The "fact" leeches suck blood is a bit of a misnomer. Many species of leeches are predators or scavengers, and only a small minority will take blood from warm-blooded animals. In fact only 2 of the 25 genera of leeches in the U.S. are blood-suckers. Blood-suckers are called parasites.
Leeches are primarily nocturnal, spending daylight hours under rocks, vegetation or debris. At night, they come out and forage on insect larvae, worms and other invertebrates. Leeches are predators. A rise in temperature at the onset of summer triggers reproduction.
Leeches (Phylum Annelida, Class Hirundinea) are related to earthworms and marine worms. Leeches are easily recognized. Their soft bodies are ringed with prominent, often circular sucker at the rear, and they have a fixed number (33) of segments. They are common inhabitants of ponds, marshes, lakes, and slow streams, particulary in the northern half of the U.S. Only a few species of leeches can tolerate pollution.