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.