Trout Are Afraid of Spiders too
Spiders are a classic for a reason. They're a simple fly, not amounting to much more than a slim body with a wisp of hackle. Stick them in the water though, especially swung in any form of current and the come to life in a flash. Breathing, dancing through the water column and generally screaming "I am delicate and in distress, please eat me!"
With a classic slash of red about it's through this spider is particularly vulnerable to trout attack, so fish it with the expectation of an eat.