Abstract: Water striders are insects living on the water surface, over which they can move very quickly and rarely get wetted. We measured the force of free walking in water striders, using a hair attached to their backs and a 3D strain gauge. The error was calculated by comparing force and data derived from geometry and was estimated as 13%. Females on average were stronger (1.32 mN) than males (0.87 mN), however, the ratio of force to weight was not significantly different. Comparedwith other lighter species, Aquarius paludum seems stronger, but the ratio of force to weight is actually lower. A. paludum applies about 0.3 mN•cm⁻¹ to 0.4 mN•cm⁻¹with its mid-legs, thus avoiding penetrating the surface tension layer while propelling itself rapidly over the water surface. We also investigated the external morphology with SEM. The body is covered by effectively two layers of macro-and micro-hairs, which renders them hydrophobic. The setae are long (40 µm–60 µm) and stiff, being responsible for waterproofing, and the microtrichia are much smaller (<10 µm), slender, and flexible, holding a bubble over the body when submerged.

Key words: force, surface tension, aquatic insect, morphology, microtrichia, setae