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First published online August 8, 2008
Journal of Experimental Biology 211, 2576-2583 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.020164
Micromechanical properties of consecutive layers in specialized insect cuticle: the gula of Pachnoda marginata (Coleoptera, Scarabaeidae) and the infrared sensilla of Melanophila acuminata (Coleoptera, Buprestidae)
1 Institute for Zoology, University of Bonn, Poppelsdorfer Schloss, D-53115
Bonn, Germany
2 Forschungszentrum caesar, Ludwig-Erhardt-Allee 2, D-53175 Bonn, Germany
* Author for correspondence (h.schmitz{at}uni-bonn.de)
Accepted 10 June 2008
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Summary |
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 TOP Summary INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Key words: insect cuticle, mechanical property, articulation, nanoindentation, atomic force microscopy, infrared receptor
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INTRODUCTION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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;
Neville, 1975). In addition to
its main function, i.e. building the exoskeleton and protecting the inner
organs, cuticle is an important component of insect sensilla.
In the exoskeleton, the mechanical properties of joints are of special
interest because articulations have to fulfil numerous tasks. For instance,
both components of the head articulation system of Pachnoda marginata
(i.e. the gula part of the ventral head and its counter surface of the
prothorax) are in permanent contact with each other and, therefore, must be
resistant to wear and friction. Simultaneously, gliding has to function with
as little obstruction as possible. Recently, the mechanical properties
(hardness and reduced elastic modulus) of the superficial cuticle layers of
the gula have been measured by nanoindentation
(Barbakadze et al., 2006).
Interestingly, the existence of softer and more compliant layers in the
exocuticle could be shown. To further investigate this phenomenon, we made
cross sections through the gula cuticle and determined the mechanical
properties of the different layers on a microscale. For the identification of
the different types of cuticle we stained the sections with Mallory trichrome
stain.
Furthermore, the structure and composition of the external cuticular
apparatus of a sensillum determines which kind of stimulus is perceived
(Altner and Loftus, 1985;
Steinbrecht, 1984;
Thurm, 1969). Thus, the
modality of a given sensillum can be deduced from the construction of its
cuticular components, which serve for stimulus transmission
(Keil, 1997). Additionally,
the cuticular apparatus often filters and amplifies a stimulus. This is
especially true for cuticular mechanoreceptors, where the mechanical
properties of the different types of cuticle determine the specific function
(i.e. perception of touch, wind speed, airborne sound, gravity, etc.) and the
sensitivity of the receptor (Barth,
1999; French et al.,
2002; Hossl et al.,
2007; Humphrey and Barth,
2008).
About 70 spherical infrared (IR) sensilla of pyrophilous
Melanophila beetles (Coleoptera, Buprestidae) are housed in special
(IR) pit organs located on both sides of the metathorax
(Evans, 1966;
Vondran et al., 1995).
Recently, it has been shown that the IR sensilla have evolved from cuticular
hair mechanoreceptors. Based on the mechanosensitive heritage of the IR
sensilla, Schmitz and coworkers have established a refined functional model of
the transduction mechanism (Schmitz et
al., 2007). IR radiation causes a brief increase in pressure
inside the spherical sensillum, which is measured by a mechanoreceptor [the
so-called photomechanic mechanism of IR perception; cf.
Fig. 6A
(Schmitz and Bleckmann,
1998)]. Because the cuticular apparatus accomplishes the
conversion of IR radiation into a micromechanical event, the determination of
the thermo-mechanical properties of the different components is of particular
interest. In a first approach, we measured hardness and modulus of different
areas of individual sensilla. Mechanical properties such as elastic modulus
and hardness of a given material are mainly determined by the bond strengths
(i.e. the interatomic forces within the material). In addition, strong
interatomic forces are associated with low thermal expansion, while weak
forces are associated with high expansion
(Newnham, 2005). Therefore,
the information about elastic modulus and hardness can provide valuable
insight in the thermal expansion behaviour of a material. A manifestation of
this correlation can be observed for example in polymers where an increase of
cross-linking leads to an increase in hardness and stiffness and to a decrease
of thermal expansion (Nielsen,
1969).
