Another cold, snowy, wonderful winter in central Ohio is finally yielding to spring. As a true fan of winter, my praise of the frozen season is genuine. Snow-covered trees, birds at the feeders, ice sculptures, ice skating, and ice hockey all make winter a uniquely fun annual phenomenon. And a good, hard winter makes us appreciate spring all the more. So with warm weather turning the those pond hockey rinks and stream-top ice skating arenas back into swimming pools, a different set of athletes will soon take possession.
Like hockey players waiting out an intermission in the locker room, adult water striders in Ohio spend the winter tucked away in leaf litter, logs, rocks, or soil awaiting their turn to retake their skating arena. Overwintering forms sometimes possess wings, which allow them to fly to more suitable aquatic environs in case conditions at the winter site prove undesirable when spring returns. Such wing dimorphism is a common trait of water striders, and the presence or absence of wings depends on the cohort and season of hatch.
As members of Order Hemiptera, water striders are true bugs. Drably colored with combinations of gray, brown, black, and white, water striders range in size from 2 – 20 mm. The family Gerridae contains the most and best known representatives, but smaller surface-skating insects in the families Veliidae and Mesoveliidae are also called by the same common name. Interestingly, the water strider moniker is only one of many such common names associated with these aquatic bugs, which are also known as pond skaters, skimmers, water scooters, water skippers, and Jesus bugs for their ability to walk across water.
The ability to glide so gracefully across the water’s surface derives from the numerous hydrofuge hairs that cover the water striders’ legs. Working in conjunction with the high surface tension of the water these hairs repel water and retain pockets of air that allow the striders to remain on top of the water surface. (You can easily test the surface tension of the water with this experiment from National Public Radio’s Science Friday).
Additionally, claws are located a short distance up the leg rather than at the end of the tarsus to avoid puncturing the water surface. Propulsion is enabled by the long middle legs that stroke the water in rowing motions like the oars of a boat. At slow speeds the push of the water repellent legs against the water provides sufficient force for forward motion. When a water strider is travelling faster than the speed of the capillary waves its motion creates, however, a unique skill is employed: the water strider’s legs deform the water surface creating a trampoline effect, which springs the animal forward. Through such activity, water striders can travel at speeds of around 1 m/s.
Hind legs act as rudders and brakes to change course and stop. A water strider’s agile legs also enable it to jump off of the water surface and land as far as 10 body lengths away. In the event of an unanticipated wave or splash that inundates the entire body, hydrofuge hairs across the whole outer integument allow water striders to right themselves on the water surface. They can even purposely dive under the water, where they breathe from a plastron – an envelope of air around the body that is held in place by the hydrofuge hairs.
While middle and hind legs are invested in locomotion, the forelegs allow water striders to capture prey. These aquatic predators feed upon terrestrial insects, spiders, and dragonflies that have experienced the misfortune of falling into the water. While the forelegs grasp the prey, the piercing/sucking tube that is characteristic of bugs is inserted into the victim to drink its internal fluids. The forelegs also help locate prey by sensing the vibrations of organisms floundering in the water. During the non-mating season, water striders will feed cooperatively in groups, attacking struggling victims like a school of piranhas. With such voracious habits, it may not be surprising to learn that cannibalism is a common phenomenon among Gerridae, especially in the instance of adults preying upon young. What goes around comes around, though, and water striders must be wary of bug-eating birds. Frogs and sometimes fish will also prey upon water striders, although scent gland secretions from water striders repel many fish.
Not only can gerrids sense prey via water vibrations, they also communicate with each other through wave action. Studies have shown that high wavelength frequencies (35 Hz or 90 – 100 Hz, depending on the species) send a repel signal, which alerts an oncoming water strider about territorial claims. A non-reply to such a signal indicates to a male that it is a receptive female approaching. A low frequency signal (2 – 5 Hz) is then used to send courtship messages. In some Gerridae species, a mated male and female remain together throughout the entire mating season. Eggs are attached to floating surfaces in the water and development of the young involves 4 – 5 nymphal instars over a period of 3 – 6 weeks Offspring from a mated couple must be wary of the cannibalistic tendencies of Gerridae, however, so must disperse as soon as possible to avoid such risks.
Only the oceanic Halobates water striders are safe from parent cannibals, as members of this genus can recognize their own offspring and will refrain from such familial cannibalism. Halobates is remarkable for several other reasons too, not the least of which is its singularity as the only insect of the open ocean. Halobates water striders have been found far out at sea, making them the most prodigious water striding athletes of the insect world.
But whether you find yourself out at sea or much closer to central Ohio this spring, keep an eye out for water striders. With the annual transformation of ice into water, these remarkable creatures are springing into action once again.
*Cheng, L. 1985. Biology of Halobates (Heteroptera: Gerridae). Annual Review of Entomology. 30:111-135.
*Hu, D.L. and Bush, J.W.M. 2010. The hydrodynamics of water-walking arthropods. Journal of Fluid Mechanics. 644:5-33.
*Merritt, R.W., Cummins, K.W, and Berg, M.B. 2008. An introduction to the aquatic insects of North America. Dubuque, IA: Kendall/Hunt Publishing Company.
*Spence, J.R. 1994. Biology of water striders: interactions between systematics and ecology. Annual Review of Entomology. 39:101-128.
*Voshell, J.R., Jr. 2007. A Guide to Common Freshwater Invertebrates of North America. The McDonald and Woodward Publishing Company: Blacksburg, VA.
About the Author: Jon Bossley is a Ph.D. candidate in the Environmental Science Graduate Program at Ohio State University. Jon is committed to science education. He has a masters degree in Education and is a 4-time NSF GK-12 fellow (2011-current).