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Cylindrical Bragg mirrors on leg segments of the male Bolivian blueleg tarantula Pamphobeteus antinous (Theraphosidae)

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Cylindrical Bragg mirrors on leg segments of the male Bolivian blueleg tarantula Pamphobeteus antinous (Theraphosidae)

Abstract and Figures

The large male tarantula Pamphobeteus antinous is easily recognized at the presence of blue-violet iridescent bristles on some of the segments of its legs and pedipalps. The optical properties of these colored appendages have been measured and the internal geometrical structure of the bristles have been investigated. The coloration is shown to be caused by a curved coaxial multilayer which acts as a "cylindrical Bragg mirror".
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Cylindrical Bragg mirrors on leg
segments of the male Bolivian blueleg
tarantula Pamphobeteus antinous
(Theraphosidae)
Priscilla Simonis,Annick Bay, Victoria L. Welch,
Jean-Franc¸ois Colomer, and Jean Pol Vigneron
Research Center in Physics of Matter and Radiation (PMR), University of Namur (FUNDP),
rue de Bruxelles, 61, B-5000 Namur Belgium
priscilla.simonis@fundp.ac.be
Abstract: The large male tarantula Pamphobeteus antinous is easily
recognized at the presence of blue-violet iridescent bristles on some of the
segments of its legs and pedipalps. The optical properties of these colored
appendages have been measured and the internal geometrical structure of
the bristles have been investigated. The coloration is shown to be caused by
a curved coaxial multilayer which acts as a “cylindrical Bragg mirror”.
© 2013 Optical Society of America
OCIS codes: (330.0330) Vision, color, and visual optics; (170.1420) Biology; (050.5298)
Photonic crystals; (060.4005) Microstructured fibers; (230.4170) Multilayers.
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1. Introduction
Some spiders are known to have evolved photonic structures for coloration. Nearly perfect
Bragg mirrors (i.e. dielectric multilayers) are found, for example, on jumping spiders [1] and
tube-web spiders [2], resulting in brightly colored metallic-looking spots. In this work, we
examine a different type of spider: a neotropical tarantula.
Tarantulas form a group of more than 800 species of spiders and are frequently large and
hairy. All belong to the family Theraphosidae, which is one of fourteen families in the in-
fraorder Mygalomorphae. Mygalomorphs are distinct from the so-called ”true” spiders (Arane-
omorphae) and differ in certain morphological details. These include the presence of patches
of specialized bristles (called”scopulae”) in tarantulas that help with the adhesion of tarsi and
metatarsi to the surface supporting the spider, or assist the functions of the tarsal claws.
Pamphobeteusantinous (Pocock, 1903) [3], under investigation here, is a very large tarantula
originating from the southern neotropical region, essentially Peru and Bolivia. The mature male
of this so-called ”Bolivian blueleg tarantula” displays, under specific illumination and viewing
directions, a vivid blue-violet color on part of its dorsal side. The specimen examined in the
present study was a mature male with a leg-span of 23 cm(see Fig.1(a)).
2. The bristles: a versatile arthropod structure
Tarantulas have different types of bristles, which serve a variety of functions, including re-
pelling water and making an efficient barrier to parasites. Some of their ”sensitive” setae are
very specialized and many neotropical tarantulas, such as Pamphobeteus antinous, also bear
urticating bristles and very rigid spines, that form part of a stridulation organ. The individual
defence strategies of American tarantulas show several phases [4]. When not running away and
hiding, a tarantula faces danger with a distinctive threat posture. If this fails, it makes intimi-
dating forward movements and some tarantulas (like the one studied here) are able to produce
a loud hissing sound from stridulatory organs [5]. Under a sustained threat, species possessing
urticating bristles may deploy them defensively by rubbing off these bristles, which break close
to their attachment point. The fine bristle-tips penetrate the opponent’s skin under a random
direction and, because of the presence of rear-pointing barbs, progresses, producing further
irritation. As far as has been observed, the discomfort is caused only by the structure of the
bristle: no irritant chemical has been identified.
Some species of tarantulas are arboreal, but many more, like Pamphobeteus antinous, are
terrestrial and typically live in burrows, from which they hunt prey. Largetarantulas need large
prey, such as small rodents, lizards and birds [6] (Victorian biologists have introduced the ver-
nacular name “birdeaters” for them). Mating is a complex process and the male first deposits
sperm under a kind of silk cloth stretched on a natural wedged groove on the ground. He then
calls for a receptive female: a process involving tapping the ground and taking expressive pos-
tures. The fact that the coloration studied here has only developed in mature adult males sug-
gests that coloration and visual effects may also be part of the mating signals.
However, the coloration of the male Pamphobeteus antinous is not only interesting for its
biological importance. The mechanism of coloration that will be explained in the next sections,
has far-reaching and interesting consequences for the possibility of coloring fibers, including
natural and artificial textile threads.
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Fig. 1. (a) The male tarantula Pamphobeteus antinous displays a vivid violet-blue color on
the three leg segments closest to its cephalothorax. (b) Blue areas on the dorsal side of the
mature male Pamphobeteus antinous. Colored patches appear on the three inner segments
of the legs and pedipalps, on the chelicerae and, less visible, on the dorsal cuticle of the tho-
rax. The blue color originates from specialized bristles covering the cuticle. These bristles
are roughly parallel, aligned along the length of the spider’s leg segment. (c) Macrophotog-
raphy of the bristles on the dorsal side of the pedipalp’s femurs, oriented at a small angle to
the symmetry plane of the body. (d) Optical microscope view of the blue setae, suggesting
a roughly cylindrical shape. At this scale, individual setae are visible and a slight variation
