The flies feet are stuck with sticky milkweed pollen pads. This is how milkweeds are pollinated. Insects land on the flowers, their feet, or sometimes their proboscis, sinks into the flowers, and when they pull free and land on other milkweeds, the pollen is transferred. Unfortunately for the insects, many can't pull free, and those that do have to fly around with these sticky things attached to their feet. This fly (a bumble bee mimic) is well and truly laden with the pollen pads.