Lecture 12, February 10
Dipteran flight: the ‘non-click mechanism’
Haltere evolution
Some sources related to dipteran flight
• Miyan J.A. & Ewing A.W. 1988. Further observations on dipteran flight:
details of the mechanism. J. exp. Biol. 136: 229-241.
• Pringle, J.W.S. 1948. The gyroscopic mechanism of the halteres of Diptera.
Philosophical Transactions B 233: 347-384.
• Roch, F. & Akam, M. 2000. Ulltrabithorax and the control of cell
morphology in Drosophila halteres. Development 127: 97-107.
• Ennos, A.R. 1987. A comparative study of the flight mechanism of Diptera.
J. exp. Biol. 127: 355-372.
• Miyan J.A., Ewing A.W. 1985. Is the ‘click’ mechanism of dipteran flight an
artefact of CCl4 anaesthesia? J. exp. Biol. 116: 313-322.
• Chan, W.P. et al. 1998. Visual input to the efferent control systems of a fly
‘gyroscope’. Science 280: 289-292. [** important]
A shows the mesothorax of the fly seen in
lateral view. The longitudinal flight muscles
within the thorax are shown (green) running
between anterior and posterior phragma.
Phragmata (plural) are transverse thin sheets
of inwardly projecting cuticle to which the
flight muscles are attached. When these
longitudinal flight muscles contract with the
start of the downstroke they make the
scutum more arched, i.e. the phragma on
which these muscles insert fore and aft, are
pulled closer to each other and the scutum
above bows upward in the longitudinal
median plane.
But because of the 'hemispherical' shape of
the scutum (and a 'scutal inflection') this
same muscle contraction creates lateral
forces in the transverse plane. That is, the
scutum actually becomes less arched in the
transverse plane, or to say it another way, the
right and left parascutal shelves on either
side tend to move away from each other,
following an outward force which acts
through the first and second axillary sclerites
onto the pleural wing process (see B).
Non-click mechanism of
dipteran flight
Non-click mechanism of
dipteran flight
And this same muscle (longitudinal flight
muscle) contraction also moves the scutellar
lever (A) down at the rear (scutellum) but up
where it reaches toward the wing base (see
the two red arrows on diagram A). So the end
of the scutellar lever at the wing base moves
upward from below against the first axillary
sclerite.
Throughout the downstroke, the pleural wing
process (PWP) (the top of the insect's side)
acts as the lower part of the wing fulcrum. But
the upper part of the fulcrum changes: during
the early part of the downstroke the second
axillary rests on the PWP (B); later, past the
half-way point, the radial stop on the
underside of the radial vein (the radius is the
main structural vein of the wing) comes to rest
on the PWP. So the initial fulcrum is formed of
the PWP and second axillary sclerite; but
about midstroke there is a new fulcrum: the
radial stop engages with the PWP.
Non-click mechanism of dipteran flight
With the radial stop resting on the PWP, the
base of the radius and first axillary act to pry
up the parascutal shelf (C). Thus the scutum
is lifted and the dorsoventral flight muscles
are stretched slightly. It is this stretch which
triggers their contraction for the upstroke.
The mechanism is designed so that the
contraction of one antagonist slightly
stretches the other and so these fibrillar
muscles mutually re-stimulate each other for
a number of contractions without the
necessity of another motor neuron impulse.
[They are myogenic not neurogenic.]
While the wing moves on down pivoting (first
order lever) about the radial stop, there is a
bending of the pleuron (the side of the
thorax). Strain energy is stored in the pleuron
exoskeleton: it is elastic and stores energy by
being deformed. This energy storage as
deformation is only appreciable if the
pleurosternal muscles are contracted so that
the side can't just move away under the
forces acting on the PWP.
So strain energy can be stored in the
elasticity of the thoracic wall and also
in the bending veins of the wing. This
happens during the bottom half of
the downstroke. This stored elastic
energy of deformed cuticle is
returned as elastic recoil as the
upstroke begins. In other words the
wing doesn't start up just because of
the contraction of the dorsoventral
flight muscles but they are helped
because the distorted thorax and
wing veins return to their
prestressed state.
Note that during the upstroke the
dorsoventrals contract. Because of its
shape this still produces lateral forces
acting outward at the wing base. But
on the upstroke the end of the
scutellar lever is moving in the
opposite direction: it is now applying
force down against the first axillary.
Non-click mechanism of dipteran flight
Haltere: reduced hind wing
functioning as a balance organ.
A club-shaped appendage with
an array of sensory organs on
its stalk base. It functions to
maintain balance during flight.
Whether a wing is the goal of
development or a haltere is
controlled by a switch gene.
“Ultrabithorax is the primary
genetic switch that controls the
differences between the wing
and haltere of Drosophila… “
“Ultrabithorax (Ubx) is the sole
Hox gene responsible for the
differential development of the
forewing and haltere in
Drosophila (Roch & Akam
2000).”
Pictures from
‘Insects of West
Virginia’
and
psmicrographs of
UK
Proprioceptors
A copy of an
old lecture for your
edification:
Allignment
of sensillae
within
groups is
adaptive re
effective
orientation
to monitor
direction of
Coriolus
forces
Evolution of the haltere from aerodynamically function hind wings
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What is the function of a haltere? How does it work? Are all fly halteres the same? Are
there any flies that don’t have halteres?
Recognize that halteres though characteristic of a species, will vary widely among the
many species of Diptera: flies will fly diversely in capturing prey or sipping nectar;
monitoring of Coriolus forces may be more important for some species less so for
others: lengths of stalk and shapes of knob (location of resilin?) varying.
Contrast haltere shape with its shape as a wing: none of the haltere’s form effective
aerodynamically: weight (mass) concentrated at the extremity; eccentric CG (centre of
gravity), lies behind the stalk axis: this may be adaptive.
Comparison of structures can give insight: relative development of sensory fields at
base of forewing and haltere (serial homologues of sensory fields at base of forewing)
Haltere stress sensor fields much more extensive: ‘hyptertrophied’.
In locusts, hind wing mechanoreceptors make strong connections with forewing
motorneurons and function to maintain a correct phase relation between the wing
pairs. The hind wings of flies have lost all aerodynamic function [and become a sensory
organ reacting to Coriolus forces], they have retained and elaborated
mechanoreceptors capable of entraining motor neurons of the forewing.