1. Any form of locomotion in air, which can be active or passive (gliding). Mechanisms of flight have evolved mainly in birds, bats, and insects: these animals are adapted for flight by the presence of wings, which increases the ratio of surface area to body weight. Birds possess powerful flight muscles: the depressor muscle runs from the underside of the humerus to the sternum and is responsible for the downstroke of the wing; the levator muscle works antagonistically, producing the upstroke. Flight in insects works in a similar fashion but the muscles that control the wing movement are attached to the thorax. A few species of mammals, reptiles, and fish have developed flight to a lesser extent. For example, flying squirrels (order Dermoptera) possess a membrane attached to the limbs that can open and function as a parachute, allowing the animals to glide.

2. Part of a survival mechanism in an animal that is generated in response to a threatening situation. A potentially dangerous situation can induce the release of adrenaline, which prepares the animal for ‘fight or flight’ by increasing the blood pressure and heart rate and diverting the blood flow to the muscles and heart. See alarm response.