Hi Leslie, they are the larvae of the black fly.
You can brush them off into the pond for fish
food. Or tie a Mosquito Dunk in the outlet to
the waterfall, they should die and get into the
pond that way for fast food.
You probably don't want them to hatch as they
bite!!

kathy :-) www.blogfromthebog.com
this week ~ introducing Miss Lily,
a painted turtle

Pond 101 page for new pond keepers ~
http://hometown.aol.com/ka30p/myhomepage/garden.html