One thing that I didn't reveal is that there were other bees nearby that looked very different than the bees I showed last night. Instead of being mostly brown and black, they were much paler overall with fuzzy white hairs, green eyes, and long antennae (see below).
Occasionally, these paler bees would "attack" the browner bees that I described last night. See the next two images for examples of action shots:
Because I didn't know any better at the time, I thought the paler bees might be a different species and that they were exhibiting territorial behavior.
However, Robbin has confirmed that both the paler individuals and the browner individuals are Melissodes pallidisignata — the paler bees are males and the browner bees are females. So the "attacks" I observed were more likely males trying to mate with females.
Here's one short sequence that was intriguing. A male flew in towards a female who then lifted her hind legs (covered in pollen) vertically.
Below, the male flew off and the female went back to digging.
But about 10 seconds later, he (or another male?) returned and this time made contact with the female.
Robbin also was very generous in sharing information about the life history of Melissodes pallidisignata. When a female burrows into loose soil, she is accessing a previously excavated burrow with a nest chamber (review the post from 31 July 2013). She'll deposit only one egg per chamber and provision the chamber with pollen (which will take several trips!). A female may stay underground for up to 15 minutes and then emerge to go on another trip to find more pollen. It's a significant parental investment!