Some mesoAmerican lizards of the genus Basiliscus are known as “Jesus Christ lizards” because they can run across the water for considerable distances, using flaps on their toes to create air pockets that help them gain traction. I actually saw this behavior in Costa Rica. Here’s a video of one:
But now there’s a Jesus Christ cricket, too (the monicker is mine). It’s a pygmy mole cricket, Xya capensis, found at the southern tip of Africa. The cricket lives in burrows near the water, and sometimes mistakenly jumps into the water. But it can get out! A new paper in Current Biology by Malcolm Burrows and Gregory Patrick Sutton (free pdf at link) tells us how:
We show that flightless pygmy mole crickets use a new strategy to jump rapidly from water. Their powerful hind legs are moved so quickly that they penetrate the surface and as they move through the water, unique arrays of spring-loaded paddles and spurs fan out to increase surface area. This enables these insects to propel a large volume of water downwards in a laminar flow, so that they are launched upwards into the air.
Here’s one of them, showing the hind leg which contains the paddles and spurs. Note how small it is: about 6 mm long, or about a quarter of an inch:
An SEM photograph of the tip of the hind leg showing the three “medial” and four “lateral” paddles, and two pairs of spurs. The bar is 500 microns long, or about 1/50th of an inch.
When these guys find themselves in the water, they hold their hindlegs above the surface and then plunge them into the water, extending the paddles and spurs which, according to the authors, increases the surface area of the leg 2.4 times and propels the cricket upwards. Then, just before the leg leaves the water, the extensions are folded up, presumably to avoid surface tension. Their bodies also emerge nearly perpendicular to the surface (84±8 degrees), which minimizes drag. Here’s the sequence (photographed from several jumps); note that the paddles are extended in the first four photos but then retracted:
How far can they jump on water? The authors say they can reach 10 cm high (about 4 inches) and 3 cm forward (a bit more than an inch, but 5.4 body lengths).
The paddles and spurs are unique to this family of insects (Trydactylidae):
For those of you who want hard data, here’s a plot of the velocity of the animal and the angles of spurs and paddles with time as the cricket jumps from the water. Note that the spurs and paddles are retracted about four milliseconds (0.004 seconds) before the cricket emerges from the water.
The photo below, taken under both white and UV light, shows the articulation of the paddles: where they attach to the leg. The blue fluouresence shows the presence of the elastic protein resilin, which, the authors say, “suggests that the folding of the paddles and spurs, and hence reduction of drag, is effected by springs.”
Finally, a video of the leap, courtesy of reader Michael:
Here’s one of the paper’s authors, Michael Burrows, talking about how he learned about these crickets and a bit about the new research:
Besides the Jesus Christ lizard, some frogs are also capable of jumping from the water, though I don’t know which ones. But the cricket has an entirely different strategy. As the authors note at the end of their paper:
This locomotory strategy differs from those of other animals that jump from or move on water. Pygmy mole crickets generate a laminar flow of the water beneath their hind tibiae and large forces from extension of their moveable paddles and spurs. The generation of thrust is possible because of the viscosity of water relative to body size; the paddles and spurs act in a similar way to the smaller hairs on the moving mouth parts of copepods . By contrast, frogs and basilisk lizards generate a turbulent flow beneath their wide and flat feet (Reynolds numbers 5000–15000) and to run on water, a basilisk lizard must maintain a pocket of air above its feet. Pygmy mole crickets and copepods therefore exploit the viscosity of water, basilisk lizards its mass, and pond skater insects, and fisher spiders its surface tension. Jumping from water by pygmy mole crickets results in a lesser performance than when jumping from land, but the price paid for overcoming the drag from the water should be repaid in higher survival rates.
The last sentence, of course, shows the adaptationist program here: the crickets can’t jump all that well from the water, but Burrows and Sutton presume that the leg adaptations nevertheless conferred a higher survival rate compared to crickets lacking them. That seems a reasonable assumption, though, given the specificity of the paddles and spurs and a reverse-engineering analysis of how they’re used.
Knowing nothing about “fisher spiders,” I looked them up, and found that these animals, in the genus Dolomedes, can run across water to catch aquatic insects and small fish, which they detect by ripples in the water. I couldn’t find a good video of one hunting, but here is one walking on water. It’s a miracle!
One of our readers Al Denelsback, has a nice post about fishing spiders on the website Walkabout; have a look. And a reader has posted two other videos of them in the comments.
h/t: Florian, Michael
Burrows, M. and G. P. Sutton. 2012. Pygmy mole crickets jump from water. Current Biology Volume 22, Issue 23, R990-R991,doi:10.1016/j.cub.2012.10.045