See leeches warm up in anticipation of feeding and seek out a host.
View these two videos of medicinal leeches in action.
Medicinal leeches are initially kept refrigerated until use to slow their metabolism. The leeches in the video above have been removed from the refrigerator and are warming up in the mobile plastic home they come in. They will become more active as they warm and swim around the plastic container.
Watch this leech searching for a meal on a sterile surface. Note how the leech will wave its head (the smaller end) in the air searching for heat and any sign of a host.