Circulation: moving fluid internally: hearts and other pumps
Course theme revisited
Course Theme reminder
Don’t take morphology for granted.
Puzzle about it. Compare and
question the morphology of an
animals, plants and their parts and
materials. Structures have evolved
under certain regimes of selection.
Squirrels have tails. Watch them in
use. Tail behaviour in squirrels.
Organs of balance on a street wire;
signalling with flicks to potential
mates, keeping warm. The tail is not
just the ‘bit left over’.
Ask why structures have evolved
with certain features of shape, size,
stiffness, segmentation, elasticity,
branching, colour, etc. And why have
these features come to exist in
certain taxa while in other taxa they
Orb-weaver spider and
the food filter/trap it
Silk fibres are very very
tiny: only 1 micrometre
in diameter: is this an
adaptation to reduce
Argiope: genus of
garden spiders, is an
Many orb-weavers add
‘stabilimentum’ to their
web: thick silk radiated
near the web hub and
before the viscid spiral
zone, quite variable in
In quotes because this
is a functional term
which favours a
particular hypothesis of
function; best to avoid
this and leave it an
open question. Call it a
‘silk mass’? Don’t
Hypotheses to explain
the selective advantage
2. Tension adjustment
4. Visual marker
this spider re
Eisner T. 1983. Spider web protection through visual advertisement:
role of the stabilimentum. Science 219: 185-187.
This picture seems to argue for stabilimentum as a cryptic
Of course different spider
species might have
evolved a stabilimentum
for different selective
reasons: more than one
hypothesis might be right
depending on the
• Observe behaviour: the structure in use, i.e.the structure behaving.
Measure the spider’s behaviour in relation to the stabilimentum.
Measure/observe the nature of the stabilimentum itself. Spiders
position themselves with respect to the stabilimentum. Spiders do
stabilimentum push-ups when disturbed.
• Functioning as an insect trap, web silk is apparently selected
to reflect light poorly in the UV, where prey insects see well;
but this is not so for stabilimentum silk which returns UV
well. So perhaps stabilimenta have never been selected to
HIDE from prey?
• Spiders do push-ups at the hub, responding to the advance
of a relatively larger non-prey item [bird, cow, human];
pushups are consistent with the visual marker hypothesis of
function of the stabilimentum; the movements would make
the stabilimentum more visually effective since vertebrate
eyes respond well to movement in the visual field.
Eisner T. 1983. Spider web protection through visual
advertisement: role of the stabilimentum. Science 219:
Tom Eisner showed that the presence of visual markers
significantly affected the time a web could last without
mechanical damage from flying birds. His experiments
support the hypothesis that a stabilimentum is a visual
marker enabling large animals and birds to avoid
blundering through the spider’s web.
Some spider species keep their webs up night & day, and
some species spin anew each evening, taking their web
down again at dawn; this latter group of species don’t make
stabilimenta and their webs were used in Eisner’s
experiment: the spider residents were captured and
removed from their webs.
An elegant experiment
• 30 natural webs without
• 30 other natural webs adorned
with artificial equivalents of
stabilimenta: triangular strips of
white paper forming an X.
• Webs with artificial stabilimenta
survived “intact through the
early morning period when birds
are on the wing”.
What is the function of moth wing scales? Imagine them with glassy
wings like bees or beetles. How would life success be affected?
Eisner’s field experiment tested the idea that moth scales are an adaptation to
make moths ‘nonadherent’ to spider webs. They are covered with “detachable
Orb-weavers create a ‘viscid spiral’ of silk with beaded sticky droplets (bottom right) as glue for
trapped prey. Moth wing scales are shown (bottom left) and adhering to a viscid spiral strand in the
middle right picture. Upper right empty scale sockets. Illustration from a book by Thomas Eisner. For
Love of Insects. Belknap Press, Harvard Univ. Press. Moth scales have evolved from articulating hairs
(mechanoreceptors) and the open sockets (upper right photo) show where once scales lodged.
Circulation and respiration make one think of lungs and hearts. Lungs and
hearts are pumps: pumps creating force as pressure to displace body fluids:
in animals --Fluid flow comes about by squeezing, dilation, bailing,
Water, blood, air even gut contents, may be regarded as pumped fluids.
Pumps circulating fluids already mentioned in lectures: Chaetopterus,
echinoderm tube feet, sap-feeding insects, filter feeding salps etc.
Echinoderm tube feet and ampullae involved pumping fluid from one part of
the organ to another. The basis of a hydrostatic skeleton is squeezing
incompressible fluid from one spot to another. The basis of heart beating
circulation is squeezing incompressible fluid from one spot to another.
