9. Insect flight muscles

Most insect species have wings as adults and are able to fly. Unlike birds and bats, insect wings are not modified fore limbs, but are extensions of the cuticle of the meso- and meta-thoracic segments. These two thoracic segments also have prominent muscles used for generating the wingbeat Flight muscles of bats and birds attach directly to the wings, and pull the wings up and down. In insects, only dragonflies and damselflies have muscles attached directly to the wing and these muscles only produce the downstroke for the wingbeat. In all other flying insects, both the downstroke and upstroke of the wingbeat are produced in response to contractions by muscles that attach to the thoracic cuticle and not directly to the wing. The downstroke is produced by a set of dorsal, longitudinal muscles attached to phragma. Phragma are articular invaginations of the meso- and metathoracic segments. The upstroke is generated by a pair of dorso-ventral muscles attached to the top and bottom surfaces of the meso- and metathoracic segments. These indirect muscles act by undergoing rapid, antagonistic changes in tensions that produce alternating changes in the length and height of the thoracic segments. These alternating changes in the shapes of the segments cause the base of the wing to move in and out over a lateral fulcrum point that flips the wing into the upstroke and the downstroke for the wingbeat. Many insects species are able to move their wings rapidly and can fly at wing beats of 100 to 700 per second. By comparison, hummingbirds fly at approximately 50 wing beats per second. Muscles that attach directly to the base of the wing cause wing folding or may control the pitch and twisting of the wing in some species. you

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