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MATERIALS AND METHODS |
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Sample preparation
Infrared organs were excised from the metathoracic segments of M.
acuminata beetles and stored either in 30% ethanol [for subsequent
embedding in water-soluble Durcupan® (Fluka, Buchs, St Gallen,
Switzerland)] or in 70% ethanol [for embedding in Epon 812 (Roth, Karlsruhe,
Germany)]. Gula plates were excised from the median ventral plates of heads of
P. marginata and processed in the same way. Some gula plates were air
dried and mounted onto holders by epoxy glue.
Specimens scheduled for embedment in Durcupan® were dehydrated through
a series of Durcupan A and water, with an increasing Durcupan A fraction, and
finally embedded in a mixture of Durcupan A, hardener, accelerator and
plasticizer (Kushida, 1964;
Stäubli, 1960). A second
set of specimens was dehydrated through an ascending ethanol series and
embedded in Epon 812 (Luft,
1961). Samples were cured at 60°C for 3 days.
At this point it should be emphasized that fixation and especially dehydration most probably have changed the material properties of the biological samples. In the Discussion, an attempt will be made to infer the mechanical properties of the native cuticular material as well as using data from the literature where hydrated cuticle (e.g. the gula of Pachnoda) had been examined.
Semi-thin sections (0.5µm thick) from the respective specimen were cut
with a Reichert OMU3 microtome (Wien, Austria) using a diamond knife (DiATOME
Histo, Biel, Switzerland). Sections were stained with a 0.05%
Toluidine-Blue/borax solution and examined with a Leitz DM RBE light
microscope (Leica, Wetzlar, Germany). As soon as a suitable cross-sectional
profile was reached, the surface was polished by decreasing the slicing
thickness down to 
150nm. In the measurements with the atomic force
microscope (AFM) and the nanoindenter, these smoothed surfaces of the epoxy
block were probed.
Staining of semi-thin sections with Mallory trichrome stain
Some of the sections were specifically stained to reveal the different
types of cuticle. Prior to the staining procedure, the Epon resin was removed
by immersion in alcoholic KOH solution
(Weyda, 1982). Chemical
treatment of the sections with potassium dichromate, acid fuchsin and
phosphotungstic acid hydrate was followed by a final staining process for 7
min with a Mallory trichrome solution
(Weyda, 1982). Non-melanized
exocuticle, which does not really stain, appears in a faint yellow or amber
whereas mesocuticle appears red and endocuticle appears blue
(Weyda, 1982). Digital images
of the sections were taken with a Nikon Coolpix 5000 (Tokyo, Japan).
Atomic force microscopy
To verify the nanoscale topography the samples were examined using an
atomic force microscope (FRT MicroProf equipped with a SiS ULTRAObjective®
AFM; Bergisch Gladbach, Germany) in non-contact mode with standard tips
(Nanosensors PPP-NCLR; Nanosensors, Neuchâtel, Switzerland).
Additionally, after the nanoindentation tests, AFM was used to visualise the
indents in order to determine the plastic/elastic behaviour of the tested
samples.
Nanoindentation
Nanoindentation offers the measurement of hardness and elastic modulus of
very small volumes of material (Bhushan and
Li, 2003; Oliver and Pharr,
1992). For nanomechanical tests, an AFM (NanoScope IV; Digital
Instruments, Santa Barbara, CA, USA) with a conjugated nanomechanical test
instrument (TriboScope Hysitron Inc., Minneapolis, MN, USA) was used. The
Hysitron nanoindenter is a depth-sensing and load-control device, which is
capable of providing measurements of elastic and plastic properties at the
nanoscale level. Because it can act like a scanning probe microscope, it
allows topography imaging and precise positioning of the tip. In the present
study, a Berkovich tip was employed as an indenter. The total included angle
of the tip is 142.3°, with a half angle of 65.35°, which makes it very
flat and efficient for a wide range of materials, including polymers and
biomaterials. The hardness (H) and reduced elastic modulus
(Er) were evaluated by the nanoindenter software from the
recorded unloading step of the depth-displacement curve, based on the method
of Oliver and Pharr (Oliver and Pharr,
1992). H can be described by the term:
 | (1) |
 | (2) |
, m and hf are
determined by the fitting procedure. In this function, hf
equals the final indentation depth, i.e. the depth where the loading force is
completely removed (F=0). Finally, the slope at the maximum
indentation depth (hmax) can be derived. By means of the
contact stiffness, Er can be calculated by the formula:
where the indentation depth (h) and the loading force (F)
are the variables and , m and hf are
determined by the fitting procedure. In this function, hf
equals the final indentation depth, i.e. the depth where the loading force is
completely removed (F=0). Finally, the slope at the maximum
indentation depth (hmax) can be derived. By means of the
contact stiffness, Er can be calculated by the formula:
 | (3) |
). In a
calibration process, the projected contact area could be evaluated and used to
calculate the Er and H of the cuticular material.
