Both up and down insect-wing movements employ a distance-increasing lever – a lever
with good speed/distance advantage [and a poor force advantage]; there is a very short
force arm, the moment arm of the effort is divided by the much larger moment arm of
the load – the centre of gravity of the wing being much farther from the fulcrum (pleural
wing process, PWP).
“A muscle... is relatively
good at producing force
and relatively bad at getting
shorter. ...any engine that gets only
20 % shorter will have to operate
with a substantial distance
advantage to move a long limb
[wing] through an angle that may
approach 180 degrees. For that
good distance advantage it will
necessarily suffer a poor force
advantage because the product of
the two must be unity...” (Vogel 2nd ,
p. 475)
Class 3 lever for both up
and downstrokes of bird
wings
As with the insect wing, the bird wing is a
distance (speed) -increasing lever. Distance
advantage will be >1, i.e., put in a small
distance get out a large distance.
Distance advantage: ratio of the distance
moved by the load (weight of the wing)
relative to that moved by the effort
(contracting muscle); this is much >1.
The effort arm in a flying bird is not longer
than the load arm, so it is not maximizing the
moment of the effort.
The hind (metathoracic) leg of the
locust is a saltatorial adaptation. In
jumping the insect moves the tibia
relative to the femur using paired
antagonistic muscles: the extensor of
the tibia and the flexor of the tibia.
The pinnately arranged fibres of
these muscles, within the femur,
originate on the exoskeleton’s inner
surface and insert on two blade
apodemes (red in diagram), one large
and one small. The apodemes ‘focus’
the muscle forces to a small locus of
the proximal end of the tibia. A
dicondylic joint ensures that the tibial
movement is in one plane. Flexion
here is the same thing as depression;
extension is the same as levation;
so these muscles and their apodemes
are also called the depressor and
levator
Pinnate muscle via apodeme: more powerful than orthogonal (direct) fibres
To avoid having to draw, and only use words for organ description:
see this wonderful old book of terminology for entomologists: TorreBueno, J.R. A Glossary of Entomology. Brooklyn Entomological Society
•
•
•
•
•
•
•
acclivous – rising gently
acetabulum – cavity into which an appendage is articulated
acicular – needle-shaped (e.g. spine of Chestnut is acicular)
aculeate – armed with short sharp points (e.g., burr of burrdock is not aculeate)
acuminate – tapering to a long point
adnate – adjoining (e.g., radius and subcosta are adnate)
alate – winged etc. Takes a while to get to ‘unguis’: one of the claws at the end
of the tarsus, plural ungues
• ETC.
• unguiform – shaped like a claw; unguiflexor – muscle flexing the ungues of an
insect; unguifer – the median dorsal process or sclerite on the end of the tarsus
to which the pretarsal claws are articulated etc. Unguiflexor lets me illustrate a
rope apodeme, one specialized to convey tensile stress as well as the fact that
not all antagonists are other muscles.
Apodemes can also be designed for tension
Locust rope apodeme and unguiflexor muscles
unguiform – means claw-shaped; unguiflexor is a
muscle flexing (depressing) the ungues (terminal
claws) into the substratum; unguifer – the median
dorsal process or sclerite on the end of the tarsus to
which the pretarsal claws are articulated. Several
small unguiflexor muscles (139 a,b,c) act to dig in the
ungues. A locust rope apodeme is specialized to
translocate tensile stress, creating effort around
corners and far away at the limb tip.
As in the scallop, the antagonist to
muscles 139 abc i.e. the partner in
the pair is NOT another muscle; it is
cuticle that has become more elastic.
Arthropod cuticle: a hierarchical composite material
Vincent J.F.V. & Wegst U.G.K. 2004. Design and mechanical properties of insect cuticle. Arthropod
Structure and Development 33: 187-199.
