In the table on p. 752, Aquatic Insects of North America, fourth edition, Protanyderus is listed as having a western distribution, and Protoplasa an eastern distribution. So basically, I'm assuming the genus ID based on a record of geographical distribution rather than morphology. Perhaps not solid enough for BugGuide.

The species and genera do not seem to be widespread.

The only species recorded for Oregon is P. vipio; it occurs from Washington to Southern California. P. margarita occurs in the Rocky Mountains. Also, this genus seems to now be a synonym of Protoplasa.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=7180&context=etd#page=345

According to Alexander(1) Protanyderus margarita is found in Oregon and British Columbia, beyond the range given in the work you cite.

Check the revision. The distribution maps seem to have been updated for these species.

I saw the maps, but I didn't see an explanation of the change from the 1965 catalog. Perhaps not all specimens were examined. I saw at least one note saying a type was not available for study.

According to the revision: "OTHER MATERIAL EXAMINED. See Map 14. USA, OR. Dayton, Willamette, 17.iv.1949, coll. K.D. Ferder [1m#]"

Though the map states the records from north of California as larvae only. Not sure where the association with the adults of P. vipio stems from... earlier publication perhaps. It also mentions that there are records from Canada that weren't included.

I had to poke through the revision a bit, but apparently this is a male specimen that was studied, so the map must be in error with regards to only larvae being known north of California. The data point is missing entirely from the map, actually. This location is near Portland.