Furthermore, E can be determined by the relationship:
 | (4) |
is the Poisson's ratio, and Ei and
i are the material properties of the Berkovich indenter. Since
we used a diamond tip with a high elastic modulus
(Ei=1170GPa) and a low Poisson's ratio
(i=0.07), the right-hand term is negligible and the formula can
be reduced to:
 | (5) |
Unfortunately the Poisson's ratio of insect cuticle has not been described satisfactorily yet. Thus, we describe stiffness and elastic modulus by means of Er.
The epoxy blocks containing the IR organ samples were glued to silicon
discs and fixed on the AFM probe-stage using a vacuum system. In a typical
nanoindentation experiment, a trapezoidal loading pattern with a maximum load
of 300 µN and a load/unload rate of 30 µNs–1
(Fig. 1A) was applied. To
minimize the effect of material creep, a hold time of 10 s was set after
maximum load was reached and before unloading
(Chudoba and Richter, 2001).
More than 260 indents were performed on 16 sensilla from four IR organs
originating from three Melanophila beetles. Indents were made in the
mesocuticle of the sphere, in the region of the lamellated shell, in the
adjacent mesocuticle and also in resin. Furthermore, we measured material
properties of the outer surface of one air-dried gula with 20 indents and
across all layers of cross-sectioned gula cuticle of one Pachnoda
beetle with 198 indents. The tips were cleaned with ethanol after several
cycles of indentation.
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RESULTS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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; Schmitz and Bleckmann,
1997; Vondran et al.,
1995) could not be identified in AFM images. The mesocuticle
continues to the inner surface of the cuticle of the whole organ. Only small
islets of presumed endocuticle could be identified in the more basal layers of
the cuticle (Fig. 2A).
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In contrast to the IR organ of Melanophila, the cuticle of the
gula of Pachnoda reveals a well-developed layer of inner endocuticle
(cf. Barbakadze et al., 2006),
which is reduced to a very thin layer of 5 µm or less at the caudal end of
the gula (Fig. 3B).
Furthermore, a second layer of endocuticle can be found near the outer surface
(Fig. 3A,C). The arrangement of
the different layers of cuticle is unusual in this area of the gula: between
the thin outer epicuticle and the exocuticle, two layers of mesocuticle are
situated, with a thin layer of endocuticle in between
(Fig. 3A–C). These layers
are not as smooth as the other layers of the Pachnoda gula, as can be
seen in the AFM topography (Fig.
3D,E). Thus, the granular topography may indicate a
micro/nanostructured material. For clear identification, we named the layers
of the gula cuticle on the basis of their staining results and numbered
equally stained layers serially, ascending from the outer to the inner surface
of the cuticle.
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Hardness (H) and reduced elastic modulus (Er) of the embedding resins used in our study (Durcupan and Epon) were significantly lower than corresponding values of H and Er of all kinds of examined cuticle (Fig. 4, Table 1). Thus, during probing, the resin could be clearly distinguished from other materials and, therefore, was used as a reference material.
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Hardness of the exocuticular shell of the Melanophila sensillum was 0.53±0.25GPa, which is significantly harder than all other types of tested cuticle (Fig. 4, Table 1). Hardness of the exocuticular shell was almost twice that of the mesocuticular core of the sphere (0.29±0.1 GPa), which is the softest of all tested embedded biomaterials.
Hardness of the mesocuticular core of the Melanophila sensillum was clearly lower than that of the mesocuticle below the spheres (0.34±0.09 GPa) but differed only insignificantly from the hardness of the outer endocuticular layer and the mesocuticular layer 2 of the Pachnoda gula. However, hardness of the inner mesocuticular layer 3 of the gula was significantly higher than hardness of both kinds of mesocuticle identified in the Melanophila IR organ and also significantly higher than the two outer mesocuticular layers 1 and 2 of the gula.
