"Some people seem surprised that I don't keep fish in the large aquarium in my home. Instead I have mud and debris, plants, insect larvae, beetles, hydras, scuds, crawdads...and leeches. I collected all these goodies from ponds and sloughs in the area. This fall, I noticed that one of the leeches, a good-sized sucker, was clinging to the side of the aquarium, out of the water. I wondered if it was trying to escape to find a winter home in less soggy mud. To find out, I went to the library and came back with a three-volume set of books all about leeches.
There are two categories of leeches. One kind sucks blood or fluid from animals it finds in the water - snails, fish, amphibians, clams, birds, turtles, cows, horses, or even people. This kind of leech can go for long periods of time - months and months - between feedings because they can engorge themselves on very nutritious food. They're very sensitive to smell and movements in the water and will move fast towards something new in their environment.
The other kind of leech is a predator and feeds on small invertebrates such as aquatic insect larvae. These leeches must eat every one to three days. They're sluggish and opportunistic rather than aggressive. They attach the rear sucker to a firm surface and wave their bodies about gracefully in the water, waiting for an encounter with something edible.
My wandering leech wasn't trying to find a winter home; it was a blood-sucking leech looking for a meal. I returned it to the wild, but a nice piece of liver or ground beef would have satisfied it, too."