This week have seen a damselfly larvae in our ponds. It is living submerged under the water and not yet ready to emerge as an adult.
We can tell it is a damselfly not a dragonfly as the body is thin and there is the opportunity to see the 3 gills that look like tails behind the hard casing when we put the larvae into a bucket to photograph. Dragonfly larvae do not have these - they have short fat bodies without gills.
When they are ready both dragon and damselfly larvae will emerge from the pond or container pond water by climbing up the taller emergent pond plants that grow out of the water.
These larvae could have been in the pond or container pond water for a couple of years until they are ready to emerge as adults. A good reason to avoid removing all the water from a pond or container pond. You may not realize you are removing the larvae from their habitat before they are ready to survive as adults.
They would have been placed in the water as eggs by an adult female hovering above the water to deposit her eggs. The container pond is as easy for her to use as a pond for this job - she is only looking for safe, still water with plant growth to hold onto as she pushes her ovipositor (backend) down into the water.