Feb 14, 2013
Bird flight completed
Insect flight: locust
Tidal flow
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
Recall from last lecture, muscles powering the flying of a bird: antagonists located
below the wing: pectoralis major and supracoracoideus
Sternum oscillates up and down during flight because
of the actions of supracoracoideus and pectoralis
major: flight is directly linked to ventilation.
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.
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.
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
Winged Wisdom
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.
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.
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”.
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).
Sct2 is the scutum of the
second segment of the
thorax; scutum is the
name given to a part of
the tergum, as is Scl2
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 & 2nd
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 (S2, S3)
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.
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.
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.
basilare
2nd axillary
PWP
Seen here in dissection, the heavily sclerotized pleural wing process with the second
axillary that contacts it above. See also the first basilare, 1Ba2, involved in wing
pronation and upstroke and apparently, primitively a leg muscle now co-opted for flight.
Seen from below the wing, some of the same veins (Sc) and axillary sclerites appear,
the 2nd axillary is crucial; it is concave and sits atop the wing process – the fulcrum.
Note that the 1st and 3rd and 4th don’t take part in the lower
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.
•
•
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 2nd axillary (red
arrow); this rotates
the proximal end of
the 2nd 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]).
•
•
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 2nd axillary is
upward (red arrow).
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?