The larvae of robber flies typically live in soil, decaying wood or under bark. They hunt soft-bodied prey such as beetle grubs and digest them extra-orally before ingesting the liquified tissues. Just like their parents.
Orthogonis

This is one of the smaller robber flies we see, but it is still larger than many. Some species are just a few mm long! As a larva, this species probably prowled beetle burrows in wood.