Photography by: Lucy Mendel, David Hu and John Bush
This page features images of quasi-static locomotion by water-walking insects. By locking into certain fixed postures, these creatures can propel themselves without moving their legs.
Figure 1. From our perspective, the surface of a pond appears flat (left). Looking closely at the surface reveals fluid menisci at the border between water and land (right). To creatures 1 mm in length, these slopes appear as frictionless mountains and they are unable to climb them using their ordinary means of propulsion.
The following films are Quicktime movies, playable on Mac or PC. Each film is ~ 10 MB, and may be downloaded for ease of viewing.
Film 1: The infant water strider
Many water-walking insects are incapable of climbing menisci using their traditional means of propulsion. Here we see an infant water strider trying in vain to row up a meniscus. Video played at 1/20 real time. Body length, 1 mm.
Film 2: The Waterlily leaf beetle
The meniscus-climbing technique of the beetle larva, a terrestrial creature not suited to walking on water. As it is circumscribed by a contact line, it can manipulate the free surface by arching its back. In so doing, it generates a torque that twists and aligns it perpendicular to the meniscus, and a force that subsequently drives it up the meniscus. Videos played in real time. Body length, 6 mm.
Film 3: The water treader (side)
Mesovelia attempting to climb a meniscus from right to left. In its first attempt, it tries in vain to scamper up using its traditional running gait. In its second attempt, it locks itself into a fixed posture, pulling up with its front and rear appendages, and thus glides up the meniscus, seemingly effortlessly. Video played at 1/20 real time. Body length, 2 mm.
Film 4: The water treader (top)
Mesovelia climbing a meniscus from right to left. The surface deflections are indicated by the shadows cast beneath the insect. Where it pulls up (with its front and rear appendages), the surface deflection focuses light into bright spots; where it pushes down (with its middle legs), light is diffused, resulting in dark shadows. Video played at 1/20 real time. Body length, 2 mm.
Figure 2. The waterlily leaf beetle Pyrrhalta feeds upon the plant for which it is named. The larva is a poor swimmer, making travel between lily pads difficult. It uses a special meniscus-climbing technique to close in on emerging (left) and overhanging (right) vegetation.
Figure 3. The larva of Pyrrhalta is circumscribed by a contact line with the water surface. To climb the meniscus, the larva arches its back pulling up on the free surface with its head and tail.
Figure 4. The deformation of the water surface near the head and tail of the larva is clearly visible. In these images, it approaches an emerging wetted leaf.
Water-walking insects are generally covered by a dense mat of hair that renders them hydrophobic. Learning to climb the meniscus was a necessary adaptation for their terrestrial ancestors as they colonized the water surface. Modern water walking insects ascend to land in order to escape aquatic predators and lay their eggs.
Figure 5. The border between land and water may appear flat to us, but to water-walking insects, there may be significant topography. Here the water measurer Hydrometra treads carefully atop slippery rocks protruding from below the water surface.
Figure 6. Meniscus-climbing by the water treader Mesovelia. a, The water treader approaches a meniscus, from right to left. The deformation of the free surface is evident near its front and hind tarsi. While covered entirely with non-wetting hairs, the treader uses specialized wetting claws to pull up on the water surface.
Figure 7: To climb the slippery meniscus, water-walking insects need to get a running start. Only by running up he meniscus and using their specialized climbing mechanism as they slide back down can they generate the speed to reach land. Mesovelia (left) and the infant water strider (right) start their sprints at the bottom of the meniscus.
Figure 8: Floating weeds (left) are attracted to the meniscus, so on occasion, Mesovelia can hitch a ride to draw itself closer to land. On the right Mesovelia pauses before attempting a second climb.
Figure 9: Left, Two water treaders making haste to climb the meniscus. Right, the water measurer prepares to climb the mensicus by drying its non-wetting claws.
Figure 10: Water measurers Hydrometra known for their plodding speed on the water surface. Left, Hydrometra next to a downward sloping meniscus at the edge of a glass of water. Surface tension both supports Hydrometra's weight and keeps the water from spilling out the glass. Right, an upward sloping meniscus generated by an overhanging plant. To asend the plant, Hydrometra must ascend the slippery meniscus.
Figure 11: Hydrometra ascending the meniscus. By assuming a static posture in which it pushes down with its middle legs and pulls up with its front and hind legs, the creature rises to the top of the meniscus. Once at the top (right), it uses its claws to haul itself upward.
Figure 12. Meniscus-climbing postures assumed by insects. Shaded spots indicate the sense of the surface deflection, light being upwards and dark downwards. a, Mesovelia. b, Microvelia. c, Hydrometra. d, Pyrrhalta. e, Anurida. Figures courtesy of Brian Chan.
Figure 13. Meniscus-climbing by Anurida maritima. By pulling up on the water surface with its wetting ventral tube and pushing down with its nose and tail, Anurida can deform the water surface. Assuming this static postures allows Anurida to ascend to land and to form colonies of 50-100 individuals.
Figure 14. To travel between two colonies, Anurida combines walking with meniscus-climbing. Meniscus-climbing is recognized by the upward deformation of the free surface, as seen around the individual on the left.
Figure 15. By assuming static postures in which they deform the water surface, two Anurida can generate forces that draw them together. Such forces are in use of generating colonies on the right.
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