Understanding Evolution

Exoskeleton strength: Totally tubular

The legs of the largest ant crumpled under the strain of holding up its body. To understand why this happened, we'll explore a simplified situation. A single section of arthropod leg is a lot like a tube holding up a weight — a hollow support structure (the exoskeleton) bears the weight of the arthropod's body.

A tube supporting a cube is analagous to the tube-like leg segments supporting an ant

Of course, there are lots of differences between a tube holding up a weight and a leg holding up an ant. For example, ant legs have joints and are usually splayed out to the side of body, as opposed to our unjointed tube, which is positioned directly underneath the weight. However, when looking at size constraints, weights and tubes turn out to be a good model for an ant body. In the cases we'll look at, the details missing in our simplification just make it even harder for ants to be big-bodied!

Tubes and cubes

Tubes are quite strong, but even tubes have their limits. The three sets of tubes and cubes below are scaled up just like the ants were: each is twice the size of the previous. Check out the sizes of the cubes. As the size increases, what happens with the weight?

Three cubes and three tubes with increasing dimensions

 

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http://evolution.berkeley.edu/evolibrary/article/_0_0/constraint_09

Understanding Evolution © 2018 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California