My wildlife pond has been leaking and, after several unsuccessful attempts at patching it up, the race was on to replace the liner before the smooth newts return to breed. Twenty-five years ago, I planted a single yellow flag iris (Iris pseudacorus) in the shallows. While it is a British native, it’s an invasive species and has taken over my modest two-metre-diameter pond, sprawling out of its planting basket, self-seeding into the silt. The thick mat of fleshy rhizomes – some as thick as my wrist – has smothered the more delicate plants such as miniature bulrushes, water mint and water forget-me-not, and displaced much of the water, leaving the pond almost devoid of life. After hacking through the irises and baling out the water, I hadn’t found a single dragonfly or damselfly larvae, pond snail or water beetle. The only remaining residents were horse leeches (Haemopis sanguisuga), and plenty of them. Several of these greenish-grey segmented worms had burrowed into the silt at the bottom, while others were hidden in the folds of the liner. While the majority of leeches inhabit fresh water, horse leeches are only semi-aquatic, and more were lurking beneath the pebbles on the pondside beach. Both parts of the species’ scientific name relate to the act of drinking blood, but unlike their blood-sucking kin, horse leeches are predators rather than parasites. These carnivores consume whole prey, often feeding on midge larvae and water snails, but moving on to land to hunt terrestrial molluscs and earthworms. Their common name is also a misnomer as they have no association with equids. The prefix “horse” is used figuratively to describe something large and strong, and while they’re slightly smaller than the famed medicinal leech (Hirudo medicinalis), they still reach an impressive size. At rest the leeches are about 5cm long, with the appearance of flattened slugs, but when disturbed they stretch out like jelly worm sweets, the largest reaching 15cm when fully extended. As I picked them up, I felt the gentle suction of their oral and posterior suckers against my fingers, but they quickly released and curled up contentedly in my palm. Country diary is on Twitter at @gdncountrydiary