back, flipping over, and walking away.
Here's another individual walking.
The Strange Part
As said, the arthropods keep on re-appearing around the plant pot. But in all those years, I never saw any of them on the pot, inside the pot, or on the cactus. I also have other cacti right next to the "infected" cactus, but their pots were never surrounded by the arthropods.
Question
What are these arthropods, where do they live, and why do they leave their habitat?
My Theory
I suspect the arthropods to live inside the dead and cracked part of the cactus. They could walk on the spikes of the cactus and jump from the ends of the spikes to the ground. If they land on their backs, they cannot get up and stop moving after some time.
I don't think that the arthropods live inside the soil. Two reasons for that:
- The arthropods could have drowned often. From time to time I put too much water in the pot, such that all soil is fully covered by water, often for more than 8 hours (yes I know that can't be good for a cactus *cough*).
- In order to get to the window sill, the arthropods would have to walk quite a while. Over time, I should have seen at least some of the arthropods on their way to the outside world.
