Effects of Microhabitat Selection on Feeding Rates of Net‐Spinning Caddisfly Larvae

Abstract

Net—spinning caddisfly larvae of the family Hydropsychidae are known to prefer microhabitats with large, stable substrate and high water flow velocity. It is often assumed that net spinners in high—velocity microhabitats have higher feeding or growth rates than larvae in less preferred sites, but there is no direct evidence to support this assumption. We hypothesized that net—spinning caddisflies would select microhabitats that offered the greatest feeding rates. This hypothesis was tested by field experiments in which we determined if net—spining caddisfly larvae preferred high—velocity sites even when substrate size and type were held constant. We then measured feeding rates of net spinners in microhabitats with different flow characteristics. High—flow positions were selected by 96% of hydropsychid arvae colonizing artifical moss substrates. Artemia nauplii released into the water column were captured by individual larvae in high—flow sites at a rate of 0.016%/m, significantly higher than the capture rate in low—flow sites. Combing this rate of prey capture with mean hydropsychid densities of 1125 individuals/m2, we estimate that hydropsychid larvae in riffles remove drifting invertebrate prey at a rate of ° 18%/m. Assuming exponential prey removal, a prey item in the drift would travel an average of only 5.5 m before being consumed. This study is one of the first to show that the distribution of a stream filter feeder is related to the feeding rates obtainable in different microhabitats.