As shown here it's male at the top, female at the bottom, for no particular reason except that's how I happened to frame the picture.
When I saw this image, nothing initially struck me as odd about this pairing. The male is clasping on the left side of the image, while the female's ovipositor is prominent on the right side of the image, and in the middle various parts are engaged as one might expect for genitalia.
But when I looked at an overview image, I was surprised to find that in the above image, the abdomens are paired "face to face", male ventral facing right and female ventral facing left.
Here, look:
So then I thought, "Well they are dead, maybe they came uncoupled while I was setting them up to photograph, and this is just an accidental positioning."
But no, that turns out to not be the answer.
The real answer is that this is a normal orientation for the pairing, and when both flies are feet-down on the same surface, there's a half-twist in the male's abdomen. The ventral direction of the abdomen points one way at the male's thorax, but it points almost 180 degrees the other way at the mating end.
This started to make sense when I realized that the mating position for most flies is with heads together, male on top, both flies facing the same direction, except that the male's abdomen is curled under at the tip so its very end is inverted. Starting there, if a mating pair with long abdomens happens to turn around so the overall position has heads at opposite ends, then the abdomens naturally develop a half-twist in the process. I don't know if that's exactly the way they get together during the mating ritual, but if not then it's still a simple way to evolve whatever behavior they actually do have.
Here's another view, almost perfectly from the side (lateral). It shows some of the internal structures better.
Shot at roughly 5X using Mitutoyo M Plan Apo 10X NA 0.28, on a Canon EF Macro 100 mm f/2.8 IS USM lens on Canon R7 body, using in-camera focus bracketing at its maximum step size 10. I did this rather than using a focus rail because it was simpler given the ways that I wanted to manipulate the subjects. Step size 10 is way too large for the 100 mm by itself, but the 100mm has to be set on f/2.8 to avoid vignetting while the optics are really stopped down by the objective. It just happens that step size 10 makes the in-camera focus bracketing use a step size that matches the effective aperture instead of the nominal f/2.8.
[Edited to add: by pixel-peeping the original source images, I see that in fact there is definite softening in the transitions between successive focus planes. Focus banding is barely detectable in the rendered images seen by themselves, but for more critical work I would use a smaller step size, yet to be determined.]
--Rik
PS. The subjects are collateral damage from the legally mandated spraying of my cherry trees to control Western Cherry Fruit Fly. So yes, I did kill them, but for once I can claim it was an unfortunate accident.
