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Dance fly (Empidid)? - Empis geneatis

Dance fly (Empidid)? - Empis geneatis
Monte Bello Open Space Preserve, Santa Clara County, California, USA
April 20, 2009
Size: Body about 3 mm
I saw a couple white balls floating across a trail in a mixed evergreen forest, clearly in flight rather than drifting on the wind. One of them landed within reach of my macro lens and cooperatively stayed for a while while I took a photo. I didn't get a good look at the front. My open-space docent colleague Debbi Brusco found photos of dance flies on BugGuide that looked pretty similar, so I'm guessing that's what it is.

Moved from Empidinae.

Thanks, Isaac, for adding to your bio page and also for lending your expertise to this wonderful group of flies! I'm sure everyone at BugGuide will be grateful for any future assistance you can provide.

Oh, and congratulations to Jack for adding a new species to the guide!

New species
It's going on my CV immediately!

Empis geneatis
great shot!
A very distinctive balooning species - dark with a hairy face.

Empis geneatis
Thanks for the ID! Would you mind adding a bit about your background to your bio page? It sounds like you know what you're talking about, but it's good for editors to be able to get a sense of contributors' credentials before making new guide pages based on their IDs. No need to reveal your identity if you don't want to. Thanks!

about me
Hi, I've finally added to my bio page. I signed up a while ago at the suggestion of Keith Bayless, who is in the same lab as me, but as you can see, I've come on the site only infrequently. I'm currently working on a project on the phylogeny of Empidinae, and am focusing on the ballooning Empis and relatives (subgenus Enoplempis) as a subproject. I've got a lot of specimen shots of from the USNM collection - you can see Empis geneatis and some others on morphbank ( and ). I plan on adding a lot more North American images soon.

About the silk vs. froth: it seems that only a few species of Hilara make balloons, using silk, while the few species of Empis that make balloons make them with froth, presumably from the anus.

Moved from Dance Flies.

Yes, male with nuptial gift!
Outstanding! Male presents this to female prior to mating. The silken ball may or may not contain prey for her to feed on. Great behavior shot, thank you for sharing!

Ball material...
Thanks, Eric, for the explanation! I agree that this is a great behavior shot and it inspired me to go look at some other similar posts. On Gary McDondald's post he quoted the following from Borror & DeLong (1): "Some species of Hilara & Empis that occur in the Northwest construct balloons, which they carry about as a means of attracting females. These balloons can be made of silk (spun from the basal segment of the front tarsi) or of a frothy material from the anus, & they usually contain prey."

To my eyes, this photo seems to show the frothy material from the anus (vs. silk). Speaking as a female -- I'd much prefer silk! (*grin*)

More on ball material
Thanks to both Eric and Harsi. I can only agree with Harsi's preference for silk over anal froth!

The material on this fly had a strong sheen (partially reproduced in the photo), which reminded me more of liquid froth than silk. In particular, you can see what seem to be bubbles beneath the wings. Hard to tell for sure, though, since some silk fabrics are pretty shiny.

There must be some interesting research on evolution, behavior, and nuptial gifts to the female. If the gifts are edible and expensive to produce (and not just a fitness display), that probably does something to redress the imbalance in parental investment between the males and females.

And it's interesting that the balloons SOMETIMES contain prey. Cheating is a common evolutionary strategy.

More reading...
In case you didn't already come across this PDF, it goes into a lot more detail on this fascinating subject.

Cheating dance flies
Fascinating paper, Harsi! I also noted that the gift pictured in Figure 1 is distinctly fibrous. If that's what a silk gift looks like, then the ones I saw weren't silk.

New to BugGuide...
Jack, I just noticed that this is your first submission to BugGuide -- Welcome! I took a look around your website and you have a nice collection of nature photos. I do hope that you will continue to submit your mystery images in need of identification. Also, if you have high-quality images of known species you can submit those directly to the guide pages. (A good tip is to first browse the current images for any given species and consider whether your image will add something to the repository of knowledge before submitting.)

Thanks for the encouragement Harsi. I'll do as you suggest.

I think figure 1-B shows the cotton ball that was given to the males by the people doing the research as a fake gift.

Good catch, John!
Question: Given Paul Beuk's comments on this page (#4 under the Remarks section), can't we at least move this image to Subfamily Empidinae?

Well, he does say mostly in Empidinae, so it sounds like we couldn't do that with 100% certainty...

Based on the wing venation this fly is in the subfamily Empidinae.

I agree
we have other images with gifts that have been at this level a long time. I'm not sure if there is a reason other than lack of a "champion" to sort or something else?

Thanks, Charley and John.

You're right
Yes, the legend clearly states that it's the ersatz gift. That's what I get for posting without reading carefully!

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