[For full-size image, click this link...then click the image again after it loads in your browser window.]

Detail of previous image, with a few salient items labelled...

First, one character to get to the tribe Cephini in Middlekauff's 1969 treatment of CA Cephidae (PDF here) is: "hind basitarsus not as long as the 3 following segments together". That's apparent in the image above.

Next, Middlekauff gives only two US genera in Cephini: Cephus and Trachelus. He states the later is an introduced pest from Europe which lacks yellow dorsal bands on the abdomen and is (or at least was) not known from California. So we can presume this is Cephus.

Now, the first couplet in the key for Cephus includes the character "abdominal sternite 8 in male apically notched" which again is apparent in the image above, and takes us directly to Cephus clavatus.

Last (just for fun), note the two trochanter segments of the hind leg. This is a key character for both the suborder Symphyta and the (apocritan) family Ichneumonidae, according to Borror & Delong. Regarding Cephidae, Goulet(1) states that: "They are unusually slender for sawflies" and "to the untrained eye adults are very much like those of Ichneumonidae".

See the 1st image in this series for link to companion post with more info.