From swimming to flight: still in fluid Hydrofoils and aerofoils: the fluid flowing faster has lower pressure; faster flow occurs on the upper surface of an aerofoil or hydrofoil and so lower pressure generates lift. Hydrofoils more robust than aerofoils: density Ladybird beetle Lecture 12 From swimming to flight: still in fluid Linden Gledhill Diversity in aerofoil shape Harris’ Hawk

Some wings don’t look like aerofoils in section

Feathers as aerofoils Rachis: tapering central support of a feather: to either side is the vane Body feathers have vanes symmetrical about the rachis but bird primaries are not symmetrical in this way: in primaries the rachis is closer to the edge that leads in flight. In strong fliers the leading vane is sometimes almost obliterated. This asymmetry gives each feather an aerofoil transverse section, i.e., the primaries of most birds function as aerofoils in their own right. Feduccia, A. & Tordoff, H.B. 1979. Feathers of Archaeopteryx: asymmetric vanes indicate aerodynamic function. Science 203: 1021-1022

* [from Wikkipedia] Keratin refers to a family of fibrous structural proteins. Keratin is the key structural material making up the outer layer of human skin. It is also the key structural component of hair and nails [and the feathers of birds]. Keratin monomers assemble into bundles to form intermediate filaments, which are tough and insoluble and form strong unmineralized tissues found in [tetrapods]. The only other [material] known to approximate the toughness of keratinized tissue is [the polysaccharide] chitin.

Supracoracoideus muscle is wing elevator; it originates on the sternum and lies beneath its antagonist, the wing levator. Load of wing is taken up by the tendon of the supracoracoideus. Stress (force) is created by the contracting muscle and tension develops in the tendon which stretches, i.e., shows strain. Drawing can be criticized for misleading re passage of the red muscle through the foramen. Direction of muscle pull is changed by the rope-like nature of the tendon.

Why are the muscles that power bird flight located below the wing Why are the muscles that power bird flight located below the wing? Flight muscles are 1/5 of the body mass. Keeping the centre of gravity low keeps the bird from out-of-control rolling.

Class 3 lever both up and down: effort is always applied between the fulcrum and the load in birds.

The flight of birds is made possible by close integration of the muscle system with their Respiratory/ventilation system: flight and breathing are linked. Costal suction pump Intercostal muscles run between ribs and contract to move ribs forward and down during inspiration which coincides with downstroke (? Check): sternum moves forward and down. Forward and down the volume of thoracic cavity is greatly increased giving reduced internal pressure and causing air inrush. Reverse happens on upstroke. Sternum moved down (and up) by supracoracoideus and pectoralis major operating during flight: flight directly linked to ventilation.

Lungs of human occupy about 5% body volume Lungs of human occupy about 5% body volume. Lungs of bird only occupy 2% of body volume. But lungs plus air sacs of a duck occupy about 20% of its body volume. Nevertheless bird air sacs are not involved in gas exchange: they are not vacularized (no blood vessels): but they are involved in air circulation. Air sacs are reconnected to the mesobronchi by recurrent bronchi. Multiple functions of air sacs: cooling system: flying produces heat and air sacs are well placed to remove this excess heat (bird has an air-cooled engine). Shock absorber: gannets, pelicans diving into the ocean from a great height (bird has shock absorbers). Resonance: sound box areas in bird sound production (birds have sound boxes). Egg laying: abdominal air sacs help squeeze egg along the oviduct? Defecation?

Tiny air capillaries within the parabronchi walls, in the (fixed-volume) lungs, are the site of actual gas exchange. Tracheae, which conduct the low density fluid (air) into the body need cartilaginous rings to support their lumen against the denser tissues (water-based) that surround. Convergence of a sort with the tracheae of insects. Bird lung section

Bird air sacs are how a bird can inhale and exhale with lungs that don’t change volume. Posterior thoracic sac There are 9 air sacs. An anterior group: interclavicular (1), cervical (2), anterior thoracic (2). Then there is a posterior group: abdominal (2), posterior thoracic (2). Unpaired interclavicular air sac in anterior midline sends diverticulae into some of larger bones (sternum, pectoral girdle): called pneumatic bones: these serve to lighten the bird for flight. Syrinx is located at junction of trachea and bronchi and is an organ for sound generation: developed elaborately in songbirds. Modulation of air flow to create sounds: respiratory system serves also in communication.

Diagram to right is simplified bird lung, ‘it ‘groups’ anterior and posterior air sacs in order to more easily visualize the air circulation. The lungs cannot change their volume, but the air-sacs do. Two cycles of inspiration and expiration (powered by the muscles of flight , including the intercostal muscles between the ribs of the thorax) are required for one breath to make its way through the system, in and out again; it is a true circulation and not a tidal system such as in mammals.

Follow one breath through the system: remember, on inhalations sacs expand, on exhalations they are compressed, this being accomplished by thoracic volume changes linked to flight muscles & rib cage [costal pump]. Inhalation 1, posterior air sacs expand & draw breath into themselves: draw this air via primary bronchus>mesobronchus. Exhalation 1, breath air displaced from squeezed posterior air sacs into posterior secondary bronchi and parabronchi; in latter the gas exchange occurs. Inhalation 2, expanding anterior air sacs draw ‘breath’ from the parabronchi into themselves. Exhalation 3, the anterior air sacs expel the air breath to the outside.

How did birds evolve flight? The use of wings in flight would seem to involve quite sophisticated changes in the in aerofoil form, in musculature in neural motor control. For the protobird ancestor to change from a scrambling-forelimb terrestrial ‘lizard’ [reptile] to a soaring eagle requires a plausible hypothesis of change: one that envisages slight changes to forelimbs that are adaptive all during the historical process. The authors of a recent paper propose an hypothesis based upon study of the development (ontogeny) of young partridge, the ontogenetic-transitional wing hypothesis. Dial, K.P. et al. 2008. Nature. A fundamental avian wing-stroke provides a new perspective on the evolution of flight. Nature 451: 985-989. Read this paper to understand their hypothesis, how they arrived at it and what experimental evidence they present that tests the hypothesis.

