How do fishing spiders and water striders walk on the water surface?

A number of students enrolled in Independent Research in Biology or participating in Vassar's Undergraduate Research Summer Institute have worked with Dr. Suter is his pursuit of answers to problems in biomechanics.

Much of data gathering for this work has involved image analysis and video from a high-speed imaging system, and some of the data visualization was made possible by Matlab, software that is particularly adept at working with matrices.

 

 

 

 

 

 

 

 

 

Fishing spiders, Dolomedes triton (Araneae, Pisauridae), propel themselves across the water surface using two gaits: they row with four legs at sustained velocities below 0.2 m/s (upper image) and they gallop with six legs at sustained velocities above 0.3 m/s (middle image). In addition, when startled, these versatile animals can also leap vertically from the water surface.

Dr. Suter's students have used a Kodak high-speed camera and imaging system to photograph spiders at 1000 frames per second, about 33 times faster than normal video speed. The resulting video, played back 33 times more slowly than normal video, reveals the details of motions that are too fast for the un-aided eye to see.

Click on the image to view high-speed images of a spider rowing across the water surface ...

Click on the image to view high-speed images of a spider jumping vertically from the water surface ...

 

Click in this image to view a similar analysis, this time in motion, on the rowing kinematics of a fishing spider.

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Vertical jumping by a fishing spider takes a fraction of a second, from start to finish. The motions, captured by high-speed video, are analyzed frame-by frame (right). Using an application called NIH Image, students use a cursor to click on the body parts that are in motion, ultimately producing a graphical representation of the spider's kinematics (below).

 

 

 

 

In the graph shown here, the motion of the center of gravity forms a nearly perfect parabola, as expected from the physics of bodies in freefall, while the leg tips (dotted lines) follow much more complex paths.

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Sometimes, two dimensions are not adequate for the visualization of data. In this case, Dr. Suter and his research students needed four dimensions to represent data on the effectiveness of attacks by fish on spiders.

In each graph, the x and y axes represent the angles and velocities of fish attacks from below the water surface, and the z axis represents an index of the probability that the spider can evade the attack by performing a vertical jump.

The fourth dimension is spider mass, with the top graph representing data from a 1.0 gram spider and the bottom graph representing data from a 0.06 gram spider.

MatLab was used to transform the raw data, contained in an Excel file, into the visual representations shown here.

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