Question: They are one stage in the life cycle for which organism?
I've heard from several of you (on-line and off) that you've hosted little families of these yourselves! It's an easy thing to do.
Congratulations to Heather for getting the correct subfamily Bonus Question. These are of the Culicine subfamily, which includes Culex, Aedes, and Mansonia mosquitoes. What they all have in common is the presence of a little siphon that allows them to breathe while hanging upside down from the surface of the water. That's how you can distinguish these from Anophiline mosquitoes which lack a breathing siphon and lay parallel to the surface of the water. Here's more detail on the siphon:
Image 1 - Culex larvae in a small container of water. (Click on image to enlarge)
Image 2 - microscopic detail of the siphon. Note that the siphon connects with breathing tubes that go into the body of the larva.
Here's a link to a great illustration which demonstrates the difference in breathing mechanisms between Anophelines and Culicines (Culicine mechanism - left, Anopheline mechanism - right)
Finally, by request, I've expanded the answer from my last case to further describe Entamoeba moshkovskii. If you're interested, please take a look below.
Images were taken at the London School of Hygiene and Tropical Medicine and shown with Permission.