Many diverse animals use arrays of hair-like structures to perform important jobs such as feeding, gas exchange, smelling, and swimming. Since these functions involve hair interactions with the surrounding water or air, analysis of the fluid dynamics of diverse hair-bearing appendages reveals how the morphology of an array of hairs affects it performance. Mathematical and physical models of flow between cylinders have shown that arrays of large, rapidly moving cylinders are leaky sieves, whereas little fluid moves through a row of small, slow rods. The purpose of the present study was to test this prediction for realistic appendage morphologies and to elucidate whether the design of a hairy leg can affect the range of speeds in which this transition in function occurs. We studied flow through hairy food-capturing appendages (second maxillae) of calanoid copepods, abundant planktonic crustaceans whose feeding on unicellular algae forms an important link in many marine food webs. Using dynamically scaled physical models, we found that hairy appendages undergo a transition between paddle- and sieve-like function at a critical range of sizes and speeds. The coarser the mesh of hairs on second maxillae, the smaller the size and speed at which this functional shift occurs. Thus, a simple increase in size (ontogenetic or evolutionary) or speed can generate a novel function (a paddle can become a filter), but the morphology of a hairy appendage determines the size and speed range at which leakiness to fluid flow can be affected by behavior or growth.