Growth and Reynolds number of lotic macroinvertebrates: a problem for adaptation of shape to drag

Streamlining or dorsoventral flattening of the body of lotic macroinvertebrates is traditionally considered to be a morphological adaptation to reduce drag. This view is rather speculative because the Reynolds numbers (Re) at which these animals live and thus the type of drag that predominantly affects these animals are unknown. Using Laser Doppler Anemometry to measure water velocity such Re were determined from flume experiments. Young, small individuals live at rather low Re. Therefore they experience mainly friction drag. Fully grown individuals live at higher Re, at which pressure drag predominates. To minimize drag differently sized animals must differ in shape. Small specimens should be hemispheric while large ones should be streamlined. Within a species, young and old instars of freely moving aquatic stages are usually of the same shape, which is frequently neither hemispheric nor streamlined. Therefore I conclude that evolution compromises between life at low and high Re.