What looks like an hourglass-shaped paint splotch on the thorax of some bees is actually pollen. In the past, I often saw these stripes — usually in yellow — and wondered what they were. The bees look like they squeezed through someone’s freshly painted woodenware.
But according to Rosanna Mattingly in her fascinating book, Honey-Maker, the design occurs when pollen-covered bees groom. The honey bee uses her two midlegs to clean pollen from her forelegs and the back of her thorax. However, there’s a place she can’t quite reach, right down the middle of her back.
She swipes each side of her thorax, removing the pollen in an arc, much like the sweep of a wiper blade on a car. The hourglass design remains after she’s reached as far as she can on each side.
From the comments below, I learned a word for these unreachable places. According to Wikipedia, the acnestis lies between the shoulders and the lower back of an animal, a place out of reach for scratching (and grooming).
Honey Bee Suite