Blackflies are small dark coloured flies with short legs and antenna. They feed on the blood of mammals, including humans, though the males mainly feed on nectar. The rate of reproduction is highly sensitive to water pollution. Eggs are laid in flowing water and the larvae that hatch and attach themselves to a smooth solid substrate, such as rocks, with tiny hooks on the end of their abdomens. They can use silk holdfasts to anchor themselves in place and threads to help move their position to more favourable positions downstream where they are best suited. The fans around their mouthparts capture debris passing through the flowing water such as algae and bacteria which they then scrape off with their mouths to eat. They pass through several larval developmental stages known as instars before they pupate in water where the emerging flying adult will exit the water in an air bubble. Blackfly larvae are widespread across the UK.