Unafraid to be yellow

Most of the crab spiders that I find are white.  Not only do I find white crab spiders on white flowers, I also find them on pink, purple, red and blue flowers.  The exception is on yellow flowers, where the crab spiders tend to be a matching yellow.  Therefore, when I found a white crab spider hunting on the very yellow flowers of a goldenrod, I decided to keep track of it for the next few days.

Day 1: A white crab spider on yellow goldenrod flowers.

On the first day that I observed it, the white crab spider was doing quite well for itself despite not matching the color of the flowers.  It had caught a large fly, thus procuring for itself a substantial meal.

Day 3: The spider is pale yellow.

Notwithstanding its previous success as a white spider on a yellow flower, the spider had soon acquired a pale yellow hue.  After another two days, the crab spider had acquired both more yellow and another fly.

Day 5: The spider has caught another fly.

How had the crab spider turned yellow?  The color change was first triggered by the crab spider seeing yellow around it.  In response, the crab spider began to synthesize yellow pigments which it then kept in cells near the surface of its body.  (To return to white, the crab spider could move the yellow pigments away from the surface to be stored until needed again.)

Day 6: The spider is the same bright yellow as the golden rod flowers.

Why had the crab spider turned yellow?  Although the answer seems simple -- to hide from potential prey and predators -- studies of crab spiders have failed to find a benefit from either increased prey capture or decreased capture by predators.  Nevertheless, the metabolic cost of synthesizing the yellow pigments suggests that a benefit must exist (or at least one must have existed until relatively recently).