The “crunch” of squishing a cockroach is caused by the exoskeleton — a mosaic of hard plates that protect and support the soft, inner body of the arthropod. Humans and other vertebrates have an internal skeleton made of bones, but arthropods do not — only their exoskeletons keep them from collapsing into soft blobs.

The exoskeleton is a great evolutionary innovation. With this jointed “suit of armor” operated by muscles attached to it on the inside, arthropods, like the pillbug above, can move around quite efficiently. Beyond support, the exoskeleton provides protection against predators and strength against prey. And it protects arthropods from the environment, for example, keeping ocean-dwellers from being affected by small changes in saltiness and preventing land-dwellers from drying out.

But as you will see, the exoskeleton also constrains arthropod evolution by introducing a big problem: how to grow.