Stereo of the back of doli's head

[BugEZ]

A ~ 10X photo of the back of a dolichopod's head. This fly inhabits the muddy bank of a stream. The male hovers over the female about 2cm back looking over her folded wings and toward the back of her head.

I puzzled over these spots till I observed the courtship and now suspect that the spots are what he uses to maintain his position. Eventually he lands in front of her, then flies back to the rear and hovers some more. If she is pleased, she waves her wings a bit and they become more intimate.

stereo of rear head by Keith Short, on Flickr

[MarkSturtevant]

These iridescent markings might be strongly UV reflective. One could check with a UV light. There are many species of flies with UV reflective markings and these are indeed used in courtship.

[BugEZ]

MarkSturtevant wrote:

These iridescent markings might be strongly UV reflective.

As coincidence would have it I photographed the face of this species with soft UV LED lights several years ago.

uv pmax by Keith Short, on Flickr

Similar view with white light

front portrait by Keith Short, on Flickr

As you can see, the fine white fuzz on the fly’s face really lights up.

In white light the face looks white in a face-on orientation, and dark from below. I am not sure if this is to allow the fly to flash white to dark by nodding, or if it helps the fly avoid alerting its prey in the mud below where it normally feeds.

I will try to check out the patches with UV. Thanks for the comment!

Keith

[BugEZ]

And here is an image of the back of the head in UV. The blue patches don’t lite up more than other areas. I don’t know if this is because the orientation allowed UV to reflect away, but I suspect not. I think the patches are blue as the eyes of the bug may be tuned to blue wavelengths for max sensitivity not UV. The puddles where these bugs feed result in the light passing through a water surface and back out. UV polarized light in water de polarizes rather quickly relative to longer wavelengths. And UV is somewhat more quickly attenuated in water than blue light. So there may be physics behind the color of the blue patches as blue light sensitivity may be advantageous for the bug.

Other species that are not mud puddle feeders may well be most sensitive to UV and key on UV flags as the water surface attenuation won’t be important.

K

uv stereo head rear by Keith Short, on Flickr

[Troels]

Very interesting and wonderful stereos!

[BugEZ]

Thanks Troels!

The back of the head UV shot is not much of an image. A bit too fuzzy for my taste.

But the result is interesting.

K

[MarkSturtevant]

That does seem to resolve the question of whether those patches would enable communication by lighting up under uv. It seems not.

[BugEZ]

During courtship the male hovers about an inch above the female and 3/4” behind her. I wonder what he targets for his stationkeeping. It might be the shiny overlapping wings, or perhaps the blue spots. Sounds like a field experiment is in order...

K