Possible pest identification question

[loyall]

I have a small collection of indoor windowsill cacti. I watch out for any signs of infestation when I water, but there are no signs on the cacti stems. Whenever I repot or add a new specimen I carefully inspect the roots, and I have found no signs of pests.
Recently, however, when emptying the saucers after watering, what looked like dust particles on the surface of the saucer water caught my attention. I noticed it in three of my 15 cacti saucers. I decided to take a closer look using a magnifying glass. The dust was composed of tiny black specks about the size of the point of an ordinary straight pin. Some of the specks clumped together in groups of five or six and were moving in and around the clusters. The movement aroused my suspicion that the specks might be microscopic bugs of some sort. Possible soil microbes?
Does anyone recognize what I am experiencing? Possible cause? Whether to be concerned? Possible further tests?

[loyall]

I take cautious optimism at the lack of comment on my topic. It would seem that, of those who have read my post, no one recognizes what I have described as any sort of known pest. I intend to carry on keeping close watch for any further symptoms or problems. I have sterilized the saucers and pot bottoms with 70% isopropyl alcohol.

[jerrytheplater]

I did read this before, but I've never heard or seen anything like this.

[Steve Johnson]

I take cautious optimism at the lack of comment on my topic. It would seem that, of those who have read my post, no one recognizes what I have described as any sort of known pest. I intend to carry on keeping close watch for any further symptoms or problems. I have sterilized the saucers and pot bottoms with 70% isopropyl alcohol.

Sorry that I didn't have a chance to respond already...

See this:

https://www.cactiguide.com/cactipests/#bugs

Black specks floating around in the water don't fit with any known cactus pest described in that link. Only possibility might be sciarid fly (fungus gnat) eggs. IMO highly unlikely, but not out of the question:

https://www.organiclesson.com/cactus-fungus-gnats/

According to the experience of other members, sciarid flies are a problem primarily for people growing cacti from seed as the germinated seedlings have to undergo a "tender" period during the first 6-12 months.