hydrostatic skeleton

Hydrostatic Skeletons
Read: Kier W.M. 2012. The diversity of
hydrostatic skeletons. Journal of
experimental Biology 215: 1247-1257 ,
Morphological diversity in treehoppers is conveyed by the helmet.
B Prud’homme et al. Nature 473, 83-86 (2011) doi:10.1038/nature09977
In the absence of a text it is very important to read and understand assigned papers,
the first being Prud’homme 2011; this is assigned re membracids. For some papers of
particular importance I will try to provide an essay online and I have done so for the
membracid paper discussed last day. The essay tries to simplify the content a little,
make it easier to follow and point out its importance. See the page on the 325 website
called ‘Source paper essays’.
What are you wearing to the tree?
Genes retained as instructions for making
the early stages of a prothoracic wing have
formed the basis for making new adaptive
• This is an important paper because it gives insight into the process
by which structures can evolve. As the authors say in their Abstract,
it illustrates “how complex morphological structures can arise by the
expression of ancestral developmental potentials”. Incorporating
genetic instructions which once made a pair of wings on the
prothoracic segment, these insects have evolved right and left
appendages into a fused helmet that covers their body like a new
‘suit of clothes’; it has then been acted upon by selection in contexts
such as crypsis, aposematism, or defensive spination -- to produce
an astonishing diversity of species-specific structures.
• Aposematic: an antipredator adaptation where a warning signal is
associated with some noxious quality. Crypsis: ability of an organism
to avoid observation or detection through camouflage
Tetrigidae: pigmy locusts: could their
odd pronota also be fused wings like
those of membracids?
Paratettix mexicanus
photo by Robert Behrstock Arizona
Saussurella sp. Malaysia
Kurt (Hock Ping Guek)
Our lab animal for many
Locusta migratoria
The composite material of which insect exoskeletons are made:
crystalline chitin nanofibres embedded in a protein matrix
Source: Vincent J.F.V. & Wegst U.G.K. 2004. Design and mechanical
properties of insect cuticle. Arthropod Structure and Development 33: 187199.
“The cuticle is ...multifunctional: it not only supports the insect, it gives it its
shape, means of locomotion, waterproofing...”
“The cuticle is secreted by a single layer of epidermal cells that covers the
entire surface of the insect, extending into the tracheal system, fore- and
hind-gut, and part of the genital system. ...it is composed of several layers
(from the outside: cement and wax, then epicuticle, then exo- and
endocuticle) most of the increase in understanding of its mechanical
properties has been in the exo- and endocuticular layers, which make up
the bulk of the thickness.”
Insect cuticle is “a composite material (e.g., as is kevlar) consisting of
arrangements of highly crystalline chitin nanofibres embedded in a matrix of
protein, polyphenols and water, with small amounts of lipid.” The combined
materials contribute differently to the mechanical properties.
Chitin is a polysaccharide akin to cellulose
D’arcy Thompson
‘diagram of forces’
Drawing on tests
Please find on the website under ‘Lecture topics’ a newly posted explanation of the
tentorium and its function from last Tuesday’s lecture, presented as an answer to a test
question: it is a suggested way of organizing your studying toward the ‘standard’ 325
question: what is structure X and what is its function?
Draw to learn
The Introduction of a paper is often the best place to find useful general information.
Introduction from Kier 2012
• “Animal skeletons serve a variety of functions in support and
movement. For example, the skeleton transmits the force generated
by muscle contraction, providing support for maintenance of posture
and for movement and locomotion. Also, because muscle as a
tissue cannot actively elongate [muscles can’t push], skeletons
provide for muscular antagonism, transmitting the force of
contraction of a muscle or group of muscles to re-elongate their
antagonists. In addition, the skeleton often serves to amplify the
displacement, the velocity or the force of muscle contraction
[mechanical amplification]. A wide range of animals and animal
structures lack the rigid skeletal elements that characterize the
skeletons of familiar animals such as the vertebrates and the
arthropods. Instead these animals rely on a ‘hydrostatic skeleton’ ...
in which the force of muscle contraction is transmitted by internal
Kier 2012 is very clearly written.
• Hydrostatic skeletons (see Kier Principles of support and movement)
“typically include a volume of enclosed fluid. The fluid is usually a
liquid (essentially water) and thus has a high bulk modulus, which
simply means that it resists significant volume change.” It is
effectively incompressible.
• “Contraction of circular, radial or transverse muscle fibres will
decrease the diameter, thereby increasing the pressure, and
because no significant change in volume can occur, this decrease in
diameter must result in an increase in length.
The body of a sea anemone is “a
hollow column that is closed at
the base and equipped at the
top with an oral disc that includes
a ring of tentacles surrounding the
mouth and pharynx” “By closing
the mouth, the water in the internal
cavity –the coelenteron – cannot
escape and thus the internal [fluid] volume
remains essentially constant. The walls
of an anemone include a layer of circular
muscle fibres. Longitudinal muscle fibres
are found on the vertical partitions called
septa that project radially inward into the
coelenteron, including robust longitudinal
retractor muscles along with sheets of parietal
longitudinal muscle fibres adjacent to the
body wall.”
Fig. 5 Kier
Phylum Cnidaria
sea anemones,
corals, jellyfish
“With the mouth closed,
contraction of the circular muscle
layer decreases the diameter and
thereby increases the height of
the anemone. Contraction of the
longitudinal muscles shortens the
anemone and re-extends the
circular muscle fibres.”
“...with this simple muscular
arrangement a diverse array of
bending movements and height
change can be produced.”
• Connective tissue fibre reinforcement
• “The walls of many hydrostatic skeletons are reinforced with layers
of connective tissue fibres that control and limit shape change. The
fibres are typically arranged as a ‘crossed-fibre helical connective
tissue array’ in which sheets of connective tissue fibres (often
collagenous) wrap the body or structure in right- and left-handed
helices. Even though the connective tissue fibres are typically stiff in
tension and are thus relatively inextensible, such an arrangement
actually allows length change. Elongation and shortening is possible
because the pitch of the helix changes during elongation...”
Phylum Annelida
segmented worms
species are mostly marine
Lumbricus is specialized for
a terrestrial existence but is
probably primitive in its fore
and aft septa segmentation
ABC News
leech is
also an
annelid but
Univ of Wisconsin
the transverse grooving
along its body does not
represent its embryonic
Transverse section Lumbricus
Trochophore larva Annelida
Trochophore larva of Annelida

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