“...we can’t draw a sharp line between thrust-producing devices of locomotion
[think of insect flight, or fish fins] and pumps that push fluid” The same organ, the
mesothoracic wing or pectoral fin could suffice for both pumping and locomotion:
bees use their wings not just for flight but to circulate air in their hives, ventilating
and cooling (an example of a fluid-dynamic pump with the wings acting as
Another example: stickleback males (parental care by males) use their pectoral fins
as impellers to circulate freshwater over the clutch of eggs in their nest and aerate
Pumping air: recall the system employed by birds in flight, the way the
movement of the skeleton draws in and expels air through the agency of air sacs
and the inextensible lungs: a costal suction pump
Costal suction pump
Intercostal muscles run between ribs
and contract to move
ribs forward and down during
inspiration: sternum moves forward
Forward and down the volume of
thoracic cavity greatly increased with
reduced internal pressure
causing air inrush.
Gill ventilation in fishes involves pumping both water and blood
Buccal force pump combined with an opercular suction pump
Unidirectional flow of water in fishes is generated by a buccal force pump combined
with an opercular suction pump. These two pumping actions operate out of phase,
to achieve a near continuous flow of water. (As the phase differences in the cycles of
the three muscular fans of Chaetopterus achieve more even flow.) The water enters
the fish’s mouth, passes through the buccal cavity, then is forced out through gill slits
in the gut wall into a branchial cavity containing the gills. The branchial cavity is
covered with an operculum and the water exits posteriorly between the edges of the
operculum and the body.
Irrigating gill filaments
The heart in a tetrapod
embryo begins as a
muscular tube formed
from mesoderm; the
circulation of blood
begins very early in the
and the heart pumps
Two muscular chambers are arranged in
series, ventricle and atrium: why not just one
chamber? Why is the atrium less powerful
than the ventricle?
The same serial
arrangement occurs in a
fish but atrium is bent to
overlie the ventricle
Problem of filling a too muscular
chamber: need to bring up low
pressure of venous blood in 2
Function of the
The vertebrate heart occupies its own fluidfilled space, the pericardial cavity, a
space of the coelom. Makes sense to have
an organ like a heart, one that changes its
shape to function, situated in its own
inviolate space. Don’t want other viscera
‘crowding in’ as the heart contracts and so
having to be pushed back out of the way
when the heart fills again with blood.
There is an adaptive linkage between the
atrium and the ventricle: as each of these
chambers contracts, the volume of the
pericardial cavity is slightly increased and
its fluid pressure decreased, i.e., there is
reduced external pressure on the heart.
This reduced pressure makes it easier to
fill whichever is the expanding heart
chamber. The contraction of one chamber
(e.g, atrium) is coupled to the distension of
the other (e.g., ventricle).
Closed high-pressure systems contrast with open low-pressure
circulatory system of many arthropods.
Insect circulatory system
• Because in insects the tracheal system does the job of moving
respiratory gases to and from the tissues, the circulatory system
proper can be low pressure, low flow.
• There is a heart, a long muscular tube, situated within a pericardial
sinus, a space defined by a dorsal diaphragm. The dorsal longitudinal
vessel (heart posteriorly, aorta anteriorly) draws in haemolymph from
this sinus through ostia (with valves) and pumps blood peristaltically
anteriorly; blood leaves the vessel in the head to circulate rearward
through sinuses. Aliform [wing-shaped] muscles are the antagonists
of the circular heart muscles.
Insects ventilate their tracheal
system using primarily the
Intersegmental muscles make
the segments of the abdomen
of a stridulating katydid
telescope in and out; the
abdomen pumps like a
bellows, supplying oxygen as
the insect is stridulating.
Air flow is made unidirectional
through timing the opening
and closing of metameric
Front to back flow in the
locust: anterior spiracles are
opened, all others closed and
inspiration by expanding
abdomen; then spiracle 10
opens while all others close,
abdomen volume reduced
Metrioptera sphagnorum, bog katydid
Tracheae are branching tubes
conducting air inward from
spiracular openings. They need to
be prevented from collapsing
against the force of the
surrounding haemolymph and
other tissue. They are
strengthened by spiral cuticular
Internally tracheae end blindly in fine intracellular tubes known as tracheoles.
“These are partly filled with fluid and the
extent to which the air penetrates down
them varies with the state of activity of the
insects. If it is very active, or if oxygen is
witheld, the air extends further down the
tubes and comes nearer to the tissues,
thus shortening the final path of slow
diffusion in water (Ramsay J.A.
Physiological Approach to the Lower
“The great advantage of the tracheal system is
that a high oxygen tension can be maintained
in the tissues without energy being wasted in
maintaining a rapid flow of blood... But on the
other hand, since the rate of diffusion is
inversely proportional to the distance, the
tracheal system is not readily adaptable to the
needs of larger animals.”