The bear who swallowed a fly

Overview

A bear is sleeping peacefully in the woods when he accidentally swallows a fly. He can't stand it buzzing around in his belly. On the advice of a squirrel, he swallows a frog... who croaks inside him; then a grass snake, whose wriggling tickles him; then a heron, whose beak prickles him. What lengths will he go to to stop this torture?