Leaving the office the other day I noticed a yellow spider sitting on a plant close to the door. Don't ask me how I spotted him, I just did. Turns out it was a Yellow Flower (or Crab) Spider. They are of the family Misumena and can either be yellow or white depending on the flower they are hunting on. During early summer young males can be quite small and easily overlooked, but females grow up to 10 mm (excluding legs) with males reach 5 mm at most.
At the time I assumed the object on this picture was an egg sack and I have a feeling I may be right. Older females require large amounts of relatively large prey to produce the best possible clutch of eggs.
Generally it is often very hard even for a searching human to recognize one of these spiders on a yellow flower so an insect wanting to land on a flower occupied by one of these better make sure his will is in order.