I was looking at this photo, and it finally struck me what had been striking me in the first place. The little orange structures in the middle of the fly! It turns out the little membranous flap is a calypter(or possibly calypters, plural, can’t tell if there are multiple ones) that cover the haltere, which in this case is the orange knob going toward the abdomen. According to the Wikipedia, the haltere is a modified hindwing (or a forewing in Order Strepsiptera) in two-winged insects that serves as a gyroscope:
Halteres thus act as a balancing and guidance system, helping these insects to perform their fast aerobatics. In addition to providing rapid feedback to the muscles steering the wings, they also play an important role in stabilizing the head during flight.
I hope everyone’s calypters are in sound condition and halteres properly functioning so you may fly high this weekend. Also hopefully they’re not too shiny or too dull; it makes for a better photo that way.