rain bugs

South Park S1 E3, Bugs that crawl up your legs and bite the inside of your ass (according to Eric Cartman).

Kyle: Hey Stan, did you see that rainbow this morning?
Stan: Yeah, it was huge!
Eric: Ugh, I hate those rain bugs things! ... you'll just be sitting there, minding your own business, and they'll come marching in and crawl up your leg and start biting the inside of you ass. ...
Stan: Eric Cartman, what the hell are you taking about? ...

Kyle: Rainbows are those little arches of color ...
Eric: Oh rain bows, oh yeah I like those, those are cool...