The hardest layers of Pachnoda were the exocuticular layer, where hardness was 0.40±0.07 GPa, and the inner mesocuticular layer 3 (0.43±0.08 GPa). The slight difference between these layers was not significant, but they differed clearly from all other tested materials. Thus, the inner mesocuticular layer 3 showed about the same hardness as the exocuticle of the gula.
In contrast to the Melanophila IR organ, endocuticle could be detected in two areas in the cuticle of the gula of Pachnoda (Fig. 3B), but the inner layer was not probed, since we placed the nanoindenter tip at the region where the inner endocuticle is reduced. Hardness of the thin outer endocuticular layer was 0.3±0.05 GPa, which only differed slightly from the hardness of the mesocuticular layer 2 of the gula and both kinds of mesocuticle of the Melanophila IR organ and, therefore, is the softest layer of the gula of Pachnoda.
Hardness of the entire cuticle of the Pachnoda gula determined from outside was 0.21±0.03 GPa. This value is significantly lower than H of all embedded types of cuticle.
The corresponding elastic modulus differences between particular material classes are somehow dissimilar to the respective hardness data (Fig. 4, Table 1). Our elastic modulus results also reveal the significant differences between the structural sections of the cuticles examined. For example, the highest measured Er of 9.9±3.0 GPa refers to the inner mesocuticular layer 3 of the gula. By contrast, Young's moduli of the exocuticle of the gula and the exocuticle of the outer shell of the sphere of the Melanophila IR sensillum are statistically comparable (8.4±3.4 GPa and 7.9±2.8 GPa, respectively). The modulus of the exocuticle of the Melanophila sensillum also did not differ statistically from the modulus of the mesocuticle below the spheres of the IR organ (6.8±1.9 GPa). However, the mesocuticle below the spheres of the IR organ is significantly stiffer than mesocuticle 1 and 2 and also the outer endocuticular layer of the gula. The most elastic material is the mesocuticle inside the sphere of the Melanophila sensillum (4.8±1.4 GPa). The modulus of this kind of cuticle is significantly lower than the moduli of all other kinds of examined cuticle except for mesocuticular layer 2 of the gula of Pachnoda, which had a modulus of 5.5±0.6 GPa and differed only slightly.
Experiments on the outer surface of the gula revealed a modulus of 6.2±1.3 GPa. Stiffness of the three outermost layers of the gula and the mesocuticle below the spheres of the Melanophila IR organ does not differ significantly from this value. The modulus of the mesocuticle inside the sphere of the Melanophila sensillum is significantly lower, and the modulus of the exocuticular shell of the Melanophila sensillum, as well as the three inner layers of the Pachnoda gula, are significantly higher than the modulus of the entire gula cuticle probed from outside.
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DISCUSSION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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; Enders et al.,
2004; Hillerton et al.,
1982). This allows a well-founded prediction of the condition in
the native (i.e. hydrated) phase. Theoretically, some of the embedding resin
could have infiltrated the cuticle, thereby changing the mechanical
properties. Since sclerotized cuticle is a dense material representing an
effective barrier to the environment, intrusion of epoxy chemicals should be
impossible but cannot be completely ruled out. Because the embedding resin is
much softer than the cuticle, the resulting effects caused by intruding resin
should be moderate and restricted to the outer layers. To avoid any further
anomalies from the native state we did not apply any chemical fixation. Due to
its composition, there is strong evidence that insect cuticle is anisotropic.
However, the details of anisotropic behaviour of the cuticles we have
investigated are unknown. As in previous studies
(Barbakadze et al., 2006;
Sachs et al., 2006), we used
formulae primarily suitable for the calculation of hardness and modulus of
isotropic materials. Because mainly relative differences between the different
cuticular layers are of interest in this study we are convinced that this
procedure is acceptable.
Recently, the mechanical properties of the gula of P. marginata
were investigated by nanoindentation
(Barbakadze et al., 2006).
Hardness and modulus were determined by probing the outer surface of the gula
plate. The authors found a sharp drop in hardness values for dry and
chemically treated samples at a depth of 1.7µm. This indicates the presence
of softer and more compliant layers in the exocuticle. This interesting
finding encouraged us to further investigate this phenomenon. Therefore, we
subjected the gula cuticle to the same procedures of embedding and sectioning
as the Melanophila IR organs.
