Ants
and Robber Flies (Posted 9 Dec. 2007, Updated 11 & 14 Dec. 2007, by
Roy J. Beckemeyer)
This page contains photographs taken by Dale Randall and observations made
by Dale Randall and Eddie Stegall in September 2007 in Roger Mills County, Oklahoma, in
the Antelope Hills. Images and notes are used here with their permission.
Eddie reports: "We were on a nature walk in a cow pasture when we came
across a well established and active ant hill. We noticed 2 or more robber
flies zipping about to and fro from it. The ants were some species of
harvester that were not winged. The robber flies were snatching them up
off the ground and then flying a short distance, landing and then
munching. Several dead ants were visible in the region around the ant
hill."
I do not recognize the species, but it looked like it might be a species of Diogmites. [After
discussing this page with Herschel Raney, I suspect that his suggestion that it
is likely in the genus Saropogon (closely related to Diogmites, both
genera in subfamily Dasypogoninae) is
correct. I have now gone through the key in Wilcox's 1966 paper on
Saropogon and read through descriptions of the species that have been recorded
for Oklahoma. The closest I can come for now is guessing that this might
be Saropogon pritchardi Bromley, 1934. I will try to find photos or
specimens for comparison.]
In the above right photo an ant is seen impaled on the robber fly's
proboscis. The fly is perched on dry grass and is supporting itself by all
six legs - an unusual feeding posture for Digomites, which usually hang
from vegetation by their fore legs [a reason for suspecting this robber is not
in Diogmites].
At left is a lateral view of the robber fly. The abdomen appears quite
dark, but is in the shadow of the wings.
Below is a photo of the robber fly and one of the dead ants, probably a
recently discarded meal. Here the abdomen color pattern can be better
seen. The black wings, abdominal terminalia, and lateral abdomen stripes
and very pale tarsi and thorax are quite distinctive. Species of Diogmites
that have dark wings include basalis and platypterus, neither of
which share these other features. The image of the robber fly is a bit out
of focus, so the number of bristles on the scutellum is not discernible,
unfortunately, but this view provides a good feel for the characters specified
in the type description of S. pritchardi (summarized below) being present.
Many species of Diogmites seem to feed preferentially on bees and
wasps, but I could not find any references in Lavigne's database on Asilidae
prey items that indicated any previous records of Diogmites feeding on
ants. Nor were there any records of Saropogon taking ants.
Very little has been published on the behavior or biology of Saropogon,
so it is hard to say if feeding on ants would be common or uncommon for
Saropogon; it would certainly be out of the ordinary behavior for
Digomites. [Note added 14 December: Bromley, S.
W., 1946, Guide to the Insects of Connecticut. Part VI. The Diptera or
True Flies of Connecticut. Third Fascicle. Asilidae, p. 25, under the
entry ofr Diogmites misellus: "An inconspicuous species found in dry
fields and pastures...I have seen it pick worker ants off grass stems, a rather
unique habit among the Asilidae." And, in Bromley, S. W., 1950,
Florida Asilidae (Diptera), with description of one new species, Annals. Entomol.
Soc. America, 4:227-239, p. 232, entry on Diogmites misellus: "Feeds
on ants which it deftly picks off grass stems."]
Here is the description of Saropogon pritchardi from the publication
where the species was described (Bromley, S. W. 1934. The robber flies of Texas (Diptera, Asilidae).
Annals of the Entomological Society of America. 27:74-110 + plates 1,2):
"Total length, 20 to 23 mm. ... the legs uniformly reddish without dark
markings. The wings are black, the abdomen black with light lateral
pollinose spots, the thorax yellowish pollinose, the scutellum with two pale
bristles.
Male. - Head yellowish gray pollinose. Antennae reddish brown.
Palpi and proboscis black. Palpal hairs straw colored. Mystax,
beard, and occipital bristles pale yellowish. Thorax yellowish brown
pollinose with light yelllowish brown bristles. Legs uniformly reddish
with concolorous hairs and bristles. Wings black, long and broad.
Halteres reddish. Abdomen black, the posterior lateral margins of segments
one to five with grayish pollinose markings.
Female. - Similar."
Bromley listed S. pritchardi for Harper County, OK (on the Kansas
border) and Randlett, OK (Cotton County, on the Texas border); Roger Mills
County is about halfway between them, on the border with the Texas panhandle.
Herschel Raney has a photo of another robber fly from a different subfamily
(Apocleinae: Efferia (Pogonioefferia) pogonias) that had
taken a winged ant on his web site at: http://www.hr-rna.com/RNA/Main%20pages/Efferid%20frame%20page.htm