cuticle and to the epicuticle, where they can move different parts of the body including appendages such as wings. The muscle fiber has many cells with a plasma membrane and outer sheath or sarcolemma. The sarcolemma is invaginated and can make contact with the tracheole carrying oxygen to the muscle fiber. Arranged in sheets or cylindrically, contractile myofibrils run the length of the muscle fiber. Myofibrils comprising a fine actin filament enclosed between a thick pair of myosin filaments slide past each other instigated by nerve impulses. Muscles can be divided into four categories: Visceral: these muscles surround the tubes and ducts and produce peristalsis as demonstrated in the digestive system. Segmental: causing telescoping of muscle segments required for moulting, increase in body pressure and locomotion in legless larvae. Appendicular: originating from either the sternum or the tergum and inserted on the coxae these muscles move appendages as one unit. These are arranged segmentally and usually in antagonistic pairs. Appendage parts of some insects, e.g. the galea and the lacinia of the maxillae, only have flexor muscles. Extension of these structures is by haemolymph pressure and cuticle elasticity. Flight: Flight muscles are the most specialised category of muscle and are capable of rapid contractions. Nerve impulses are required to initiate muscle contractions and therefore flight. These muscles are also known as neurogenic or synchronous muscles. This is because there is a one-to-one correspondence between action potentials and muscle contractions. In insects with higher wing stroke frequencies the muscles contract more frequently than at the rate that the nerve impulse reaches them and are known as asynchronous muscles. Flight has allowed the insect to disperse, escape from enemies and environmental harm, and colonise new habitats. One of the insect's key adaptations is flight, the mechanics of which differ from those of other flying animals because their wings are not modified appendages. Fully developed and functional wings occur only in adult insects. To fly, gravity and drag (air resistance to movement) have to be overcome. Most insects fly by beating their wings and to power their flight they have either direct flight muscles attached to the wings, or an indirect system where there is no muscle-to-wing connection and instead they are attached to a highly flexible box-like thorax. Direct flight muscles generate the upward stroke by the contraction of the muscles attached to the base of the wing inside the pivotal point.