Arthropods including insects and arachnids are characterised by their jointed limbs and bilateral symmetry with a fixed number of metameric segments e.g. head, thorax, abdomen.
They have both a mouth and an anus as part of a digestive tract with specialised regions (the caecae are appendages that contain gastric juice). The arthropod body form has enabled vast diversification of this animal group, including flight i.e. because the extra limbs could change without affecting the function carried out by the rest.
Their outer body is supported via the exoskeleton, which is a strong chitin structure.
This has led to arthropods, more specifically insects, dominating other animals in terms of highest number of different species as well as overall number of individuals. Extensive evolutionary potential was encased in this body form, giving rise to the most diverse animal group on the planet.