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Kleptoparasitic fly? - Cacoxenus indagator

Kleptoparasitic fly? - Cacoxenus indagator
Brooklyn, Kings County, New York, USA
May 14, 2011
Size: about 1/8 inch
Last year, I set out bamboo canes as habitat for solitary bees. Several Japanese Hornfaced Bees nested in the canes, which was very exciting and interesting. While setting up to take photos of the bees coming and going, I noticed this tiny fly hovering in front of the cane bee condo. It was very intentional in the way it was hovering and checking out the canes. Sure enough, when one of the bees flew out, the fly scurried into the cane. I had taken a workshop on bee parasites and kleptoparasites and suspect that this fly may be a klepto. I don't know if it shows in the photo, but the abdomen of the fly was striped dark charcoal grey and lighter grey. This spring, when I split the canes to remove and clean the hibernating bee cocoons, I did find several chambers that had some kind of non-bee larvae in them. I don't know if they were larvae from this fly, or some other invader. I'd love to know what this little fly is, specifically, if possible.

I looooooooooove your site. This is the first image I've submitted for ID. I hope y'all can help.

Best wishes,


Moved from ID Request.
Ross' suggested ID seems reasonable, until/unless someone suggests an alternative.

Cacoxenus indagator…
I suspect this may be the species of fly in this image. There is a fascinating blog about this drosophilid and its relationship to Osmia bees here. BugGuide does not currently have a reference to this genus or species, but you can find an image of a female here.

The species is described as being about 3 to 3.5mm long, with a light-gray thorax, a black abdomen with light transverse bands, and large eyes that are brown to dirty-red in color. Females lay their eggs on the pollen piles within Osmia nests, which their larvae apparently feed on. Your hunch that this is a klepoparasite is certainly correct. Great find, Jess!

Cacoxenus indagator
Wow. Thanks for the speedy reply, Ross. You're the best!

Thanks for the links to the blogs discussing this critter. Apparently, it's a well-known "red-eyed devil", as one of the bloggers described it.

One thing that I've noticed when cruising the internet, looking for more info...It looks like this fly is considered a native of Europe, although it's clear from the blogs that it is now found in Canada and in the US (unless there's a native fly that is a dead-ringer for C. indagator). So, I'm wondering if this kid was introduced to the US along with the Japanese Hornfaced Bees or as a tag-along with some other imported insects? Any ideas? Maybe I'll ask the folks...that's a great site. Thanks for that link.

All best wishes and thanks, again, for your fabulous site. :-)


Found this in Swarthmore, Pennsylvania
Here's the iNaturalist observation in case of interest:

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