Adult Leaf Miner flies are 2-3 mm long and most often black. They sometimes have yellow areas on their legs or body.

They lay eggs on or in leaves which the larvae then feed on after hatching.

Leaf Miner  larvae are small headless maggots, with yellowish-green colouring. 

The larvae live in leaf tissue, creating mines/tunnels as they feed. They live in many plants, including most brassicas.

They create mines when feeding within the leaf. There are often large damaged windows in the leaf, with the outside layers remaining intact.