Eva Crosson of Stirling asks :-

What do leeches eat and do they occur as far south as Otago?

Anthony Harris, an entomologist at Otago Museum, responded.

In New Zealand, there are about ten species of freshwater leeches and a further eight that attack marine fishes. Leeches are related to earthworms. They have a small sucker that contains the mouth at the front and a large sucker at the posterior end. Some species feed on the blood of warm-blooded vertebrates, including humans. At least one species of leech attacks the legs of wading birds. Six of the freshwater leech species that occur in New Zealand are predaceous, they eat small animals that they capture, and are not parasites.

All leeches have 43 body segments. Leeches have a characteristic manner of moving, looping end over end by holding on with the rear sucker, arching the body over, and grasping the surface with the front sucker, then letting go with the rear sucker and arching the body over to grasp the surface with the rear sucker.

A greyish leech, over 2.5 cm long, adheres to the legs of people who wade in the northern arm of Tomahawk Lagoon, Dunedin. A freshwater blood-eating leech that grows to 1.5 cm long, Placobdella maorica, also occurs in Otago and throughout New Zealand. A smaller species occurs in large numbers in many Otago ponds, but these tiny leeches feed mostly on fresh water snails and insect larvae.

The most spectacular leech that regularly attacks humans in New Zealand is Richardsonianus mauianus. This is quite common in the North Island and reaches 7 cm long. It has ten eyes and is bright green, yellow and orange, with five greenish stripes on its back alternating with yellowish bands. The underside is orange. It is locally common near Auckland and Wanganui.

In some parts of the country, there are feral medicinal leeches, Hirudo medicinalis, from Europe. These originated when some local doctor threw his bag of leeches into a pond around 1900, when bleeding became unfashionable, and the leeches have survived ever since, feeding on the legs of cattle that enter the ponds.