Backyard Biology

Nature stories from my backyard and beyond

Nature Stories

A Well Dressed Spider

Tuesday, August 7, 2012

Good things come to those who wait.

This old saying came to mind as I watched the bumblebee. It was sitting on a goldenrod flower. Perfectly still. Nothing was moving. Not an antenna or a wing. The question I asked myself was - what was the bumblebee waiting for? Upon closer inspection, I discovered that the bumblebee wasn’t waiting. The bumblebee was the good thing that came.

It had come to a goldenrod crab spider. The spider, had done the waiting. And it was rewarded by a meal in the form of the bumblebee. The bumblebee had not seen the spider, or ignored it - hard to tell which - and paid the ultimate price. One bite from the spider and the bumblebee was instantly immobilized, only waiting to be devoured by the spider.

The goldenrod crab spider has a completely transparent outer skin or exowhite crab spiderskeleton. There are no color pigmemts underneath the exoskeleton, so light shining on the spider travels right through the exoskeleton and reflects off white crystals under the skin - giving the spider a white color.
This is fine when the spider is on a white flower. But the spiders are often found on goldenrod and other yellow flowers - hence the name goldenrod spider. A white spider on a yellow flower would certainly stand out. So, in a matter of days, the white spider changes to a yellow spider. To do this, yellow granules are formed just under the outer layer of skin. Now there is a yellow spider on a yellow flower. And if the spider should end up back on a white flower, the granules decompose and once again we have a white spider on a white flower.

crab spiderIt has been demonstrated that this color change is purely a visual effect - if the spider sees it is on a yellow flower, it changes to yellow. If it sees it is on a white flower - a change to back to white. If the eyes of the spider are painted over so the spider can not see the flower it is sitting on, no color change will take place.

It is easy to see what the spider is doing. Much harder is figuring out why it is doing what it is doing. Why does the spider change color? What benefit does this action provide the spider? Creating and destroying these granules requires energy on the spider’s part. So color changing must serve some vital purpose.
Of course the obvious answer is camouflage. If the bumblebee sees the spider, it will stay away from that flower, and not provide dinner for the spider. Also, a well camouflaged spider will be better protected from its predators.
What other reason could there be for the color change. It seems so obvious that that must be the reason. In fact, this is what people believed for a hundred years.
However, keep in mind that not all animals see the same colors as we humans. For instance, many insects, including bumble bees, see ultraviolet light that we can not see. Also, while the yellow color of the flower and the spider may seem to match perfectly to us, to a bee they do not.

Trying to figure out what purpose the color change plays in the spider’s life, a group of researchers performed a series of experiments with spiders and flowers of different colors. They came up with three possible scenarios that they could test.
1. Crypsis - the color change makes the spider invisible to prey. Insects are more likely to visit flowers with camouflaged spiders than flowers with spiders that stand out.
2. Spider avoidance - flowers with crab spiders are avoided regardless of whether or not the spider matches the color of the flower.
3. Flower visitation attraction - flowers with spiders are visited more often than flowers without spiders.

Here is what they discovered:

Crypsis: Flowers with different colored spiders (e.g. white flower and yellow spider) were visited just as often as flowers with the same colored spiders. Clearly, camouflage was not working in the spider’s favor.

Spider Avoidance: Some insects, such as solitary bees and hover flies either avoided flowers with crab spiders or spent less time at the flowers. Honeybees and bumblebees, however, seemed to ignore spiders on flowers, irregardless of the color - often to their own cost. Not surprisingly, honeybees and bumble bees are the primary food source of the goldenrod crab spider.

Flower Visitation Attraction: As one would expect, flowers with crab spiders did not attract more visitors than flowers without crab spiders.
However, a study of a different type of crab spider showed that bees were more likely to visit flowers with this type of crab spider present. The center of daisies absorbs ultraviolet light. Since we humans don’t see ultraviolet light, we can not notice this. Bees, who can see ultraviolet light, notice the center of a daisy as a black bullseye - a target for them to land on the nectar producing part of the flower. To a bee, this bullseye means a sweet meal.
It turns out that the abdomen of this crab spider also absorbs ultraviolet light. Therefore, when the spider sits on the pedals of the flower, the bee sees two bullseyes - the flower center and the spider abdomen. Twice the sweetness. Of course, in this case, the sweetness all goes to the spider.

So, back to our goldenrod crab spider. What is the purpose of the color change? At this point, no one really knows. At this point, your guess is as good as anyones. Personally, I like to think the goldenrod crab spider likes to be color coordinated. Just a well-dressed spider.