Worms in window sill pond

Worms in window sill pond

Observation - Worms in window sill pond - UK and Ireland. Description: I have a mini-pond on an external window sill in north London rearing some tadpoles. When I originally furnished it with pebbles I also, for no good reason, popped in a horse's knee bone found in the Thames mud. I found this some time ago, well over a yea

I have a mini-pond on an external window sill in north London rearing some tadpoles. When I originally furnished it with pebbles I also, for no good reason, popped in a horse's knee bone found in the Thames mud. I found this some time ago, well over a year, and it has been 'bone' dry in a shoe box since. The bone has not been totally submerged, except briefly after very heavy rain, and the worms are on a dry part above the water. (The wetness seen in the photos are from splashes from when I topped it up). Today I noticed these small worms, they're about 5mm long with black heads and are segmented into about 12 sections. They are very active, sometimes reaching into the air. I think they may have emerged from the hole seen in the pics. The water itself has been replenished with both pond water and supermarket mineral water.
So I'm wondering what they are, what they'll turn into, where they came from, and will they make good food for the frogs?!