In contrast to the power muscles the steering muscles are synchronous, i.e.,
there is a 1:1 correspondence between neural spikes and muscle
contraction. No less than some 22 pairs of steering muscles are
involved in the force transmission; a few of these indirectly
modulate the output by affecting the resonating properties of the
thorax, while others are directly attached to the sclerite elements of
the hinge mechanism [6,15]. Three small muscles (b1–b3) are
attached to the basalare plate that is directly involved in wing
articulation (Figure 3). The actual wing sclerites (Figure 2) are also
controlled by specific steering muscles, also with the function of
moving the sclerites in relation to required wing motion. The main
control function of the hinge mechanism appears to be of the
downward movement of the wing, i.e., the angle at the turning
point at end of downstroke. For a detailed review about the
steering muscles and their function see Dickinson and Tu [6].
To date, the function of the steering muscles has been revealed
mainly by electrophysiological studies on tethered subjects.
Tethering means that the animal is glued to the end of a thin
rod, often with force sensors attached to it, and then stimulated to
‘‘fly.’’ In many insects this can be achieved by simply blowing at
them or placing them in a wind tunnel. On the tether the insect
can either be presented with a visual stimulus or be rotated, which
flies can sense via their halteres (hind wings modified to sensory
gyroscopic sensory organs) [16]. By inserting electrode wires into
the steering muscles, the neural impulses are measured at the same
time as the wingbeat kinematics is recorded [13,17]. What we
know about the function of the steering muscles comes from the
meticulous studies of correlations between muscle activity and the
associated wing movement, including how the hinge mechanism
works [6,18]. Needless to say, such experiments are extremely
difficult to achieve in small insects like blowflies and fruit flies that
flap their wings at high frequencies. Recent studies of the wing and
hinge kinematics provide some support for the hypothesis that the
hinge may have a gear function that affects stroke amplitude, as
well [18]. However, there are still many open questions regarding
the exact function of the steering muscles and how they help in
generating laterally asymmetric forces during a fly’s flight
maneuver