enlarge   Milkweed flowers have a unique anatomy.

5 fused anthers and stigmas form a central "corona" atop 5 reflexed petals.

Pollen are packed into sacs called "pollinia"; pairs of pollinia hang like saddlebags within pockets of the anthers.

Hidden within the corona are also stigma slits to receive pollinia.
  A pollinating insect, such as this Honey Bee visiting a Prairie Milkweed (A. sullivantii), often gets its legs trapped in the anther pockets.

When she pulls her legs out, the pollinia are carried off, and may drop down the stigma slit of another milkweed blossom, thus achieving pollination.
  The pollinia burden can be quite heavy; this bee rests awhile with her luggage.

Bald-faced Hornets may also be an unwilling pollinator.
  Sometimes a Honey Bee fails to free her legs and becomes permanently trapped, eventually dying of dehydration.

A close-up of one of her legs reveals a pollinium sac still attached.
  Japanese Beetles may also get trapped; this pair tries out a new love position.

Occasionally, a milkweed turns into a mass murderer, trapping numerous insects like these Japanese Beetles on the right.
The milkweed nectar is so fragrant that insects are willing to risk life and limb harvesting the liquid, each intoxicating step potentially their last.

Other visitors to milkweeds include:
Milkweed Tussock Moth
Red Milkweed Beetles
Large Milkweed Bugs
Great Golden Digger Wasp
Hummingbird Clearwing
Copyleft Peter Chen