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Large Black & Red Robber Fly - Ospriocerus vallensis

Large Black & Red Robber Fly - Ospriocerus vallensis
Southeast of Adel, Lake County, Oregon, USA
July 18, 2012
Found in a sandy/rocky alkali valley bottom dominated by greasewood (Sarcobatus vermiculatus)...for a view of the habitat from above and afar, see the panoramic image with this post.

I was thrilled when I first spotted this large fly from a I thought it might be a mydid (a favorite group of mine). Although it turned out not to be, it was still a fascinating find. A BugGuide search using the phrase "red and black robber" led quickly to genus Ospriocerus. Upon finding Martin's 1968 revision of the genus (available here), I had to adjust my presumptions regarding some of the antennal terminology for this genus. The "excision" on each long 3rd antennal segment is not a cut as I at first presumed, but a long shallowly-cupped longitudinal excavation along one side of the antennal segment (see the figures on pg 407 of's the one for O. vallensis). Also, the "style" is not a conspicuously sharp-pointed projection as I initially presumed, but rather a small hemispheric or hood-like terminal nub, with a cleft or circular opening, housing a tiny microscopic spine within (see the figures on pg. 402 of Martin). The antennal excisions and the tiny, cleft styles are both well-illustrated in this detail image of the holotype specimen of O. vallensis. Other images of that male holotype can be seen here.

So, once I understood the terminology, I proceeded through the key as follows:

1) Style at the tip of the 3rd antennal segment about 4/5 as high as wide (visible as a tiny nub in the full-size version of the image above).........2

2) Bristles on the hind tibiae and femora predominantly black (vs. white)......8

8) 3rd antennal segment not strongly angulate (and specimen not from Guerrero, Mex.)......9

9) Thorax black.....11

11) Hind femora entirely black (and not from Sinaloa or Sonora, Mex.)......12

12) Bristles on metanotum and scutellum black.....15

15) Hair on proboscis and below eyes white (vs. black)....16

16) Style short, not extended apically; mystax black (Idaho, Oregon).......O. vallensis

Note that the range is now known to be much larger than ID and OR (see info page).

The BugGuide posts of O. abdominalis looked close to the asilid here (though they seemed to have more red on the abdomen), but the key (and figures 1 and 47 of Martin) make it clear that O. abdominalis has no apical style, whereas a small nub-like style is visible (at full size) in most my images with enough resolution/focus to see it. Also, the description of O. vallensis in Martin states that the setulae on the dorsum of the thorax is black (as in my images), whereas those setulae are said to be red in O. abdominalis...though that's not borne out on the BugGuide posts of the latter. (None of those images show the tip of the antennae with enough detail for me to make out the presence or absence of a style.)

As always...comments, corrections, or confirmation are welcome and appreciated.

Images of this individual: tag all
Large Black & Red Robber Fly - Ospriocerus vallensis Large Black & Red Robber Fly - Ospriocerus vallensis Large Black & Red Robber Fly - Ospriocerus vallensis

Excellent series of photos an
Excellent series of photos and comments on Ospriocerus vallensis and Epicauta prey! It is noteworthy how often various Ospriocerus spp. are seen to be attracted to aggregations of blister beetles (Meloidae: Epicauta, Lytta & other genera) and to prey on these insects. John Pinto (my former major professor at U.C. Riverside) and I came up with the idea that Ospriocerus may be able to sequester the poisonous chemical cantharidin after feeding on the meloids, and utilize it in their own defense. The aposematic coloration of the Ospriocerus would assist in this strategy, especially against predation by birds (Fisher 2009). Other insects are known to use this technique (Carrel & Eisner 1974); as far as I know, the possible use of cantharidin by Ospriocerus has not yet been tested.

Thank you, Eric
Glad you enjoyed the posts, and that you shared the very interesting hypothesis that Ospriocerus spp. might sequester cantharidin as a deterrent to predation, and that their conspicuous colors might serve as an aposematic signal to predators, especially birds. Those are the kinds of natural history gems I love to learn about :-)

I looked up the interesting article by Carrel & Eisner (PDF available here). While it detailed experiments demonstrating the deterrent effect of cantharidin for insect predators of various meloids, I couldn't find reference to the idea that other insects with a tolerance for feeding on meloids might procure a secondary defensive benefit by sequestering their cantharidin. Perhaps that's been addressed elsewhere...if not, it would make an interesting research project. It's certainly a plausible idea. (I couldn't find your 2009 paper, would love to read it.)

I'm hoping Dr. Pinto might get a chance to review the Epicauta post. And I've got more asilids in my backlog of photos to post...many of which I probably won't be able to fortuitously ID on my own, as happened here. (Glad you're around! :-).

Thanks Aaron: yes, it seems t
Thanks Aaron: yes, it seems that I probably needed additional references on the uptake of cantharidin, as the Carrel & Eisner one is just the beginning of this interesting subject. I believe there are more specific studies now available on insects (and vertebrates) that do utilize this strategy (unfortunately, I can't produce one at the moment!). What really is needed is for a student, conveniently located in the Southwest (where Ospriocerus with aposematic coloration are most abundant) to investigate this phenomenon further.

My reference "Fisher 2009" is the "Asilidae" chapter 45" from the "Manual of Central American Diptera, 1" (1), which I will email a pdf of to you.

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