Comparison of the relative position of trichobothria (red arrows) on the elytra of Hydrobius. A Trichobothria positioned in the intervals between the 2nd and 3rd row of serial punctures, and between the 4th and 5th row. Typical positioning of trichobothria in H. arcticus, H. fuscipes fuscipes and H. f. subrotundus, here represented by a specimen of H. f. fuscipes B Trichobothria positioned in or very close to the 3rd and 5th row of serial punctures, which is characteristic of H. f. rottenbergii.