Functional aspects of the mechanical properties of the gula of Pachnoda
Barbakadze and co-workers determined the hardness of the entire dried gula
measured from outside as 0.49 GPa and the elastic modulus as 7.5 GPa
(Barbakadze et al., 2006). In
principle, these data are in the same order of magnitude as our data for
hardness (0.21 GPa) and modulus (6.2 GPa) measured from outside. Our somewhat
lower values can be explained by a slightly higher water content of our
specimens because, unlike their study, we did not dry the cuticle in an oven
for 24h at 40°C. Furthermore, the difference in hardness between the
sectioned material of the outer mesocuticular layer (meso 1,
H=0.36GPa) and the gula probed from outside (H=0.21 GPa)
could be explained by a change in load direction. A possible explanation may
be a pronounced anisotropy of the mesocuticular material. Thus, our data are
comparable to the data of Barbakadze et al., which also demonstrates that
results obtained at the sectioned cuticle are reasonable too.
Our results show that the cuticle of the gula plate is built with higher
complexity than has been described in previous studies
(Arzt et al., 2002;
Barbakadze et al., 2006;
Dai et al., 2006;
Enders et al., 2004). The
Mallory trichrome stained sections revealed that additional thin layers of
meso- and endocuticle overlie the exocuticle
(Fig. 3A–C). Fundamental
aberrations of the standard cuticular layering have rarely been described
(Richards, 1967) and seem to
be restricted mainly to larval or pupal stages or to specialized regions in
adult insects (e.g. near intersegmental membranes). The histological findings
are corroborated by the measurements of the mechanical properties: hardness
and modulus of the three outer layers of meso- and endocuticle were
significantly lower than the values for the exocuticle
(Fig. 4,
Table 1). These differences
will become even larger in the native phase. Hard cuticle, such as exocuticle,
contains only about 12% water whereas softer cuticle can contain 40–70%
(Vincent and Wegst, 2004).
Mesocuticle is sclerotized to a lesser extent than exocuticle; therefore, the
water content is expected to be higher than that of exocuticle and lower than
that of endocuticle, which may be due to deposition of different types of
proteins in the various layers. Moreover, the water content strongly
influences the mechanical material properties, and an increased portion of
water makes the cuticle softer and more elastic
(Arzt et al., 2002;
Barbakadze et al., 2006;
Enders et al., 2004;
Hillerton et al., 1982;
Vincent and Hillerton, 1979;
Vincent and Wegst, 2004).
The unusual layering of the gula cuticle can be explained by the specific
mechanical requirements of this articulation between the head and the
prothorax; when the head is moved relative to the thorax, the gula glides over
the prothoracic counter surface [see red arrows in
Fig. 6B, and
fig. 2 in Barbakadze et al.
(Barbakadze et al., 2006)].
Comparative measurements of dried samples of the head articulation of
Pachnoda have shown that the head part of this contact pair is harder
than the prothoracic counterpart (Arzt et
al., 2002), but no histological data to estimate native hydrated
material hardness and elastic modulus have been collected. Our results show
that the outer surface of the gula does not consist of exocuticle but is
covered by a `pad' consisting of a mesocuticular encasement enclosing a thin
layer of endocuticle. Our measurements show that the meso- and endocuticle of
the pad are softer and less stiff than the underlying exocuticle. It can be
postulated that in the living beetle, where the water content of the
mesocuticle and especially the endocuticle is much higher than in dried
material, hardness and stiffness will be reduced additionally. Because
frictional investigations have suggested that for frictional behaviour the
articulation material is one of the most important factors
(Dai et al., 2006), we
postulate that, in principle, the pad may function as fibrous cartilage. In
this type of cartilage, an outer envelope containing fibrous bundles (here the
mesocuticular encasement reinforced by chitin fibres) encloses a gelatinous
core (the endocuticle). If the pad really does fulfil the function of a layer
of cartilage, it provides the articulation with high elasticity and
compression strength, which facilitates frictionless movement. Additionally,
compressive stress as well as friction and shearing forces are reduced. Fluid
components of the epicuticular wax layer could take on the lubricating
function of the synovial fluid but this has to be demonstrated. Further
investigations have to show to what extent these possible capacities are
necessary for the function of the head articulation in Pachnoda.
