dark blue fly

Observation - dark blue fly. Description: Dark (navy) blue fly. About 2cm long.Long slim abdomen. Quite long ribbed looking antennae. Transparent blue tinted wings. Appears every year in garden in spring on Jacob's Ladder plant which is then eaten by grey/green caterpillars. I assume the caterpil

Dark (navy) blue fly. About 2cm long.Long slim abdomen. Quite long ribbed looking antennae. Transparent blue tinted wings. Appears every year in garden in spring on Jacob's Ladder plant which is then eaten by grey/green caterpillars. I assume the caterpillars are the larvae of the fly.