This one certainly intrigued me when I found it in the late afternoon on the native mint Monardella exilis. Like other BG posters, I thought it was a strange hymenopteran...with flamboyant antennae and oversized tegulae. It allowed me to approach quite closely...so close that I began to wonder whether it was alive. It was windy, so I reached out to hold the plant still, and soon was able to actually touch the "wasp". It was still alive, but very lethargic...and it seemed to be stuck to the flower head. Later I realized it was a beetle of the bizarre genus Ripiphorus, whose members are very short-lived, and it was apparently ovipositing on the flower head while on its last legs.

Once I got the idea (from the antennae) that it might be a ripiphorid, a quick check on BugGuide cinched the notion, and a reading of Linsley & MacSwain(1)(1951) indicated this was R. vierecki. Then I ran into comments and this forum post by BG contributor Edward Ruden, pointing out that R. vierecki and R. rex are very similar looking, and that while Linsley & MacSwain(1) didn't include the species R. rex in their treatment, the 2006 field guide by Evans & Hogue(2) indicates it occurs in southern to central CA. Edward also noted that Evans & Hogue(2) mention (without reference) that R. rex "is unique among the species of Ripiphorus in having a serrated outer edge of the middle tibiae", and Edward provided a wonderfully detailed image of a serrated tibia edge to illustrate that character on a post he made of R. rex.

While I don't have an image of the detail (or quality) of Edward's, I do think I can barely make out the presence of what may be similarly serrate outer edges on the tibiae of my specimen (see 6th image in this series). Edward's forum post also referred to a 1955 paper by Vaurie(3), and I noticed that she stated another distinguishing character of R. rex not mentioned in Linsley & MacSwain(1), Evans & Hogue(2), or the earlier 1929 treatment by Rivnay(4). Namely, among all the specimens of R. rex Vaurie had studied, one of that species significant characters was "the densely hairy elongate projection on the inner side of the front coxae at apex". I think I may see that structure in the full-size versions of some of my images. (See, for instance, the 5th image and the cropped detail in the last image of the series.)

All this seems to point to a determination of R. rex here, although I'm not certain all my interpretations are correct, and it seems these two taxa...and the genus as a whole...could benefit from clarification and revision (e.g. see =v='s comment here). Nevertheless, considering all the above, it seems reasonable to place this under R. rex, and move it elsewhere if future comments suggest that's the right thing to do.

Two last remarks. The one-sided tapering processes ("flabella") on the antennae here, indicate this is a female. And, it may be of interest to note the Ron H.'s nice post of an ovipositing ripiphorid was also photographed on the flower head of a native Monardella.