[drosera]

The dragonfly larva defnitely could have come in on the plants you last bought, or with plants, decor such as driftwood, or live food, from anywhere up to 3 months ago. You only noticed it now because it was large enough to be noticed. They are very small, and often translucent during their first few weeks of life.

It definitely could have eaten your shrimp. But I also wonder what it must have been sustaining itself on until now. When very young, they will feast on the normal microorganisms in a tank (worms, Cyclops, etc) But it would take a heck of alot of them to sustain older larvae. It' is to be noted that they will eat each other as well, so it is unlikely that you have many more.

I agree that you shouldn't do anything major to the tank. Keep observing to see if there are others, and just remove them if you see them. It's handy to look at night with a flashlight, as that's when they tend to venture out more.

TAke care!