Carnivorous, but mostly sanguivorous, leeches have been reviled for centuries even though they were used for bloodletting as medical treatment for perhaps 2000 years. Even today, these interesting annelids are used as medical adjuncts. In this regard, reducing the accumulation of tissue blood and venous congestion is accomplished better using leeches than by other methods, especially in confined areas such as the fingers or muscle flaps. Although voracious feeders when provided an opportunity, most leeches will dine only once every few months. Of course, when they do feed, they may increase in size by an order of magnitude. But, how do they locate their prey and what part does vision play?

The medicinal leech, Hirudo medicinalis (fig 1), among species of other leech, does have eyes and does see, albeit poorly, as sight is not its primary sensory modality. Nevertheless, these maligned creatures have a surprisingly sophisticated photoreceptive sensory system coupled with an ingenious and highly successful lifestyle.

Vision usually has a key role in predatorial lifestyles, although certainly this is not an exclusive criterion. Leeches prove …