Please refer also to the following link for details on this group:
Therevidae = Link 1
This is a small family, the adults of which are predaceous mainly on other Diptera. Larvae live in sandy soil where they feed on earthworms and various soft-bodied insects. Larvae of Phycus brunneus Wied. occurring in bat dung in India feed on dermestid larvae (Isaac 1925). However, Goetze (1932) questioned the supposed predaceous habit of therevid larvae when he noted that a rye field showed extensive root injury in which only therevid larvae were present in large numbers. However, laboratory tests failed to confirm this kind of damage.
Immature stages of Therevidae are similar to Asilidae. First instar larvae are vermiform, with 20 apparent segments, due to "intersegments" between the first 6 true abdominal segments. Prothoracic and caudal spiracles are present. Mature larvae of Psilocephala sequa Wlk. are also elongate and bear spiracles at the posterior margin of the first thoracic segment and at the anterior margin of the 4th "segment" from the abdomen's end. Pupae have heavily sclerotized processes on the antennal sheaths and at the base of each wing sheath. Abdominal segments each have a ring of long spines at the posterior margin, and the final segment is developed into a pair of large, heavily sclerotized processes (Clausen 1940/62).