Most caddisfly larvae build cases of silk and a variety of collected materials. Multiple functions, including protection from predators, resistance to entrainment by high flows, and improved respiration, have been suggested for caddisfly cases. We investigated the functional role of cases built by Dicosmoecus gilvipes, a limnephilid caddisfly. In this species, the 1^st- through 4^th-instar larvae build cases with plant material and attach Douglas-fir needles as lateral extensions that resemble vanes on an arrow. We tested whether the lateral extensions and entire case deterred predators by manipulating lateral extensions and case presence for larvae exposed to large steelhead trout. No larva with a case (with or without lateral extensions) was consumed during the experiment, whereas all larvae without a case were consumed. We tested whether lateral extensions provided stability against overturning and entrainment by manipulating presence of lateral extensions and subjecting larvae to turbulent flow conditions. Once dislodged, larvae with lateral extensions experienced fewer revolutions and regained their footing faster than those without extensions. Our results suggest lateral extensions provide stability against overturning in fast flow and may improve the ability of larvae to forage efficiently in turbulent flow conditions. Other caddisfly species build lateral extensions on their case, and the extensions may provide similar benefits for these taxa.