This is the mating ritual of the flies where the female fly puts her ovipositor into the genital opening in the males abdomen .The red colour tube like structure that is seen to be going from the fly on top (female) into the abdomen of the fly in the bottom (male) is the ovipositor. The mating ritual takes hours togather, we usualy see both the flies stuck togather and flying around. The ugly truth is that the male fly dies during the ritual and gets carried around by the female as baggage. Evening the fertilization takes place and the female lays thousands of eggs.It's a sacrifice that is made for pleasure and purpose.