I never would have figured that one out. What doesn’t make sense is how it made it all the way up to the pool. It is about 50 feet from the river/lake area and up a 15 foot rock wall. What is really interesting is we had massive thunderstorms all last night. Reading on Wikipedia just now saw this;
“Although the larvae spend most of their lives under rocks below water, locals along Virginia and Pennsylvania rivers have reported emergences, known as "hellgrammite crawlings," during thunderstorms.”

I'd like to see a "hellgrammite crawling." Sounds cool.