In the QCA Scheme, this topic is Unit 4A for Year 4. This Unit is almost exclusively focused on the bony skeleton of vertebrates, and suggests that invertebrates are typified by lacking rigidity.

The majority of animal species, i.e. the insects and other arthropods (crustaceans, centipedes, millipedes, scorpions, spiders, mites and ticks) have a jointed exoskeleton. The principle of movement by contraction and relaxation of muscles is similar to that of the vertebrate body, but in insects and other arthropods the elements of the skeleton are on the body surface. Unlike bone and cartilage, the hardened chitinized plates of an insect’s exoskeleton cannot grow, which is why a developing insect undergoes a series of moults to shed successive skins as it increases in size.

The following features of some of the species in our ‘Insect Index’ illustrate the properties of an insect’s jointed exoskeleton and the way its muscles operate in combination with this exoskeleton:

• the robust hardened bodies of beetles such as the Seven-Spot Ladybird and the Cockchafer (and the noise the latter makes if it flies into a lighted window!);
• the narrow segmented straight bodies and stiff wing membranes of the Banded Demoiselle and the Four-Spotted Chaser;
• the power in the hind legs of an adult Common Froghopper that can accelerate it to 14 km/h in a millisecond; and
• the ability of the Drone-Fly to hover with great precision using one pair of membranous wings operated by rapid muscle contractions acting on a complex system of skeletal plates.