Is the mobility of stream insects related to their diet?

The relationship between mobility and diet in eight common stream insects was investigated in a New Zealand mountain stream. Propensity to drift, distance drifted (return rate to the streambed) and propensity to colonize patches of stony substrata were used as measures of mobility, while diet was assessed by gut contents analysis. The eight insect species formed two well-defined groups on the basis of diet; leaf-eaters (shredders) with homogeneous diets, and grazer-collectors with more heterogeneous gut contents dominated by elements of epilithon and fine particulate detritus. With the exception of the eased caddisfly Olinga feredayi, shredders (two trichopterans, a plecopteran and a scirtid beetle) had a high propensity to drift, settled from the drift slowly, and were strong colonizers of substrate patches. In contrast, grazer-collectors (two ephemeropterans and a plecopteran) exhibited a much lower propensity to drift, returned to the streambed rapidly and colonized stony patches less readily. Despite being very similar in size, body form and mouthpart morphology, the two stoneflies had very different diets and mobility characteristics. By comparison, the two mayflies differed strongly in body form and general behaviour (crawler v swimmer) but had very similar diets and levels of mobility. We propose that the contrasting mobilities of the two groups of insects reflect differences in the distribution (patchiness) of their main foods (leaf detritus and epilithon) on the streambed.