Can insects, crabs, spiders, and other animals with an exoskeleton become overweight or obese?



If so, how does it work? Do they just molt into larger skeletons?

In: Biology


Vertebrates usually can’t swap in a larger skeleton when they get too heavy, and so they just expand outwards and get fat.

Arthropods *can* molt into a larger exoskeleton when they’re getting a bit snug, and so instead of getting fat they actually get taller and stronger.

Your typical American lobster is about a foot long and five pounds, but once in a great while fishermen will catch a three foot long, forty pound monster that has been outcompeting everyone else for decades.

These beasts get thrown back in, as they’re the kings and queens of the breeding cycle and keep the population up.

Land arthropods are more limited in size because their heavy armor and inefficient respiratory systems don’t scale well.

Former tarantula owner here. Tarantulas can be overfed, which is visible by the size of their butt. It can get dangerous when an overfed tarantula tries to climb and falls, as the butt can rip open and the animal can bleed to death from such a wound easily, as their butt contains most of their blood and also their heart. The butt is not enclosed by a rigid exoskeleton, but rather a very soft skinlike one that can expand.

If your tarantula’s butt is smaller than its body, you can give it a snack. If the butt is the same size or about 1.5x the size of the body, you have a happy little spider. If the butt gets larger than that, your eight legged friend needs a break from food. In adult tarantulas it’s entirely possible to wait several months until the next feeding, they are adapted to survive scarcity of food.

Most of the time, an adult tarantula will only have a thin butt after changing into its new skin – but always wait for at least 3 days or even a week before feeding a freshly molted T, as their exoskeleton will be very soft and fragile, showing white fangs and joints that can easily be damaged if the animal attempts to hunt. Let your spider friend do their yoga stretches for a few days and then give them a juicy cricket 🙂

Edit: Whoa, thanks for all the awards people! This really blew up 😀 I just love spiders and sharing what I know. Have a grand day, internet strangers! <3

For lobsters if they get too big they become physically unable to get out of the old shell at molting time and die.

I remember learning about an experiment in college where an animal with an exoskeleton (a mantis or roach maybe?) has neurons disabled that allowed it to essentially feel the pressure inside its body to tell if it should eat more or not. They continued to eat until their shells cracked open and they died. So I’m guessing there’s only a certain tolerance and they likely have a way to prevent it from happening through overeating.

Hey I posted about this on r/askscience a while ago.

I have a PhD in insect physiology specializing in insect metabolism.

Insects can develop obesity like symptoms and what looks like type 2 diabetes. The extra fat doesn’t make them bigger in their exoskeleton, but does get packed in there. Here’s a paper on dragonflies that get obese!



I had a black forest scorpion that I accidentally overfed. He basically expanded out the sides of his armor with what looked like black skin. He was like that for a few months without any obvious health problems. I gave him away when I moved, so I don’t know how long he lived.

They absolutely can! For some exoskeleted creatures they can get so fat that they start to get gaps between the pieces of their exoskeleton where you can see skin/fat. See this pic of an obese emperor scorpion!

[Fat Scorpion](

Edit: hmmm it appears that that particular one is pregnant, but you get the idea.