05 July 2014

Of pollinia, pollinaria, and milkweed blossoms

Yesterday I was fortunate to observe a phenomenon that I have never witnessed before, though I had read about it years ago.

As I left the house on an errand, I saw a small butterfly frantically quivering its wings, but unable to escape from a milkweed blossom.  I've seen this before when butterflies have their legs entangled in spiderwebs; at a milkweed blossom it could also have encountered a lurking crab spider.

In this particular situation the debate as to whether to "let nature take its course" was irrelevant, and I paused only a few milliseconds before walking over to free the innocuous butterfly from the wicked spider.  Much to my surprise (and intellectual delight), I discovered that the butterfly was entrapped not by a spider or a web, but by the blossom itself..

Milkweed is not a carnivorous plant, but its pollination system can be lethal to small
insects.  Unlike most other flowering plants, milkweed pollen is not free as a powder, but is stored in rather large sacs.  The legs of visiting insects can become entrapped in the structure of these plants; when the insect leaves, it pulls away the entire sac, which then can be transported to the pistils of the next blossom.

An encounter with a pollinarium is not a problem for large insects like Monarch butterflies, bumblebees, or even robust solitary bees.  But the butterfly I saw was one of the small "hairstreaks" which are only the size of one's fingernail (see the embed at right of a different hairstreak).  It lacked the propulsive force necessary to pull the pollinarium away from the blossom, and would have perished there from exhaustion or fallen victim to a roving predator.

I removed from the large milkweed blossom the individual flower holding the hairstreak; it was still unable to escape until I grasped its wings with my other hand and pulled; it then flew away.

The video at the top has excellent visuals of this phenomenon.  A detailed text description with illustrative photos was posted at Eye On Nature in 2012.

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