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I'm posting this series of 3 images in reference to the discussion on this forum relating to the distinctions (if any) between Rhipiphorus rex (original 1891 description by Champion, in "Biol. Cent.-Am., Col. IV, pt. 2, p. 360."---not available to me [Update on 1/22/13: I found it! See here.]) and Ripiphorus vierecki (original 1907 description by H. C. Fall, as Myodites vierecki, available here).

The image above shows a female specimen from the California Academy of Sciences (=CAS) that was labelled as Rhipiphorus knausi Pierce (Pierce's original 1920 description, as Myodites knausi, is here). Note that this specimen was collected in 1927, and that afterwards Myodites knausi was reassigned as a junior synonym for Rhipiphorus vierecki in the revision of Rivnay(1)(1929), and similarly treated in the later bulletin of Linsley & MacSwain(2)(1951).

To the best of my understanding, it appears that all the characters for R. vierecki in the keys from Rivnay(1) and Linsley & MacSwain(2) can be verified in this image and the preceding image...and Rivnay's description of R. vierecki fits well. In particular, we have:
  * First segment of hind tarsus obliquely truncate and emarginate at tip, and shorter than all others combined;
  * Female antennae pectinate, 11 segmented, rami very long, decreasing in length toward apex;
  * Female shining, head and thorax dark; abdomen straw-yellow variegated with brown-black.

Regarding R. rex, the 1955 reference by Vaurie(3) states that Champion's original 1891 description was based on a single male specimen from Mexico, and that Rivnay had not seen any specimens of R. rex when he wrote his revision. This helps explain Rivnay's minimal remarks, the most salient one being that R. rex could be "separated from allied species by its carinate tergites". Linsley & MacSwain(2) did not mention R. rex, mostly likely because it was not thought to occur in California at that time. Based on 28 female and 8 male specimens of R. rex, Vaurie(3) stated three "significant characters" for the species:
  1) a deep and very oblique apical emargination of the first segment of the hind tarsi;
  2) a densely hairy elongate projection on the inner side of the front coxae at apex; and
  3) a median carination of the dorsal segments of the abdomen.

In hand and under a microscope, 1) and 3) above were evident for the R. knausi specimens here...and a median carina (or "keel") is fairly discernible (via flash highlights) on most the specimens in the full-size version of the 1st image in this series. It was difficult to discern character 2) for these specimens. I had a hard time getting a clear, unobstructed, and well-illuminated view of the inner bases of the front legs on most the pinned specimens...and I'm not certain how to interpret character 2), since I have no reference point for the relative sizes and shapes of trochanter and coxa in this genus. But I did see relatively long, hairy, conically-shaped projections at the inner bases of the fore-legs (which I'm presuming were the coxa), and these looked similar to those in my field specimen. I didn't have the time (or curatorial dexterity!) to make a careful study of these "coxal projections" for very many specimens, during my late afternoon visit to the CAS collection. But from what I recall, the "conical projections" varied somewhat in their hairiness and color among the 8 or so specimens of R. knausi and R. vierecki I looked at...and perhaps none were quite as hairy as my field specimen, or quite as light-yellow in color. (Here are specimen images exibiting darker "coxal projections".) So this character showed some promise of distinguishing R. rex from R. vierecki...though not as clear-cut and decisive a criterion as I'd like. (Where do you draw the line between hairy and "densely" hairy? And color was not mentioned here by Vaurie(3)...is it a consistent and significant character?)

Moving forward in time from the literature up to 1955, the relatively recent 2006 field guide by Evans & Hogue(4) mentions that R. rex does indeed occur in southern and central CA...and that "it is unique among the species of Ripiphorus in having a serrated outer edge of the middle tibiae". It's not clear how recently this character was noted, or how widely it is (or has been) known. But one can clearly see serrated outer edges on the mesotibiae in the full-size image of our female specimen herein...and such meso-tibial serration was definitely manifested in other specimens I examined from the CAS box of knausi specimens seen in the first image of this series, as well as some labelled as R. vierecki...like the specimen imaged here.

So summarizing, all the above indicates that these CAS specimens, labelled as R. knausi, seem to conform for the most part to the various published characters determining R. rex. (The same statement held for some other CAS specimens I examined that were labelled as R. vierecki.) It's probable that the 1927 specimens were initially labelled before the synonymy of R. knausi with R. vierecki was known, and before the serrate tibia character for R. rex was known. I'm guessing that over time, with few or no experts in this group available to curate the collection, it was decided that the genus was not well-understood, and that it was best to leave the specimens labelled as R. knausi until the time came when significant progress was made.

Of course, the question arises as to whether R. vierecki and R. rex may turn out to be synonyms? In this regard, it was interesting to note that, in searching the specimen databases for various museums (e.g. CAS, Essig, LA County Museum, MCZ) I could find specimen records for R. vierecki...but none for R. rex (save for Champion's type at the British Museum). Yet the CAS specimens of R. vierecki which I examined seemed to agree with R. rex!   This made me wonder whether specimens labelled as R. vierecki in other collections may also turn out to agree with the characters of R. rex? Or perhaps I was misinterpreting those characters? For instance, maybe the tibial serrations and dorsal carina of R. rex are VERY pronounced, compared to the relatively subtle instances I saw. This could be addressed with bonafide specimens of R. rex in hand. Vaurie(3) was affiliated with the AMNH...so hopefully they have bonafide specimens of R. rex...but I was not able to access their collections database to check. Arnett(5) mentions here that widespread synonymy has long been a problem in Ripiphorus, and that neither Rivnay(1) or Linsley & MacSwain(2) adequately addressed the problem. J. M. Campbell, in a brief summary of the family Ripiphoridae (PDF here), states that "species of Rhipiphorus, the largest North American genus of the family, are poorly known and cannot be reliably determined". Despite that cautionary statement, I'm presuming (or at least hoping!) that specimens of some of the more common sorts can be meaningfully placed to species (or at least species groups) on BugGuide and elsewhere. Overall, as has been stated in various Ripiphorus posts on BG, all this points to the pressing need for an effective revision of this genus.