Do the critters hang out at the top of the water until they are disturbed? Is
your water 'slow' enough for the critters to stay in one place?

Mosquito larvae breathe at the surface and it is the quickest way to tell what
they are.
I've seen damselfly nymphs, some worms and other critters flip around.

Try these two websites, hope they are working...

http://www.mp.usbr.gov/geospat/olymp...e/macroid.html

http://www.bio.umass.edu/biology/con...s-general.html

If your barrel is on the smallish side you can try two rosie red minnows, sold
as feeder fish in many pet stores to eat up any critters.


k30a