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Female swarms of Phalacrotophora (Phoridae)
In June and July, I spotted a swarm of Phalacrotophora (Phoridae) flying about the bases of Acer pseudoplatanus and Fraxinus excelsior trees in mixed woodland near Norwich, England. Phalacrotophora swarms have been recorded before, but their function has always been a bit of a mystery. I identified the flies as Phalacrotophora delageae Disney, a species that has not been found in Britain before.
I will be writing a short paper adding this species to the British fauna, and describing my observations, but Diptera.info seems to be the perfect place to post a series of photographs which will be too costly to publish in a journal!

The swarms consisted of twenty to thirty females flying between 0.2 and 1 metres above the ground, and within 0.2 metres of the trunk. When undisturbed, up to ten females would settle on the tree trunk, in a ?head-down? posture, eaching one staying like this for up to a minute before settling elsewhere or joining the swarm again. In the ?head-down? posture, the flies extend the abdomen and point it torwards the tree, exposing the membranous patch at the base of the fifth tergite

This photo shows the 'display posture' of a Phalacrotophora delageae female.


The unchitinised part of the tergite appears to glow brightly. It contrasts with the black tergites 2 to 4 in front and the orange of the remainder of the fifth segment.


If the overall brightness is digitally reduced, you can see how much this little fly appears to glow in the dark! I did examine some live specimens in the dark, and under an ultra-violet lamp, but there was no evidence of luminescence or enhanced reflection with UV.


While sitting in this pose, the females vibrate their wings, presumably creating a distinctive sound. Visually this appears to be exactly the same as the wing-waving of Drosophila when it is 'singing'. This sequence shows the position of the wings at different points in the 'song'.


After a while, the females stop their display and sit on projections on the tree trunk (in this case a snail shell) where they interact in a casual way. Then they join the swarm again before landing in the display posture for another session.


I did not see any males at the swarming sites, and could not find any concentrations of coccinellid larvae or pupae close by. So the purpose of these female Phalacrotophora swarms is still a bit of a mystery, but I think that the visually striking display posture, combined with the 'song' and possibly pheromones must be to attract a mate.

After all, to a male Phalacrotophora this female can only be saying 'Come and get me!'

Comments
#11 | firefly on 12 October 2007 20:48:06
It is many people here thinking that?s a bioluminescent diptera...
Am I wrong, but did you never said that was a glowing fly?
I only read that she?s bright but not bioluminescent...
Am i correct? But I would really like to see a glowing one.
I never saw a glowing diptera here. Maybe someday who knows. Grin
#12 | Paul Beuk on 12 October 2007 21:42:07
Bioluminescens is known in the larvae of some mycetophiloids.
#13 | firefly on 13 October 2007 19:06:22
Yes I know. That?s why I still expect to see one someday.
I already saw glowing centipedes, earthworms, ( of course) fireflies and glow worms, mushrooms, plankton... So a fly would be something new and interesting for me. Orfelia fultoni and Arachnocampa luminosa are some that have glowing larvae ( blue light!).
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24 June 2021 20:51
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03.06.21 11:11
@Tony Irwin Thank you Tony! I've emailed you there Pfft

02.06.21 22:26
Rob - can you PM me with an e-mail address, and I'll send it over.

02.06.21 13:16
Hello chaps! Does anyone have a copy of: Revision of the willow catkin flies, genus Egle Robineau-Desvoidy (Diptera: Anthomyiidae), in Europe and neighbouring areas ?? I'd be ever so grateful! Rob

31.05.21 14:31
The part that got deleted is "longer, dark median stripe on thorax"

31.05.21 08:37
You are correct, the copula also is phragmitidis. Corrected in the Gallery.

30.05.21 23:08
... look identical to me, the pictures of L. nigropunctata don't have the black spot on the scutellum and lack the other features you described.

30.05.21 23:07
Thank you for the answer Maks! Unfortunately the second part of your message seems to be deleted? I looked at the pictures of the 2 species in the gallery, are they correctly identified? Because they

30.05.21 20:06
@FliegenFranz Apart from differences of the hypopygium and ovipositor L. nigropunctata has darkened wing tips, mostly dark front femora, a black spot/stripe on the scutellum and a more pronounced, lon

29.05.21 18:48
What's the difference between Limonia nigropunctata and L. phragmitidis? Can someone help me? Thanks

26.05.21 11:34
Looking for material? See here: https://diptera.i.
.._id=100716

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