This is almost surely Notogramma stigma. There are numerous specimens of this species in the Florida State Collection of Arthropods (FSCA), and they range from the Everglades in the south to Orange Co. in central Florida (also Highlands Co.). Presence in Florida is not surprising, as Steyskal listed Cuba and the Bahamas as part of its distribution. Notogramma purpuratum probably does not occur in Florida. The FSCA does not have any Florida specimens of N. purpuratum, and Steyskal stated that “No specimen of N. purpuratum has been seen from “American Islands” east of Mexico.” Another species, N. cimiciforme also is present in Florida, probably less commonly than N. stigma, as only 3 specimens are present in the FSCA (Dade and Lee Counties). The photos do seem to show a clear spot in the costal band and lack of punctate color pattern on the thorax laterally, which would exclude N. cimiciforme.

According to Steyskal (1963), the clear spots at the end of R1 make this N. purpuratum as does the cactus habitat. Also, he did not find any specimens from the United States outside of southern Texas. Cimiciforme is found in Cuba, not far from Florida.

In the U.S., Otitidae and Ulidiidae are considered synonyms, or so I thought. Can someone please enlighten me?

In spite the name Otitidae Aldrich 1931 was used for a long time for the family as a whole in the USA, the name Ulidiidae Maquart 1835, which was in use simultaneously for a part of it (as a subfamily name) or the family as a whole, has a priority according to the Code. This use of the names was justified and explained in details by Kameneva and Korneyev (2006).

For pdfs, see here: http://sites.google.com/site/seioptera/list-of-references

So, in the manual of nearctic Diptera Ulidiinae is considered being a subfamily of the family Otididae. The family is divided in two subfamilies, the other one being Otitinae. The Otitinae are predominantly of north temperate distribution, whereas Ulidiinae is well represented in the tropics.
In Europe however, most dipterologists saw the two as separate families, so Otitidae and Ulidiidae! The latest change in ideas is that there is only one family, Ulidiidae, of which Otitinae is a subfamily.
So Eric is quite right in saying these are synonyms!
I thought you guys were still following the nearctic manual classification! :-)))
Greetings,
Gerard Pennards

This image shows better wing patterns, the dark color located along veins instead of spots in cell, again just a guess - Chen

So I guess that unless I can get one of them to sit still for a better portrait we're stuck at Otididae?

You're stuck with Ulidiidae that is! :-))
There is already a page about this family in the guide, you can put it there!
Greetings,
Gerard Pennards

Ahem. I'd prefer you experts do the organizing of nomenclature. I can be the shutterbug for now. Maybe later . . .

Moved to Ulidiidae; even not knowing much about nomenclature you could move the images yourself next time;it is easy after somebody tells you where they belong.