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Photo#1642617
Black Fungus Gnat on Scoliopus bigelovii - female

Black Fungus Gnat on Scoliopus bigelovii - Female
Pulgas Ridge Open Space, San Mateo County, California, USA
March 13, 2019
Size: About 2.5 mm
Full-size image here (click image after it loads in your browser to see fully enlarged version).

This nemotoceran was found late in the day probing exudate from the sepals of the interesting plant Scoliopus bigelovii. It keyed to family Sciaridae in the Manual of Nearctic Diptera(1).

Fungus gnats (Sciaridae and Mycetophilidae) are often described as the primary pollinators of Scoliopus...and the odor and parallel purple stripes of the sepals are thought to mimic gilled mushrooms frequented by fungus gnats. Indeed, the 1980 paper by Mesler, Ackerman, & Lu entitled "The Effectiveness of Fungus Gnats as Pollinators" indicated that fungus gnats where found to be effective pollinators of S. bigelovii at their study sites in far northern CA (i.e. Humboldt Co.).

However, I also photographed a larger muscoid fly on Scoliopus years ago (see the CalPhotos posts here and here) and those photos appeared to present a more closely-adapted morphological "fit" in terms of placing pollen from the Scoliopus anther precisely on the dorsum of the fly's thorax as it positioned itself to feed at the nectary of the associated sepal. In comparison, Dee Hime's photo below nicely illustrates how the smaller size of the fungus gnat results in its thorax not reaching the pollen-bearing surface of the anther while nectaring:

     

Note that, in the paper Moldenke(1976), a brief table entry indicated his field observations suggested muscoid flies were "major" pollinators of S. bigelovii, although he found the (self-incompatible) flowers were "seldom visited" and had "low seed set".

The aforementioned paper Mesler et al(1980) reconciled their observations with those of Moldenke as follows:

"The muscoid flies observed by Moldenke are presumably more effective pollinators of S. bigelovii than fungus gnats, on an individual basis, because of their larger body size. The differences in fruit set reported by us and by Moldenke probably reflects differences in pollinator abundance and visitation frequency at our respective field sites."

So presumably both types of flies (and perhaps others?) act as pollinators of Scoliopus, though which figures more prominently in a given instance may depend on local ecological factors.

Images of this individual: tag all
Black Fungus Gnat on Scoliopus bigelovii - female Black Fungus Gnat on Scoliopus bigelovii - female Black Fungus Gnat on Scoliopus bigelovii - female Black Fungus Gnat on Scoliopus bigelovii - female