Understanding Evolution

Molting and the exoskeleton: A double-edged sword

Arthropods' rigid exoskeletons provide protection and the basic building material for an arsenal of tool-like appendages. But the exoskeleton also has its downside. Since the exoskeleton is hard and its outer layer is non-living, it cannot grow bigger by small increments as the human skeleton does. Instead, arthropods must go through the delicate process of shedding the old exoskeleton and expanding to a larger size before the new exoskeleton hardens. This process is known as molting.
cicada
Here we see a cicada emerging from its molted exoskeleton.

Coconut crab The land-dwelling coconut crab weighs in at 5 kg (over 10lbs). For these giants, molting is a serious commitment: they may spend a whole month in a deep burrow wriggling out of the old skin and waiting for the new one to firm up!

 

View this article online at:
http://evolution.berkeley.edu/evolibrary/article/constraint_04

Cicada photo provided by Gladys Lucille Smith © California Academy of Sciences; Coconut Crab photo taken by Mr Robert Thorn, Parks Australia Cocos (Keeling) Islands

Understanding Evolution © 2015 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California