Insect Physiology - Muscular System

Muscular System

Many insects are able to lift twenty times their own body weight and may jump distances that are many times greater than their own length. This is not because they are strong but because they are so small. Muscle power is proportional to its cross-sectional area. Because the mass (the insect's body), that is moved is in proportion to its volume and the fact that they also have a better leverage system than we humans do, they can jump remarkable distances. (Elzinga, 2004; Triplehorn & Johnson, 2005).

The muscular system of insects ranges from a few hundred muscles to a few thousand (Triplehorn & Johnson, 2005). Unlike vertebrates that have both smooth and striated muscles, insects have only striated muscles. Muscle cells are amassed into muscle fibres and then into the functional unit, the muscle (Elzinga, 2004). Muscles are attached to the body wall, with attachment fibres running through the cuticle and to the epicuticle, where they can move different parts of the body including appendages such as wings (Gullan & Cranston, 2005; Triplehorn & Johnson, 2005). The muscle fibre has many cells with a plasma membrane and outer sheath or sarcolemma (Gullan & Cranston, 2005). The sarcolemma is invaginated and can make contact with the tracheole carrying oxygen to the muscle fibre. Arranged in sheets or cylindrically, contractile myofibrils run the length of the muscle fibre. Myofibrils comprising a fine actin filament enclosed between a thick pair of myosin filaments slide past each other instigated by nerve impulses (Gullan & Cranston, 2005).

Muscles can be divided into four categories:

1. Visceral: these muscles surround the tubes and ducts and produce peristalsis as demonstrated in the digestive system (Elzinga, 2004).
2. Segmental: causing telescoping of muscle segments required for moulting, increase in body pressure and locomotion in legless larvae (Elzinga, 2004).
3. Appendicular: originating from either the sternum or the tergum and inserted on the coxae these muscles move appendages as one unit. (Elzinga, 2004) These are arranged segmentally and usually in antagonistic pairs (Triplehorn & Johnson, 2005). 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 (Triplehorn & Johnson, 2005).
4. 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 (McGavin, 2001; Gullan & Cranston, 2005).

Flight has allowed the insect to disperse, escape from enemies, environmental harm, and colonise new habitats (McGavin, 2001). One of the insect’s key adaptations, the mechanics of flight differ from other flying animals because their wings are not modified appendages (McGavin, 2001; Elzinga, 2004). Fully developed and functional wings occur only in adult insects (Gullan & Cranston, 2005). To fly, gravity and drag (air resistance to movement) has to be overcome (Gullan & Cranston, 2005). 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 (Gullan & Cranston, 2005).

Direct flight muscles generate the upward stroke by the contraction of the muscles attached to the base of the wing inside the pivotal point. Outside the pivotal point the downward stroke is generated through contraction of muscles that extend from the sternum to the wing. Indirect flight muscles are attached to the tergum and sternum. Contraction makes the tergum and base of the wing pull down. In turn this movement lever the outer or main part of the wing in strokes upward. Contraction of the second set of muscles, which run from the back to the front of the thorax, powers the downbeat. This deforms the box and lifts the tergum (Gullan & Cranston, 2005).