“Our data suggest a default or basal wing-stroke is used by young and adults and may exist in all birds…. The fundamental wing-stroke described herein is used days after hatching and during all ages and over multiple behaviours (that is, flap-running, descending and level flight) and is the foundation of our new ontogenetic-transitional wing hypothesis. At hatching, chicks can ascend inclines as steep as 60° by crawling on all four limbs. From day 8 through adulthood, birds use a consistently orientated stroke-plane angle over all substrate inclines during wing-assisted incline running (red arcs) as well as during descending and level flight (blue arcs). Estimated force orientations from this conserved wing-stroke are limited to a narrow wedge (see Fig. 3b).

When the wing movement is considered relative to the body’s longitudinal axis it seems to vary a lot for locomotory learning behaviour, but considered relative to gravity it is actually the same relatively simple wingstroke for both up-incline and fluttering down incline. “Blue and black outlines represent the positions of the bird and wing at the start and end of downstroke, respectively. a, In the vertebral space, the mean wing-stroke plane angle shifts more than 30° from a more antero-posterior orientation during flap-running to dorso-ventrally in flight, implying different wing-strokes are used to execute different locomotor modes. The wing-stroke path is consistently oriented, however, in both the (b) global and (c) gravitational coordinate spaces over diverse locomotor behaviours, illustrating a simplified wing-stroke that is multi-functional. Data for juveniles are presented from 8- to 10-day olds.”

“a, When wing-stroke plane angles are viewed side-by-side in both the vertebral and gravitational frames of reference, the wing-stroke is nearly invariant relative to gravity whereas the body axis re-orients among different modes of locomotion. Red lines represent the wing-tip trace in WAIR (flap-running) and blue lines represent the wing-tip trace in level flight. b, Wing-strokes are estimated to produce similar aerodynamic forces oriented about 40° above the horizon during WAIR, level flight and descending flight. Error bars are s.e.m. c, Representative traces of AOA through a wing-beat for an animal flap-running vertically (red) and in horizontal flight (blue) demonstrate the similarities of AOA among behaviours. The similarities are further clarified by examining wing cross-sections and mean global stroke-planes in the first, middle and last thirds of downstroke. Here, the orientation of the aerodynamic force (Faero) is estimated from the middle third.”

Turning now to flight in insects: some anatomy is necessary: tergum, sternum, pleuron are ‘top’ ‘side’ and ‘bottom’ of each insect cuticular metamere. Insects are descended from metameric, i.e., segmented animals and the thorax is a locomotory tagma comprised of three metameres. Segments of the abdomen contrast with those of the thorax. The thorax is a tagma functioning as a locomotory ‘box’, giving a firm base on which muscles can originate and pull, muscles for walking (legs below) and for flying (wings above). The mesothorax + metathorax: the two segments specialized for bearing the wings and for flight are together called the pterothorax. Muscles involved in flight in insects (with exceptions, e.g., in Odonata) insert on the exoskeleton of the thoracic box and NOT on the wings directly; they move the wings by distorting pterothorax shape and are referred to as indirect flight muscles. LONGITUDINALS DOWNSTROKE; TERGOSTERNALS UPSTROKE Attitude of the wings (pronation, supination, camber etc.) is achieved by the elastic interplay of the veins and membranes with the air flow. The wings don’t just go up and down and maintain elevation they must scull through the fluid (air) in ways rather like some fish fins.

Locust flight {Source: R. E Locust flight {Source: R.E. Snodgrass The thoracic mechanism of a grasshopper, and its antecedents. Smithsonian Miscellaneous Collections 82, pp. 111. } [This reference is given just for completeness; it is not something you should try to obtain and read, but it is the source of much of the information given here and in the lab.] Locusts are strong fliers. The flight-powering muscles of the locust are indirect and have their effect upon the wings by distorting the pterothorax and by tergal tipping of the second axillary. (The pterothorax is the flight tagma (just segments 2 & 3, not the prothorax.) There are two antagonistic muscle sets: longitudinals (downstroke), and tergosternals (upstroke). contraction of the longitudinals wings go down contraction of the tergosternals wings go up

Apodemes are Inflections of cuticle, phragma are a kind of apodeme. Sct2 is the scutum of the second segment of the thorax; scutum is a region of the ‘top’ sclerite (tergum) as is Scl2 which is a tergal region called the scutellum. Muscle 81 is an indirect longitudinal flight muscle pulling between phragma 1 and 2; 112 is another, pulling between phragma 2 not labelled, and 3Ph, pharagma 3). These increase the arching of their respective terga, creating forces at the wing bases (PWP & 2nd axillary sclerite, see next slide). The longitudinals are situated high up in the pterothorax. Partially obscured behind them, arrayed against the pleuron, are the many tergosternals (83, 84, 89 etc.), running between the sterna (S2,S3) and the terga (Sct2, Sct3). The axes of the tergosternals all lean headward (the insect’s anterior is to the left). Notice how the upper end of the tergosternals insert on the terga where their contraction can reduce the convexity of this region. Reducing tergal convexity is associated with elevation of the wings.

More diagramatic views: Snodgrass drew the phragmata (Aph anterior phragma, Pph posterior phragma) of Fig. 129 purposely distorted, so as to show their interconnecting longitudinal muscles both ahead and behind: notice the critical placing of the second axillary, 2Ax, atop the pleural wing process, WP.