Perhaps you've seen what appear to be tiny, excessively fuzzy hummingbirds tapping flower nectar this spring. These hummer wannabes hover in front of dandelions, spring-beautys and other flowers, extracting the goods via a disproportionately long proboscis. These are the bee flies, in the genus Bombylius. I believe John Howard's fine photo above, and mine that follow, depict Bombylius major, a common species in these parts. Don't stick a fork in me if I'm wrong, though - it's a huge family, and I'm no Bombyliidophile.
This is the nearly mythical whale of northern seas known as the Narwhal. One of its teeth is modified into a greatly elongated tusk, making it a unicorn of the sea. Skip back to the first photo, then revisit this Narwhal photo. The bee fly might be called a "fly-whal", and indeed it has been.
The Narwhals (females rarely have tusks) apparently have these protuberances as an outward sexual characteristic. The bigger the tusk, the bigger the stud, and the higher its status in Narwhalian social hierarchy.
The bee fly, on the other hand, uses its "tusk" to suck nectar from flowers.
A female bee fly on the hunt lurks around the proximity of nesting solitary bees, awaiting the perfect moment. As the bee finishes its work in the burrow, and backs out to prepare to seal the entrance, the bee fly springs to action. She moves near, and flicks her eggs down into the chamber. If successful, the bee fly eggs get sealed in as well. You can guess the rest. The bee fly egg spawns a grub which eventually eats the host solitary bee's grub. And, apparently, the pollen, too.
Bee fly = 1. Solitary bee = 0.