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Mantis shrimp shoulder their evolutionary baggage and bluff :

The exoskeleton: the good, the bad, and the creepy-crawly

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.

pillbug
The hard, outer covering on this pillbug is the only skeleton it has.

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.

suits of armor

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


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The arthropods

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Growing up in a suit of armor


Pillbug photo © 2004 Joyce Gross

Mantis shrimp
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