They are a tremendously successful group. A few species are even marine. Insects also show huge variety in shape and form. Almost the only condition their group does not attain is very large body size. A number of features, however, are shared by most kinds of living insects.
Insects have a complete and complex digestive tract.
Their mouthparts are especially variable, often complexly related to their feeding habits. Aquatic forms may exchange gases through the body wall or they may have various kinds of gills. Excretion of nitrogenous waste takes place through Malpighian tubules. The nervous system of insects is complex, including a number of ganglia and a ventral, double nerve cord. Yet ganglia also use sensory output.