Bee/Fly Flight Motor

[From Bruce Abbott (950523.1115 EST)]

Earlier I mentioned that insect flight depends on an oscillator mechanism to
generate the rhythmic motion of the wings. Here is a little information
about "flight motors," which I gleaned from Werner Nachtigall's (1974) book,
_Insects In Flight_.

There are two basic types of insect flight motor. The first, typified by
the locust motor, has the main wing spar extending past the wing joint, with
a muscle attached both inboard and outboard of the joint. When the outer
muscle contract, it pulls the wing down; the inner muscle then contracts
(and the outer relaxes) to pull the wing up again. Synchronization of these
contractions is accomplished by a neural oscillator circuit. The locust
motor's wing beat frequency is rather low, on the order of 20 to 25 beats
per second.

The second type of insect flight motor, typified by the bee or fly motor,
does without the neural oscillator and is able to achieve frequencies on the
order of 200-300 beats per second. In these insects the top of the thorax
is like a stiff lid attached to the box below by means of a stiff membrane
hinged at its inner attachment to the lid and outer attachment to the box.
Pressure exerted against the sides of the thorax makes the lid "pop" either
upward or downward; the center position is unstable. The powerful
"indirect" flight muscles pull down on the lid, snapping it into the lower
position and jerking against a set of longitudinal muscles. The jerk
triggers these muscles into contraction, popping the lid into its upper
position and jerking the vertical muscles, triggering them to contract and
initiating another cycle.

To continue contracting, the flight muscles must have calcium ions, which
enter the muscle cells following neural stimulation. Asynchronous neural
stimulation acts as the throttle by governing the rate at which calcium can
enter the muscle cells; when the bee or fly wants to stop flying, it
suppresses neural activity to the flight muscles, the motor runs out of
calcium and, after a few beats, stops. The signal to stop flying comes from
sensors on the foot pads. If you glue a stick to a fly's back, the wings
will beat until you give the fly something, like a piece of cork, to hang onto.

The "click mechanism" can work only if the lid is under pressure. The bee
or fly can alter the pressure on the left and right independently by
contracting a muscle anchored to the bottom and side of the thorax. By
releasing pressure on one side while maintaining it on the other, the insect
can alter the wing-beat amplitude asymmetrically and turn sharply.

As mentioned, the indirect flight muscles are stimulated into contraction,
not by neural impulses, but rather by being sharply jerked. So how does the
bee or fly start the motor? The answer is that these critters come equipped
with a "kick-starter." If you try to swat a fly bear-handed, the looming
image of your hand will trigger a volley of neural impulses that travel down
special large-diameter (fast) axons to a muscle attached to the base of the
middle leg at one end and to the thorax lid at the other. When this muscle
contracts it accomplishes two things: (1) it jerks the center leg downward,
propelling the animal into the air, and (2) it jerks the flight-motor muscle
to kick-start the motor. The whole sequence is completed in about 40-50
milliseconds, and the fly is "outta there" before your hand hits the table.

The next time you try to swat a fly, you might want to think about the
marvelous compexity and elegance of the mechanism you are setting out to
destroy.

Regards,

Bruce