Not insulted, always ready to be corrected with factual material, the detritus thing is some thing I seem to remember from a book, hence "believe"

http://www.bbc.co.uk/nature/animals/...feature3.shtml

http://www.naturegrid.org.uk/pondexp...ery/leech.html
an interesting one as I have seen many of the cocoon like objects and thought that that was what they were.

http://www.uaex.edu/wneal/Pond_Manag...s/leeches.html

these all result from a google for "pond leeches" and I would assume there are many more interesting pages.