1. Feb 14, 2013 Bird flight completed Insect flight: locust

2. Lungs can be imporant for other functions besides respiration; in frogs they also play a role in calling and hearing: frogs make sound by modulating an outflowing airstream (as we do); they also detect water-borne sounds through their lungs where these lie against the skin, conveying these to the rear of the eardrum internally. Rana sylvatica wood frog Jim Harding Dept. Nat. Resources Michigan Tidal flow

3. Recall from last lecture, muscles powering the flying of a bird: antagonists located below the wing: pectoralis major and supracoracoideus

4. Ventilation by costal suction pump: ventilation is movement of water or air across gas exchange surfaces. Intercostal muscles run between ribs and their contraction moves ribs and sternum forward and down during inspiration/inhalation. This motion increases the volume of the thoracic cavity and so air is drawn into the air sacs. Reducing volume of the thoracic cavity as a result of the converse rib cage movement acts on the air sacs to expel air. Their interconnection circulates the inspired air through the sacs and parabronchi.. Birds have no diaphragm. Sternum oscillates up and down during flight because of the actions of supracoracoideus and pectoralis major: flight is directly linked to ventilation.

5. Trachea forks (at syrinx [bird’s sound organ]) into two primary bronchi, one going to each lung; as each primary bronchus passes through the (right or left) lung its name changes to mesobronchus. At the mesobronchus’ anterior end within the lung arise a secondary bronchi, so also at its posterior end. These anterior and posterior secondary bronchi are connected by parabronchi. Bones of birds contain air in sacs not marrow There are 9 air sacs: an anterior group: interclavicular (1), cervical (2), anterior thoracic (2) –a posterior group: abdominal (2), posterior thoracic (2). The unpaired interclavicular air sac in the anterior midline sends diverticulae into some of the larger bones (e.g., humerus): these are called pneumatic bones: this adaptation serves to lighten birds for flight. Winged Wisdom

6. Tiny air capillaries in the walls of the parabronchi are in close proximity to blood vessels; this is the site of gas exchange – not the air sacs. Airflow through the parabronchi of a bird is one-way, not tidal as with the alveoli of a mammal or an amphibian, so diffusion gradients are kept steeper.

7. This diagram shows a simplified model of a bird respiratory system; ‘it ‘groups’ anterior and posterior air sacs in order to more easily visualize the air circulation and one-way travel through the parabronchi. 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; it is a true circulation and not a tidal system such as in other tetrapod vertebrates.

8. Following one ‘breath’ through this system: the posterior and anterior air sacs expand on inhalation and constrict on exhalation (inspiration and expiration are alternative terms), this being caused by the motions of the sternum and rib cage during flight. On inhalation (1) all sacs expand and new air (a ‘breath’) moves [mostly] directly into the posterior air sacs along the mesobronchus. At the same time the expanding anterior air sacs draw air forward from the parabronchi. On exhalation (1) all the sacs are constricted again and this pushes the air of the posterior sacs (the breath) forward into the parabronchi. Now the flight motion brings about air sac expansion, inhalation (2); all sacs expand and the expanding anterior sacs draw the air into them (the breath we are following) forward from the parabronchi; finally we have exhalation (2) and the breath moves from the anterior sacs back to the outside. Mammalian lungs expand and contract during each cycle of inspiration and expiration: this is the ventilatory cycle. During a bird’s ventilatory cycle the “air sacs suck and push gases through the rigid tubing of the lungs”.

9. 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 here and in the lab.] Locusts are strong fliers. The flight-powering muscles of the locust are indirect: meaning they don’t insert on the wings. They 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).

10. Sct 2 is the scutum of the second segment of the thorax; scutum is the name given to a part of the tergum, as is Scl 2 which is scutellum. Muscle 81, e.g., is a longitudinal flight muscle pulling between phragma 1 and 2, 112 is the same pulling between phragma 2 and 3. These increase the arching of the terga creating forces at the wing bases (PWP & 2 nd axillary sclerite). The longitudinals are situated in the upper half of the pterothorax. Behind them, closer to the pleuron, are the many tergosternals, running between the sterna (S 2, S 3 ) and the terga. Their axes all lean headward (the insect’s anterior is to the left) and there are many of them: 83, 84, 89, 90, 113 etc. 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.

11. More diagramatic views: Snodgrass has drawn the phragmata of Fig. 129 somehat distorted so as to show the longitudinals between the Aph and the Pph (anterior phragma and posterior phragma): notice the critical placing of the second axillary atop the WP.

12. The wing is a double-layered outfolding of cuticle. At the wing base are 4 axillary sclerites and 2 median plates (m, m’) linking the basal/proximal ends of the veins (costa, subcosta, radius, median) to the margins of the tergum. The tergum is to the left. The third axillary serves in flexing the wing over the back when the insect is not flying.

13. Seen here in dissection, the heavily sclerotized pleural wing process with the second axillary that contacts it above. See also the first basilare, 1Ba 2, involved in wing pronation and upstroke and apparently, primitively a leg muscle now co-opted for flight. basilare PWP 2 nd axillary

14. Seen from below the wing, some of the same veins (Sc) and axillary sclerites appear, the 2 nd axillary is crucial ; it is concave and sits atop the wing process – the fulcrum. Note that the 1 st and 3 rd and 4 th don’t take part in the lower

15. Explanation of how the prealar arm stores elastic energy: basilare muscle pulls on basilare sclerite which pulls on the ligament, stretching the prealar arm resilin. The two resilin springs, prealar arm of phragma 1 and hinge at top of PWP, store energy by tension during the upstroke, energy derived from the flight muscles.

16. Shape change in the tergum (reduced convexity centrally, with outward and downward movement at the tergal margins) is brought about by the tergosternals. So over a short-distance a downward force acts on the near end of the 2 nd axillary (red arrow); this rotates the proximal end of the 2 nd axillary around the pleural wing process (PWP) and raises the wing that is linked to the axillary. Elastic energy from the upstroke is stored in the wing hinge resilin (as well as the prealar arm [not shown]).

17. During the downstroke energy returns from the wing hinge and prealar arm contributing to the rebound of the wing. The longitiudinals, antagonists of the tergosternals are now changing the shape of the tergum back to more convex and the force acting on the proximal 2 nd axillary is upward (red arrow).

18. Thoughts Animal flight needs adaptations for both lightness and power, jjust as keeping mass low and power high are the concerns of engineers designing aircraft. Perhaps the small size of insects could be seen as a pre-adaptation for flight: their smallness means they simply don’t weigh much. Could tracheal air sacs be more prevalent in insects that are good fliers (like locusts) and absent from those species that don’t fly? Could air sacs be adaptive for the locust just as they are for birds and give locusts an air-cooled engine? Hollow bones can be almost as strong as solid cylinders of bone, because the capacity to resist applied eccentric forces is enhanced farther away from the bone’s central axis. Bird bones are hollow (no marrow but in some cases air sacs) and one might expect that bird bone as a material has less density than the bone in something like an elephant. Of interest is imagining the respiratory system of the dinosaurian animals from which birds arose. What sort of air sacs did Archaeopteryx have? And how did the dinosaur’s lungs work?