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Assassin or robber flies (Asilidae) and bee flies (Bombyliidae) are two of the largest and most diverse fly families with over 7400 and 4700 described species worldwide, respectively. Adult assassin flies are predatory, feeding on smaller insects captured on the wing, and larvae predate on soft‐bodied arthropods in the soil or rotting wood. Adult bee flies feed on nectar and pollen. Almost all bee fly larvae are external or internal parasitoids of other insect larvae, usually in the soil. Although the interaction between bee fly larvae and their hosts has been rarely observed, we recently discovered assassin fly larvae hosting bee fly Lepidanthrax linguatus (Anthracinae: Villini) endoparasitoids in riverine sandy soil in southern NSW, Australia. While bee fly larvae have been known to attack other fly larvae, Asilidae is an uncommon host group. Furthermore, to find bee fly larvae inside the host is even less common because most bee fly larvae are ectoparasitoids. Here we describe and illustrate the host assassin fly (Bathypogon nigrinus: Bathypogoninae) larva and pupa, as well as the first instar bee fly larva (planidium) though careful external examination of the surface of the asilid larva, which is translucent, allowing us to see the internal parasite. This is the first larva and pupa from Bathypogoninae to be described. We also illustrate the bee fly larva in detail based on scanning electron microscopy of a larva in situ, after dissection from the host. Finally, we describe the late instar larval and pupal exuvia of the endoparasitoid. Like other endoparasitoid bombyliid larvae in the Villini, the first instar larva causes the host to produce a breathing tube that surrounds the posterior half of the parasitoid, allowing the parasitoid to maintain contact with atmospheric oxygen. [ABSTRACT FROM AUTHOR]