How does having an exoskeleton limit growth in arthropods? originally appeared on Quora: the knowledge sharing network where compelling questions are answered by people with unique insights.
Answer by Adriana Heguy, Professor of Pathology at NYU Langone Medical Center, on Quora:
I'm going to make abstraction of anything else that limits growth (such as the respiratory and circulatory system) in arthropods, for the sake of speculation, but obviously what ultimately limits growth of one type of animal is not just one factor, it's a series of factors, as organisms are not a collection of parts but rather a harmonious (or more or less harmonious) system that has to work well as a whole.
This isn't my area of expertise, either, but it's a fun and interesting subject, so I'll speculate a bit.
1. Exoskeletons are heavy, even the lighter versions without calcium that insects have, comparative to bones (that can be very light, and still sturdy—think of birds, for example), not just because of chitin but also because it needs to cover the whole animal. If a locust or a spider were as big and heavy as a human, their muscles couldn't be large or strong enough to support themselves. They wouldn't be able to move much. Exoskeletons work for insects because they are small, and small muscles are stronger than bigger muscles in proportion (i.e. a muscle that is half as big has more than half the strength of the muscle that is twice as big).
2. The molting problem. It is a very costly process, biologically speaking. It costs a lot of energy, and also creates a huge vulnerability. Can you imagine a cockroach as big as a human trying to hide somewhere while it molts? Interestingly, molting is a very complex process. The muscle attachments to the inside of the exoskeleton need to re-form every time the animal molts. Here is a fascinating article looking into the process.
The biggest terrestrial arthropod is the coconut crab, Birgus latros, is considered to be the size limit for land arthropods. It's a big animal, but it weighs only about 4 kg. Not anywhere near as big as a human. But big enough to hunt and eat rats!