There is some excellent information on aquatic insect tolerance at http://course1.winona.edu/nmundahl/Segment2.pptx (you'll need PowerPoint to view this information). Note that he shows staoneflies and mayflies in the most sensitive group and caddis flies a step down (being somewhat more tolerant of pollution). However it also appears as though there is quite a bit of variation within both mayfly and caddis species -- eg, Baetidae are more tolerant than Ephemerella.