Parachute BWO - Calf or Poly Post As an alternative to the Classic BWO, Blue Winged Olive Parachutes work just as well and may be much easier to see on the water, especially if fish are "sipping the foam". Our Blue Wing Olive - Parachutes come with either a Poly or poly post. Both are effective, but older, more experienced anglers often prefer the standard, Poly post. BWOs are a Mayfly pattern in the Baetis family. Often small in size, their numbers can be staggering, and only realistic patterns will do the trick. Mayflies typically hatch in the spring and the fall, but are available as trout food all year around, and are one of the most abundant mayflies, making appearances in nearly every moving body of water. Sizes: # 12 - # 22