Insects are well adapted to a terrestrial environment because they have an exoskeleton which helps minimize water loss, trachae that permit gas exchange with minimal water loss, and wings that provide many advantages without sacrificing a pair (or more) of legs.
Insects have a unique skeletal system: Their skeleton is on the outside of their bodies. This type of structure, an exoskeleton, helps prevent water loss from an insect's body, allowing it to survive well in a terrestrial environment.
Its rigid design also protects it from weather and predators found on land. The patterns and colors of exoskeletons can help with camouflage and even mimicry of objects such as leaves and sticks found in an insect's natural environment.
The development of wings and the ability to fly was a huge part of the success of insects. Most orders have wings, usually in two sets found on the thoracic segment of the body. Diversity exists among insect wings, from the equal-sized pairs of more primitive insects such as dragonflies to the most advanced version of hardened forewings in beetles.
Flies developed their second pair of wings into hovering structures called halteres, creating a faster, more efficient way to fly. Insects use the air in a way different from larger animals.
For more info on how are insects adapted to living on land visit Sciencing.com.