The first suspects would be Sarcophaga, if there were any females in the trap. Otherwise look through the trapped flies for larviviparous species of the right size. It should be obvious how they got there.
Thanks Tony! The problem is that the larvae were not sorted as Brachycera from the beginning but as Nematocera instead I think. And that makes it difficult to know which other flies were in the sample. There were some Sarcophaga males in the same tube so there may have been females in the sample as well.
