The white bits are grains of sugar used to attract the fly and keep it busy while I gat a few shots.
This fly has a mouth / tongue full of sugar crystals

Nikon D800, AF-S Nikkor 105mm F/2.8 micro VR, Kenko 12mm extension tube, R1C1 flash kit, tripod
Exposure = 1/200 sec; f/20; ISO 100@105mm with flash, manual spot metering, manual focus
Flash info = two R200 flash units on lens ring, iTTL 0EV, distance to subject 33cm
White balance preset via white reference card.