It might come as a surprise but 98 per cent of the animals on Earth don’t have a backbone, and 95 per cent don’t have any bones at all. So how do all these creatures support and protect themselves? Well, many invertebrates – and all arthropods – have a protective external casing called an exoskeleton. This literally means ‘outside skeleton’ and its role is to cover the animal’s soft tissues and also provide a rigid structure to which the creature’s muscles can attach.
Insect exoskeletons are made of chitin, which is embedded into a kind of tough protein matrix. Chitin is a nitrogen-based biopolymer – similar, at least in function, to keratin, which is the stuff our hair and nails are made of. Arthropods such as crustaceans, meanwhile, have additional calcium carbonate in their exoskeletons for extra armour plating.
As well as supporting and protecting the creature, an exoskeleton also creates a watertight barrier that prevents the animal from drying out. The exterior of an exoskeleton can also contain sensory hairs or bristles, while some animals can secrete various pheromones and chemicals onto the surface of their shell as a means of repelling predators.
Though an exoskeleton consists of flexible leg joints to enable the creature to move about, once it’s formed this armour does not expand with the rest of the body. Therefore, the animal will eventually outgrow it. At this point a process called ecdysis, or moulting, takes place whereby the creature will shed its overly tight outer skin in order to make way for a new one.
There are three main types of skeletal system in the animal kingdom: exoskeletons (on the outside), endoskeletons (on the inside, like humans) and hydrostatic skeletons, which are a bit different as they have no real framework but rather maintain their shape by the pressure of fluid in their bodies. Examples of creatures with hydrostatic skeletons include slugs, worms and jellyfish.
Creatures with exoskeletons
Scorpions: Baby scorpions start out all soft and squishy, riding around on their mothers’ backs, but their exoskeletons soon harden. One unusual trait about the scorpion’s exoskeleton is that it glows fluorescent under ultraviolet light.
Spiders: The discarded cuticle left behind after a spider has outgrown its exoskeleton and wriggled out is complete with all the legs and you can even see the fine hairs on its body.
Crabs: A crab’s broad protective plate across its back is called the carapace. The decorator crab’s exoskeleton comes in handy for disguise too, as it features tiny hooks onto which coral attaches. The coconut crab (above), meanwhile, is so big it can spend a whole month shedding its shell and waiting for the new one to harden.
Lobsters: Crustacean exoskeletons are reinforced with calcium and made up of plates. Hormones that originate from the lobster’s eye tell it when it’s time to shed its shell. When a chef boils a lobster its outer skin will turn from blue-black to bright red. This is because certain pigments in the exoskeleton turn red when exposed to heat.