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Ecdysis – Crabs Do It, Insects Do It, What Is It?

Photo courtesy of my Great Nephew Trey Robichaux

I recently had a revelation about something that I assumed everyone knew. I posted a photo of the discarded skin (the exoskeleton) of an insect (shared by a great nephew) and was surprised that more than one person responded that they did not know that insects shed their skins. What? They must have been asleep in Entomology Class, right? Well, shame on me for assuming, and no fault of my friends. This one is on me! I dropped the ball somehow. So here goes a bit of penance (I am a good Catholic after all) for my sins of assumption and failing to teach one of the important facts about all arthropods – that they must shed an old skin to grow – through the process of ecdysis.

Arthropods make up one of the largest groups of organisms in the animal kingdom. They are defined by their jointed appendages (arthro=joint, pods=feet/legs), segmented bodies, and by an exoskeleton. Flexible in some, rigid in others, this external structure defines the shape and way of life for these animals. Crabs, shrimp, insects, spiders and several minor groups all belong to this group of animals. The exoskeleton offers protection, supports a variety of sensory organs, and most importantly is what muscles attach to, giving these creatures the ability to walk, fly, swim and move about their worlds. But this rigid covering has its limits. It does not allow its owner to grow.

The solution is the process of ecdysis – the shedding (molting) of the old exoskeleton in favor of a new, larger one, into which the animal grows to fill it. Think of it as finding a bigger room to put more stuff – in this case larger muscles and other internal pieces and parts. For coastal folks, crabs give us the best example of this process. I love soft shell crabs, but why are they soft? They are in fact a newly molted crab that has just discarded its old shell and is waiting for the new one to harden. Here is how it works.

As the internal parts of the crab grow, they fill the existing shell to the point where a new shell is needed. At that point, a new soft version of a future exoskeleton forms beneath the old one. This new layer is folded upon itself so that once out of the old shell, it can be expanded to allow the crab to grow. At this point, the old and new layers separate, and the crab ingest water to force the old shell to break open and allow the crab, with its new soft shell, to wiggle free. Once free, the crab continues to ingest water to stretch the folded new exoskeleton to its new, larger size. Chemical reactions over a few hours act to harden the new shell, that is as much as 1.5 times larger than the old one – with room to grow. Repeat.

The size difference is surprising to many of us. Finding a single large crab in a crab trap with what appears to be a much smaller crab shell is confusing, unless you understand what just happened. This process is much the same in all arthropods. The old exoskeleton of a spider in a spider web, or the discarded exoskeleton of an insect on a bush is evidence of this same process. Certainly, some insects like caterpillars can stretch their softer exoskeletons, but to a point, before they too must molt. And for some insects, with complete metamorphosis, the transition from a caterpillar, to a chrysalid, to a butterfly is dramatic. But ecdysis is at work all the same.

Other animals also shed, like snakes and lizards, but that is only the skin and is typically done all at once, leaving a “shed” of the entire animal. We humans also shed, but not all at once. Our skin is constantly shedding dead cells. We grow by adding muscle and skin that lies outward of our internal skeleton, that also grows along with us.

So, there you have it! I hope that my explanation was understandable enough. Your first lesson about Arthropods is now complete.

Hope to see you in our great outdoors!


Written by Mark W. LaSalle, Ph.D.

Mark is a naturalist and wetland ecologist, providing expertise on wetlands, water quality and environmental impacts of humans. He has also developed and conducted a number of environmental education programs and workshops for youth, teachers, realtors, and the general public on a variety of subjects including wetlands, natural history, and environmental landscaping. Mark is a graduate of the University of Southwestern Louisiana (B.S. and M.S. degrees) and Mississippi State University (Ph.D.). Mark is the recipient of the Chevron Conservation Award, the Mississippi Wildlife Federation Conservation Educator Award, the Gulf Guardian Award, and the Boy Scouts of America Silver Beaver Award.


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