Robber Fly, similar to the Bombus vosnesenskii, yellow-faced bumblebee - 227/365
Somebody recently asked me if I had started naming my bees. As if. They aren't actually my bees. I just find them. It's not my place to name them. But sometimes they tell me their names. This is Mildred. I found her on NitWit trail. She's a bumblebee of the genus Bombus, the Latin word for buzzing or humming sound.
In general, bumblebees are not aggressive. They do not swarm, however, they can sting multiple times if annoyed. When foraging on a flower or resting, a bumblebee will raise one of its middle legs when it feels threatened. It's a sure sign that you should back off! There are multiple YouTube videos circulating the Internet of people claiming a bumblebee is giving them a high-five when in fact the bee was likely saying, "Step off dude!"
There are 250 species of bumblebees worldwide. This particular species may be Bombus vosnesenskii, or yellow-faced bumblebee. It's a common species native to west coast North America and is a very important pollinator in commercial agriculture, especially for greenhouse tomatoes. B. vosneskenskii is quite large, ½” - ¾” long, their bodies and wings are mostly black but their head and thorax are yellow. Typically bumblebee queens do not forage, but in this particular species, she does, toward the end of the season as worker bee numbers decline.
While bumblebee numbers have been declining worldwide since the early 1990's, some species even thought to be extinct, the yellow-faced bumblebee are among the species whose numbers are stable or possibly increasing. While reasons are unclear, scientists do know that they can live in urbanized areas. So plant a garden to attract bees! Enjoy them with your eyes but avoid giving them high-fives.
Update 10/8/2015: I just recently learned that Mildred above is not a bumblebee but a robber fly! I had no idea. Robberflies can be decpetively similar to bumblebees, especially the yellow-faced bumblebee. But the educational information above still stands.