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Photo#233726
single-cell nest of a resin bee - Dianthidium

single-cell nest of a resin bee - Dianthidium
Joshua Tree National Park, San Bernardino County, California, USA
October 11, 2008
Size: height: 1 cm
This was found on a bush about two feet from the ground. I'm fairly certain that those are pebbles.

Moved
Moved from Pebble Bees.

Moved
Moved from Dianthidium.

Moved
Moved from Anthidiini.

thanks!
Thanks for everyone's assistance on this.

Well...
Stephen Hart has found a bunch of these in Washington recently, e.g.

and we're now convinced that they're spider agg sacs, but nobody knows for sure what kind. I know that some agelenids cover their egg sacs with debris, but there are probably others that do something similar. Someday someone will have to collect one of these and raise the spiderlings to see what they are--but not from a national park of course.
Edit: I see now that Stephen believes this unidentified spider

is responsible for the ones in Washington. I should add that yours seems to be substantially smaller, but I can see some silk peeking out, and I can't imagine anything other than a spider pulling this off: any kind of larva making a cocoon would be building it from the inside, so there would be no way for the larva to finish it off with a layer of pebbles on the outside.

 
Not related.
THIS image is of the single-cell nest of a resin bee. Unmistakable. Family is Megachilidae, not sure of the genus, but something in the Anthidiini. Very nice find!

 
Aha!
Well, I thought I was seeing silk in there (along the stem), but I guess it's something else. I stand corrected.

 
Another example
There only seems to be one other example of one of these on BugGuide, hidden among the jumping spiders:

It more clearly shows the resin that's holding the pebbles together.

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