Lecture 12, February 10

1. Lecture 12, February 10 Dipteran flight: the ‘non-click mechanism’ Haltere evolution

2. 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 Drosophilahalteres. 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]

3. 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 axillarysclerites onto the pleural wing process (see B). Non-click mechanism of dipteran flight

4. 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 axillarysclerite.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 axillarysclerite; but about midstroke there is a new fulcrum: the radial stop engages with the PWP.

5. 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.

6. 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

7. Pictures from ‘Insects of West Virginia’and psmicrographs of UK 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).”

9. Proprioceptors A copy of an old lecture for your edification:

11. Allignment of sensillae within groups is adaptive re effective orientation to monitor direction of Coriolus forces

13. Evolution of the haltere from aerodynamically function hind wings • 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.