As with most leeches, the eye pattern is a crucial ID feature. Theromyzon tessulatum has 8 eyes, arranged in pairs along a midline on the head, which is distinctive for this species. It is olive green in colour with immature specimens having six rows of yellowish longitudinal spots. At rest it is ovoid in shape, but almost parallel sided when fully stretched out. It grows to around 25mm long. Young leeches are ectoparasites in the nasal or buccal passages of water birds, but can be found on vegetation. Adults can be found on vegetation or on stones.

where the eye pattern is clearly visible.

Good photos are required showing the whole animal plus the eye pattern.

Freshwater.

All year.

Common and widespread.