• Many animal materials are composites: a material made by combining two (or
more?) other materials: soft composites are made of a “rubbery matrix reinforced by
fibres. A “…material that is composed of two quite different materials… can have
better properties than either material on its own” (Ennos 2012).
• My fiberglass canoe is made of a composite material, isolated glass fibres embedded
in a continuous resin matrix. Cuticle, the integument of all animals in the phylum
Arthropoda, functions as exoskeleton and is a composite material.
• Arthropod cuticle consists “…of arrangements of highly crystalline nanofibres
embedded in a matrix of protein, polyphenols and water, with small amounts of lipid.”
• “The protein has to produce a matrix of varying mechanical properties, which will also
interact with and stabilize the chitin.”
• It is a material, the basis of the arthropod skeleton, “praeternaturally (it means
surpassing the ordinary) multifunctional” (Vincent 2004).
• Matrix or ‘ground substance’: a 3D-network in which fibres are embedded; a matrix of
‘mortar’ is created in a brick wall which embraces and surrounds all the bricks (cf. nachre).
Fig 1 from Vincent &
Wegst 2004
Chitin: a “…polysaccharide akin to cellulose. The monosaccharide units
link to make the molecule very straight and ribbonlike. Then nanofibres
are 3 nm in diameter, 0.3 micrometeres long, in the figure they run into
and out of the plane of the screen. “The fibrous composite cuticle
derives its properties from its components, which can be varied in
orientation... and volume fraction to produce the wide range of
mechanical properties: chitin nanofibres, type of protein, water content
and degree of cross-linking of the protein [sclerotization], lipid, metal
ions, calcium carbonate.”
“Praeternaturally multifunctional” (Vincent & Wegst 2004)
Cuticle/exoskeleton of animals in the phylum Arthropoda
• “The cuticle… not only supports the insect, it gives it its shape, means of locomotion,
waterproofing and a range of localised mechanical specialisations such as high
compliance, adhesion, wear resistance and diffusion control. It can also serve as a
major barrier to parasitism and disease ...the insect cuticle also has to form sensors,
joints, wear-resistant mandibles, devices for elastic energy storage, effective
attachment systems...” (Vincent & Wegst 2004)
• Cuticle composition changes topographically throughout an animal’s skeleton altered
to perform these various functions: it varies in strength, toughness, elasticity,
dimension, shape – so it can give broad surface to muscle fibre origins (as a blade
apodeme) – so it can act as a brace or strut (tentorium) - so it can translocate force
around corners like a rope (unguitractor). It becomes very thin in gills so allowing the
gas exchange of aquatic insects, or becomes tanned/sclerotized to resist
compression and abrashion in a crushing mandible. [Sclerotization and tanning are
chemical processes that toughen cuticle by creating stable cross-linkages between
the nanofibres.]
• Adaptive form (the theme of 325) extends to the very material of the body parts: there
is a hierarchially organized structure to materials – and they show adaptation
throughout this hierarchy. Cuticle’s microstructure – the structure of “crystalline chitin
nanofibres embedded in a matrix of protein, polyphenols and water” (Vincent & Wegt
2004) has evolved to allignments and linkages to handle force appropriately.
• “The tensile and shear stiffnesses and strengths… are much larger when fibres are
alligned parallel to the applied load.”
• 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 can be as thin as 1 micrometre in the hindgut and
over gills [where transport matters] and as thick as 200 micrometres “(wing-covers,
of large beetles) [where mechanical protection strength and toughness are
needed].
• The cuticle “…frequently is multilayered with a plywood-like structure “ – the grain
of successive glued wood sheets is running in different directions. Wood fibres are
more effective in resisting force perpendicular to the length of the fibre than along it;
think of splitting a wood: hit the grain of the log on the end.
•
Plywood analogy: “If high stiffness in more than one direction is required , as is the
case in most parts of the cuticle, ‘laminating unidirectional layers in a variety of
directions produces the desired properties.”
• Quotes from (Vincent & Wegst 2004)