Sometimes I read a paper because the methods catch my eye. I can just imagine some scientists sitting around a table with a beer and saying, “I wonder what would happen if we just dropped a bunch of spiders from the tops of trees.” An article published online yesterday did just that.
Barro Colorado Island is a man-made island is located in Gatun Lake, created by filling of the Panama Canal. It is covered in tropical rainforests, and its inhabitants have been studied extensively. It would be a mistake to look at a forest as only ground habitat. The canopy supports a tremendous abundance of life, particularly arthropods. These critters are particularly tasty to predators and must find a way to escape within their decidedly hazardous habitat that is located over 90 feet (30 meters) off the ground. Falling is a bit of a risk. But what if you do fall? You could land in the understory or on the ground which, if it doesn’t kill you, is both unfamiliar and full of predators. To avoid this potentially lethal scenario, many wingless arthropods have developed the ability to orient their bodies (via visual cues, appendages, and other structures) such that they are more likely to fall towards tree trunks.
The genus Selenops is a large and common group of nocturnal spiders. They are easy to find and collect, often hiding under bark or in crevices. The researchers went out into the forests and collected a bunch of these spiders. The spiders were then weighed and photographed. The images were then analyzed for the horizontally projected areas of different segments and appendages. Then these data were put together to give “effective wing loading.” Here’s where we get to the fun part. The spiders were put into individual plastic cups and taken up into the canopy. The cups were held at a known distance from a tree trunk, inverted and tapped to release the spider. Geronimo! These drop tests were filmed at 60 frames per second so that glide index (ratio of horizontal distance from the tree trunk to the total distance traveled) could be calculated. The videos also allowed for the measurement of how the legs were being used to maneuver. The spiders were also scored in terms of their performance, either directly reaching the tree, indirectly or irregularly gliding towards the trunk, or failing completely and landing elsewhere.
Most of the spiders had a successful, directed decent without the aid of draglines or balloons. They fall several meters and then glide to a trunk. During this fall, they were observed to adopt body postures that orient their bodies to descend head first with the forelegs out to the side and slightly forward and the rest of the legs out and back. This foreleg asymmetry was shown to significantly change body heading meaning that they are using their legs to control their glide trajectory. Also, glide index was shown to decrease with increasing body mass. They hypothesize that this negative relationship means that larger spiders must accelerate under gravity to airspeeds where aerodynamic lift becomes significant relative to body weight.
This is an interesting result because other arachnids do not show it. These spiders have developed the ability to control their glide trajectory. This means that they have evolved novel mechanisms of body righting and maneuvering. Gliding spiders….cool.
Stephen P. Yanoviak, Yonatan Munk, & Robert Dudley (2015). Arachnid aloft: directed aerial descent in neotropical canopy spiders J. R. Soc. Interface, 12 : 10.1098/rsif.2015.0534
(image via Toy Story screencap)
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