Megapropodiphora arnoldi Brown is a contender for the smallest fly in the world. Described in 2018 from Brazil, it is only slightly smaller than Euryplatea nanaknihali Brown, 2012 from Thailand, with a body length of 0.395 mm. M. Arnoldi is a very distinct little fly, and has remarkable forelegs that are extremely enlarged, just as its name sake Arnold Schwarzenegger.1
From its general body and head shape it can be deduced that the species is a parasitoid (an insect whose larvae live as parasites inside their hosts and which eventually kills them).
Euryplatea nanaknihali is also a parasitoid, believed to lay their eggs inside the heads of Crematogaster ants. The larva consumes the inside of the ant’s head, and pupates inside of it, before emerging as an adult2