... of the inflated ptilinum. You really have to be "in the right place at the right time" to catch something like this!

The bizarre-looking form of the head here is very similar to the dipiction on an emerging schizophoran fly given on page 501 of Joyce Laing's scholarly paper on the ptilinum (scroll down 5 pages in the linked PDF to see the figure).

If you examine your image carefully, you can see a paler whitish yellow-orange "horse-shoe shaped" area on the left of the bulging head. That's the actual "face"...with the two antennae dangling down from near the top. After the ptilinum is fully deflated and retracted, the juncture between the edges of the more whitish face and the more yellow-orangish ptilinal membrane will become the "ptilinal suture (or fissure)", as seen in the image below:



...and labelled here (in an image of a fly from a very different family in Schizophora).

I just noticed that Stephen Luk has another amazing series illustrating inflation of the ptilinum for the same species as yours:

....that takes the cake.