Massive Black Fly aka Mexican Cactus Fly
It was a Mexican cactus fly, Copestylum mexicanum, feeding on nectar, and this was the first time I had seen them around the Museum!
Here's what Flower Flies of Los Angeles County book has to say about them:
"This is the largest flower fly in Southern California, with a body length of 18mm. It gets its name from the larvae that feed in wet decaying prickly pear cactus. Adults are commonly seen feeding on flowers. Although it resembles some carpenter bees this species looks more like a large horse fly. The Mexican cactus fly is found from the southern USA to Central America, but related cactus-feeding flower flies are found in Mexico and South America. Many other species of the large genus Copestylum are found in our area. None of them resemble the cactus fly; instead they mimic bees and other stinging insects. All feed on decaying organic matter."
*Thanks to Museum Curator of Entomology, Brian Brown, for identifying the fly!