by ColinBattle » Thu Nov 11, 2010 3:04 pm
It's difficult to judge the scale, but these look like horse leeches, Britain's largest leech species, which can grow to over 15 cm in length. They are not particularly uncommon, and despite their size and name, they do not suck mammalian blood, instead feeding on invertebrates and decomposing flesh. Or they could be one of a number of smaller leech species, which feed on invertebrates, fish, amphibians or waterbirds.
The medicinal leech is the only species that is likely to attack humans, and this is rare and endangered, known to survive at only a couple of locations in Scotland. So if you find that a leech has actually attached itself to you, it's important to report it - it's likely to be a previously unknown population of this protected species.