Box- and whisker-plot showing morphometric differences between morphotypes of Hydrobius. Top and bottom of boxes represent first and third quartile; dark bands represent the second quartile (median); whiskers show the maximum and minimum values not including outliers (white points). a Shape of mesoventral process. H. arcticus is the only morphotype with a blunt process (indicated by the higher values) b Relative position of trichobothria in relation to the 3rd and 5th row of elytral serial punctures. The trichobothria of H. f. rottenbergii are positioned closer to the serial punctures than in other morphotypes (indicated by lower values).