Composition and mechanical properties of the cuticular apparatus of the Melanophila IR sensillum: consequences for the possible function
Mallory trichrome staining of the cuticle of the IR organ of
Melanophila revealed that darkly pigmented outer exocuticle, as well
as a distinct layer of inner endocuticle, is missing. The cuticle under the
spheres consists of mesocuticle, which also builds the interior thin layer of
the dome-shaped bulge covering each sphere. The outer layer of the bulge,
however, consists of colourless exocuticle, which is supposed to be
β-sclerotized, and is finally covered by a very thin epicuticle
(Schmitz et al., 2007). The
sphere, in which the tip of the mechanosensory dendrite is situated, consists
of an outer shell of β-sclerotized cuticle containing an inner
mesocuticular core. In this respect, the results of earlier studies were
corroborated (Schmitz et al.,
2007; Vondran et al.,
1995). However, the hardness and the modulus of the mesocuticle
inside the sphere are significantly lower than those of the mesocuticle under
the spheres (Fig. 4,
Table 1). As we have already
outlined in the Introduction, there is a close correlation between mechanical
and thermo-mechanical material properties. Thus, harder and stiffer cuticular
material most probably has a lower coefficient of thermal expansion than more
elastic and softer cuticle. According to our model of photomechanic
transduction by a photomechanic IR sensillum this would mean that, in the case
of IR absorption by the cuticle of the IR organ, the resulting thermal
expansion will not be uniform. In fact, thermal expansion of the mesocuticle
of the inner core will be restrained by the harder and stiffer exocuticular
shell (Fig. 6A). Taking into
account that the microcavities inside the mesocuticular core are filled with a
fluid, which also shows a much higher coefficient of thermal expansion, this
will result in a distinct increase in internal pressure inside the shell of
the sphere. The pressure will be transmitted by the incompressible fluid
through the canals of the spongy mesocuticle to the inner pressure chamber,
where the mechanosensitive tip of the dendrite is situated. Due to the
hydraulic function of the fluid, a cross compression of the dendritic tip will
take place (cf. Fig. 6A). This
is adequate stimulus for an insect mechanoreceptor.
The exocuticular shell of the sphere, therefore, functions as an outer
pressure vessel. To act as an expansion-restraining structure, the shell is
reinforced by layers of chitin fibres, giving the shell a highly lamellated
appearance (Schmitz et al.,
2007; Vondran et al.,
1995). The large standard deviation encountered when we measured
hardness may be explained by the variation of the angle between the probing
direction and the orientation of the chitin fibres. Additionally, in a few
cases, layers of chitin fibres of the shell were most probably approached
completely perpendicularly. In these cases, we measured a much higher
hardness, with a mean value of 0.67 GPa. Therefore, direction of applied load
may play a pivotal role for this kind of highly ordered cuticle, and material
characteristics are supposed to be anisotropic. Because chitin fibres are
extremely resistant to mechanical stress and strain, we postulate that the
main reason for the incrustation of chitin fibres in the shell is to secure a
minimal thermal expansion of the shell in case of IR absorption; all
displacement caused by thermal expansion should be `concentrated' onto the
membrane of the dendritic tip inside the inner pressure chamber.
To further develop the current model of photomechanic transduction, thereby
improving its efficiency, even a decrease in the volume of the sphere is
imaginable due to a negative coefficient of thermal expansion of the outer
shell. Several long-chain polymers and carbon-containing fibres (e.g. carbon
fibres) show a thermal contraction along their chain axis but expand radial to
the chain direction (Newnham,
2005). Because the chitin fibres are arranged circumferentially,
the outer shell would shrink because of the longitudinal contraction and
radial expansion of the chitin fibres (Fig.
6A). The resulting decrease in size of the inner lumen inside the
shell would additionally increase the rising pressure inside the fluid-filled
mesocuticular core. Admittedly, the case of chitin is different from the
examples mentioned because chitin fibres typically consist of a bundle of
chitin chains that are linked by hydrogen bonds. However, a similar behaviour
might be possible and would enhance IR receptor function. This deserves
further investigation.
LIST OF ABBREVIATIONS
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Acknowledgments |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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