Thursday, September 27, 2012

Tube Flowers and Their Pollinators

Butterfly bushes (Buddleja sp.) are very well-named.  The ones in our outdoor classroom are usually surrounded by several butterflies flying from flower to flower and filling their butterfly bellies with nectar.  If you look closer, you'll notice that lots of types insects like butterfly bush nectar, and even hummingbirds have been known to drink from these flowers.  Because of the shape of butterfly bush flowers, not all nectar-feeders are able to use these plants.  All butterfly-bush-feeders must have long, thin mouthparts that fit into the flowers.  Notice the long curved proboscis on the skipper in the photo below.  The proboscis works like a silly straw, curving and extending into the base of each flower for a sip of nectar.
A skipper sipping nectar on a butterfly bush.
Below you can see one individual flower of the butterfly bush.  The green bit is the base of the flower where nectar is produced.  The purple petals form a tube that opens at the top of the flower.  The tube shape does a good job of keeping out insects that steal nectar without pollinating the flower.  Insects that can reach their mouthparts into the flower receive a dusting of pollen as they sip nectar.  When the insects move to the next flower, they drop off the pollen, allowing the flower to produce seeds.  Nectar-sippers and flowers have a trade-off where each organism benefits from the arrangement: flowers are pollinated and the pollinators get food.  This relationship is called a mutualism, and it is a type of symbiosis where two organisms benefit from the interaction.
A single tube-shaped flower of the butterfly bush.
The purple tube-shaped flowers have orange centers to help insects find their way into parts of the flower where the nectar is produced.  Most flowers have nectar guides in their centers.  Usually nectar guides are yellow or orange regions with lines pointing to the center of the flower.  Next time you are at the outdoor classroom, look for nectar guides in butterfly bush flowers and any other flowers you'll see.
Yellow/orange nectar guide inside a butterfly bush flower.
Below you can see two more organisms that are mutualists with butterfly bushes: bees and longhorn beetles.  Both pollinate the flowers and get fed in the process.  There are some insects that 'cheat' the butterfly bushes out of their pollen.  Some types of caterpillars, beetles and ants chew through the base of the flower, drink the nectar, and leave without pollinating the flower.  Look carefully at our butterfly bush flowers for some crime-scene evidence:  if you see holes chewed through the base of their tubes, you know the nectar has been stolen with no pollination payment in return!
Bumblebee and longhorn beetle working the butterfly bush flowers.
Interesting side not:  The bumblebee in the photo above was not actively feeding.  It was just holding on and resting.  This time of year, the temperatures are dropping, and insects that don't overwinter are nearing the end of their lifespan.  It could be that this bee is slowing down because it is old.  Alternately, the bee could be finding its home for the night, since I took this photo in the early evening.  Bumblebees often sleep in large flowers or near small flowers so they have a food source in the morning (wouldn't you love to sleep in a flower?).  A third possibility is that the bee is just slow because the temperature is lower.  Some bees just slow down when the temperature drops, stay in a torpor through the cold winter, and then speed back up again in the summer when it's warm.

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