I was inspecting some beets that have been growing in a box on the balcony for a couple of weeks now and discovered a few small worm like insects crawling about on the leaves. There didn't appear to be many aphids on the leaves that had these things so I watched them for a while and was pleasantly surprise to see them viciously attack and devour aphids when they came across them.
These are syrphid or hoverfly larvae. Hoverflies are small bee like insects that hover around flowers looking for nectar. You can find them everywhere in my part of the world. Even up on my balcony.
The larvae are fun to watch. They hunt around for aphids and when the catch one they hold it up in the air to eat it and within a few minutes, are finished eating and go looking for another. The one's I found are not very long: just a few millimeters.