Exoskeletons, Endoskeletons and Hydroskeletons

Recently I did a project on the different kinds of skeletons:

Skeletons

Exoskeletons

Exoskeletons are hard body parts that are on the outside of creatures such as insects, spiders, crabs, snails and shellfish.

They are made mainly of chitin in insects, spiders and crabs. Shell exoskeletons are mainly calcium carbonate.

Crab carapace (exoskeleton)
Crab carapace (exoskeleton)

Insects, spiders and crabs must moult to grow. Over a period of time they grow another exoskeleton underneath the old one, which is then shed. This is when they are most vulnerable, because their outside is soft and predators can easily attack them. The soft new exoskeleton allows the animal to grow a bit, then it hardens, and they can go about their everyday business.

Green vegetable bug exoskeleton (underneath)
Green vegetable bug exoskeleton (underneath)
Green vegetable bug exoskeleton (top view)
Green vegetable bug exoskeleton (top view)
Green vegetable bug after it had moulted
Green vegetable bug after it had moulted
Cicada moulting (ecdysis)
Cicada moulting (ecdysis)
Cicada exoskeleton
Vacant cicada exoskeleton

Seashells and snails grow by extending the edge of their shells.

Shells (exoskeletons)
Shells (exoskeletons)

Endoskeletons

Endoskeletons are internal skeletons in mammals, reptiles, fish and birds. Sea urchins, such as sand dollars, have endoskeletons as well.

Piece of a sand dollar endoskeleton
Piece of a sand dollar endoskeleton

Bones are mainly made of collagen (a protein), plus calcium phosphate and calcium carbonate.

Hedgehog skull
Hedgehog skull

Bird bones have an intricate lattice structure that makes them strong but very light, so they can fly. Sharks have an endoskeleton, made mainly of cartilage, which is rather flexible.

Chicken bones
Chicken bones

Hydroskeletons

I think hydroskeletons are the coolest kind of skeleton.

A hydroskeleton is a chamber with flexible walls, a bit like a water balloon. The fluid inside is held under pressure. When you squish the balloon, it extends in one direction. When you let it go it pops back to its original shape.

Muscles do the squishing – they increase the pressure and stretch the chamber out.

Eathworms, nematodes and flatworms all have hydroskeletons.

Flatworms (hydroskeleton)
Flatworms (hydroskeleton)

Collecting Exoskeletons

You must wait until the creature you want to get an exoskeleton from sheds its exoskeleton. The moulting is called ecdysis.

If you keep arthropods in a terrarium you are more likely to find exoskeletons, and you might get to see the moulting.

Praying mantis exoskeletons. Note: They are a series from the same mantis as it got bigger.
Praying mantis exoskeletons. Note: They are a series from the same mantis as it got bigger.

Or you might find an abandoned exoskeleton somewhere, for example in a spider’s web (most likely a spider exoskeleton in that case).

A rather dusty daddy longlegs spider skeleton
A rather dusty daddy longlegs spider skeleton

If you are near the sea, you might find a crab exoskeleton and there are usually shells.

If you find an arthropod exoskeleton, manoeuvre them with a paintbrush otherwise they get completely wrecked. Keep them in a container, away from breezes – otherwise they will blow away!

You can’t extract them from a dead creature by using acid to dissolve the soft parts – it would ruin the exoskeleton.

Collecting Endoskeletons

You must find a dead animal. Don’t kill anything for it – it might achieve better results, but just don’t!

One way to remove the flesh is to boil it in water. If it is rotten you should boil it outside. If you do it inside, don’t tell anyone I didn’t warn you!

A bird in a old pot on the camp stove
A bird in a old pot on the camp stove

Definitely use an old pot that you won’t ever put food in again. Boiling softens the flesh so you can get it off the skeleton. But it is a really smelly job, so you might want to get another (fresher) subject.

Extracting bird bones from the feathers and the meat
Extracting bird bones from the feathers and the meat

Another way, which is less smelly, is to bury specimens in a tub of earth and sand and leave them outside for about 6 – 12 months for the flesh to rot.

Bird about to be buried
Bird about to be buried

Then, dig them up and carefully sift through the sand for all the bones.

Extracting bones from soil and sand mix
Extracting bones from soil and sand mix

Once you have the bones, put them in a jar of turpentine for a couple of days to get rid of any grease. Then put them in a bleach solution to sterilise them.

Bones from a hedgehog in turpentine
Bones from a hedgehog in turpentine

Finally, rinse, dry, display and enjoy them.

Some of the finch bones we extracted
Some of the finch bones we extracted
Part of a finch skull
Part of a finch skull

What does a skeleton do?

  • Provides a framework for the body – gives it shape.
  • Hard skeletons protect soft tissues. For example your ribs protect your heart, lungs and other organs.
  • Provides a place for muscles to attach, and for them to pull against.
  • Exoskeletons help stop the animal from drying out.

Close-up Photographs

We used an iPhone with an olloclip1 lens to get the really close magnification. The iPhone is attached to a small tripod using a glif2. A FischerTechnik scissor-lift positions the subject so we can take a series of photos at different focus points. Then we process the image stack with Zerene Stacker3 software. We used a piece of cooking paper to get nice even lighting.

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