the eggs could have been in the soil. you could even have brought them in on your clothing/shoes...walking in the grass is enough to do it.

Sounds like possible (springtails) which will spring up-wards when the soil is disturbed or watered.
Letting the soil dry out more might help since they love to be in moist soil. They eat the decaying material in the soil but won't hurt your plant.

There a few things you can try to either keep their numbers down or eliminating them all together.

Letting the soil dry between waterings,(never seen them in with cactus)
Adding some a few drops of some mild dish soap to the watering can.
Running lots of water through the soil (outdoors of course) causing the critters to surface to the top and spill out over the sides.You will lose soil doing this, just add new if needed.
Or, you can change the soil completely and not water as much,just not so much that the plant is dropping from lack of water.

It really doesn't matter how your plant is watered, from above or below, if the soil is moist and you have these little buggers wherever they came from, they will find the atmosphere delightful!

I have a few plants that need constant moist soil like palms and such, and they don't have these pest. The pest you have could have been in with the soil already when bought. I have heard people discovered these things in newly purchased bags of soil!
Wouldn't it be great if there was no such thing as indoor plant pest?



Susan