[2] The adult fly is a bee-sized insect about 10 to 12 millimetres (3/8–½ inches) long, slightly hairy with a banded, dark grey body and dull yellow head and legs.

The female then deposits a few larvae, while on the wing, within a tiny mucous drop, directly into a nostril of the host animal.

[3][4] If the fly successfully places eggs in the nostril of sheep the animal may feel the larvae after a few days and attempt to remove them by tapping their muzzles on the ground.

[3] Once the larvae have infested the nasal passage and sinuses, usually up to 15 larvae but can be up to 80, they cause irritation to the mucosa, which causes mucous discharge, swelling of the internal membranes of the nose, possibly impairment of breathing but largely discomfort and distraction to the sheep who may reduce or stop grazing and subsequently lose weight and condition.

[4] This can in some cases lead to malnutrition and death [3] Sometimes mature larvae are unable to escape from the nasal sinus and die.

[5] Most commonly they are shepherds living in close proximity to the sheep[6] but there are records of hapless visitors being subject to infestation and carrying the parasites home to their native country.