In what seemed to me unusual behavior, this fly, which spent a great deal of time buzzing futilely on its back, proceeded to spurt out a considerable quantity of milky white fluid in a series of abdominal contractions that had the effect of propelling it a millimeter or two further around the "racetrack" groove in the bottom of the plastic deli container. I was afraid it would completely circle the container in this fashion and begin making a smeary mess. What is/was this fluid and why did it contain so much of it?