Bodenheimer (Animal Life in Palestine, 1935) has this to say:
The conspicuously coloured, slender and wasp like Conopidae visit umbels and other flowers. The maggots develop as parasites in the bodies of bees and wasps. The eggs is laid on the abdomen of the host, the maggot enters the same, and pupates there. It is sometimes known to leave its dead host, even when the latter has been pinned in an entomological collection for months. Enslin reports the hatching of Physocephalla vittata from the bee Xylecopa aesuans to have taken place 9 months after he had returned to his home from Palestine.

From Wikipedia:
Conopidae, usually known as the thick-headed flies, is a family of flies within the Brachycera suborder of Diptera. Flies of the family Conopidae are distributed worldwide except for the poles and many of the Pacific islands. About 800 species in 47 genera are described worldwide, approximately 70 of which are found in North America. The majority of conopids are black and yellow, or black and white, and often strikingly resemble wasps, bees, or flies of the family Syrphidae, themselves notable bee mimics. Conopids are most frequently found at flowers, feeding on nectar with their proboscis, which is often long. The larvae of all conopids are internal parasites, most of aculeate (stinging) Hymenoptera. Adult females aggressively intercept and deposit eggs on their hosts in-flight, and the female's abdomen is modified to form what amounts to a "can opener" to pry open the segments of the host's abdomen as the egg is inserted. The subfamily Stylogastrinae, including the genus Stylogaster, is somewhat different, in that the egg itself is shaped somewhat like a harpoon, with a rigid barbed tip, and the egg is forcibly jabbed into the host. Some species of Stylogaster are obligate associates of army ants, using the ants' raiding columns to flush out their prey. However, Stylogaster has been collected as far north as Canada, which is devoid of army ants. More research is needed to determine the life histories of most Conopids.

---