Insects insects (from Latin insectum)are pancrustacean hexapod invertebrates of the class Insecta

Exoskeleton

The insect outer skeleton, the cuticle, is made up of two layers: the epicuticle, which is a thin and waxy water resistant outer layer and contains no chitin, and a lower layer called the procuticle. The procuticle is chitinous and much thicker than the epicuticle and has two layers: an outer layer known as the exocuticle and an inner layer known as the endocuticle. The tough and flexible endocuticle is built from numerous layers of fibrous chitin and proteins, criss-crossing each other in a sandwich pattern, while the exocuticle is rigid and hardened.^{[36]: 22–24} The exocuticle is greatly reduced in many insects during their larval stages, e.g., caterpillars. It is also reduced in soft-bodied adult insects.
Insects are the only invertebrates to have developed active flight capability, and this has played an important role in their success.^[36]: 186 Their flight muscles are able to contract multiple times for each single nerve impulse, allowing the wings to beat faster than would ordinarily be possible.
Having their muscles attached to their exoskeletons is efficient and allows more muscle connections.

Internal

Nervous system

The nervous system of an insect can be divided into a brain and a ventral nerve cord. The head capsule is made up of six fused segments, each with either a pair of ganglia, or a cluster of nerve cells outside of the brain. The first three pairs of ganglia are fused into the brain, while the three following pairs are fused into a structure of three pairs of ganglia under the insect's esophagus, called the subesophageal ganglion.^[36]: 57
The thoracic segments have one ganglion on each side, which are connected into a pair, one pair per segment. This arrangement is also seen in the abdomen but only in the first eight segments. Many species of insects have reduced numbers of ganglia due to fusion or reduction.^[62] Some cockroaches have just six ganglia in the abdomen, whereas the wasp Vespa crabro has only two in the thorax and three in the abdomen. Some insects, like the house fly Musca domestica, have all the body ganglia fused into a single large thoracic ganglion. ^[63]
At least some insects have nociceptors, cells that detect and transmit signals responsible for the sensation of pain.^{[64][failed verification][65]} This was discovered in 2003 by studying the variation in reactions of larvae of the common fruit-fly Drosophila to the touch of a heated probe and an unheated one. The larvae reacted to the touch of the heated probe with a stereotypical rolling behavior that was not exhibited when the larvae were touched by the unheated probe.^[66] Although nociception has been demonstrated in insects, there is no consensus that insects feel pain consciously^[67]
Insects are capable of learning.^[68]