Guiding BeeThe Evolution of Flight

The Flight Muscles

There are two basic systems of muscles for flapping the wings: direct and indirect. The descriptions which follow are greatly simplified, but the underlying mechanism presented is employed by insects. There are many complications due to the large number of muscles involved in flapping and twisting the wings and the nature of the attachment of the wings to the thorax.

The Direct Muscle System

As the name implies, the muscles in this system are directly attached to the wings. The figure shows a schematic cross-section of a thorax. The small circles are hinges where the wings are attached to the top of the thorax (the tergum). In reality, these joints can rotate out of the plane of the diagram, but such motions do not need to be considered here. The direct musculature has a pair of muscles for the up-stroke (top of diagram) and one for the down-stroke (bottom of diagram). The contracting muscles have a darker shade. When the inner muscles contract, the wings rotate about their hinges and flap upward. When the outer muscles contract, the wings are pulled downward again.

The disadvantages in this musculature are that the insect's brain must instruct the flight muscles with each wingbeat to contract and to relax and that the wings, working completely independently of one another, must be perfectly synchronized by the brain if the insect is not to crash. Also, insects employing this muscle system must have low wingbeat frequencies because of the coordination of the signals from the brain. The locust uses a direct musculature and exhibits notoriously clumsy-looking flight.

The Evolution of Flight...3, March 1996