of color, from blue to violet is observable. Other, uncolored bristles also appear in the field.
3. Naked-eye perception and optical microscopy
The blue-violet iridescent coloration covers the dorsal side of the legs segments closest to the
cephalothorax, the “coxa”, “trochanter” and “femur”. The other segments (“patella”, “tibia”,
“metatarsus” and “tarsus”) are covered with dark brown setae, showing no iridescence. The
first three of the six segments that form the pedipalp are also colored in a similar way. Traces
of blue can additionally be seen on the chelicerae (those mouth parts used to grasp and crush
food and to inject venom to paralyze prey) and at different places on the dorsal side of the
cephalothorax (see Fig. 1(b)). Macrophotography and optical microscopy (see Fig. 1(c) and
1(d)) offer the advantage of color rendering and show that the blue coloration originates from
special bristles covering the dorsal face of the spider’s cuticle. This hairy region is covered with
setae that are roughly parallel to each other and parallel to the cuticle’s surface. Examination
of the bristles under an (Olympus BX61) optical microscope, used in reflection mode, shows
colors in the range of blue and violet. The blue coloration is most easily seen from a direction
close to the normal to the cuticle surface. Under larger incidences or viewing angles, the col-
oration becomes violet and completely fades away with further angle increase. This iridescence
suggests a structural coloration.
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Fig. 2. (a) The different types of setae, as are revealed by scanning electron microscopy.
Most of the setae are asymmetrical in cross section, with one side being cylindrical (A) and
the other side flattened and sculpted (B) in a way that recalls the standard ridge-crossrib
structure displayed by relatively unspecialized scales in other arthropods. Bristle C shows
the sharp extensions typical of an “urticating bristle”, designed to penetrate soft surfaces
and produce irritation. (b) The whole cross-section of a blue seta, showing the layered outer
cortex that surrounds the structure, the thick homogeneous substrate and the cylindrical
cavity, all of which are centered on the axis of the cylinder. (c) Detail of the multilayered
structure : a few (around 4) cylindrical sheets are repeated radially. The insert shows details
at a larger magnification which suggests that the space between the sheets is essentially
empty. (d) On this fractured cross-section, some sheets are protruding in such a way as to
show their outer surface. This image also suggests a solid/air interface. (e) This view of
the perforated part of the bristle reveals that the multilayer is also present there, providing
blue-to-violet iridescent coloration.
4. Submicron morphology
4.1. Cross-section by Scanning Electron Microscopy
Figure 2 provide information on the structure of the blue setae, as acquired from scanning elec-
tron microscopy. Samples were prepared by cross-sectioning a femur from a foreleg, creating
a sharp surface perpendicular to its axis, while keeping all the bristles attached. The operation
was conducted at the temperature of liquid nitrogen, low enough to harden the chitinous mate-
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rial and increase the chances of a neat fracture of the setae. The femur’s segment was mounted
at or upright, using silver paint, on a metallic sample holder for viewing the setae’s lateral sur-
face and cross section as clearly as possible. The whole mounts were coated in metal (20 nm of
gold) in order to ease charge elimination and then introduced in a JEOL 7500F high-resolution
field-emission scanning electron microscope. About two dozen fractured cross-sections could
be examined in this way.
Figure 2(a) provides a low-magnification view of the setae. On seta labeled A, the exposed
surface is clearly a cylinder, but the presence of a more structured side appears on the fractured
cross-section. The seta labeled B is just the same as seta A but turned over, so that it clearly
shows the non-cylindrical region. Indeed, the cross section of this seta is similar to that ob-
served on seta A, with a large angle of rotation. The bristle labeled C is much more complex,
with barbs pointing backwards. Their oriented geometry suggests that the function of this struc-
ture is to penetrate soft surfaces under random motions: a mechanism cited for the urticating
bristles. This view indicates that the bristles responsible for the coloration (A and B) can only
be approximately described as cylindrical. The whole seta can be viewed as a long strip of
chitinous material curved cylindrically perpendicular to the longitudinal direction and closed
by a narrow net of “ribs” and “crossribs”. Note that these characteristics make these specific
setae rather similar to the appendages found in other arthropods, such as butterflies scales [7].
This complexity suggests multiple functionalities.
Figure 2(b) shows a nearly complete cross-section. The region near the center of the apparent
structure, on the fiber axis, is hollow (making the whole bristle topologically akin to some
kind of sack), while the outer part is a clearly distinguishable cortex, structured as a quasi-
cylindrical multilayer and terminated by a thick protective layer. Between the outer multilayer
and the inner cavity, the volume of material appears to be essentially homogeneous. Under
grazing incidence and at grazing viewing directions, the bristles on the cuticle assume a dark
brown coloration. Consequently it can be inferred that this bulk chitinous material contains
many absorbing pigments (the coloration suggests melanin).
As seen in Fig. 2(b) and 2(c), the cylindrical multilayer has three or more “bilayer” periods
(146±10 nm for each period). Each period is formed by a layer of a high secondary elec-
tron emitter, which we assume to be a hard chitinous compound (“sheets” 100±15 nm thick)
and a layer which appears very dark and unstructured, that we identify as essentially empty
(46±15 nm). Bridges across this empty layer, between sheets, are frequently seen in high mag-
nification conditions (see the inset in Fig. 2(c) and in Fig. 2(d)). These material bridges, which
would not be needed if any another solid material was filling the space, contribute to rigidify the
structure while acting as spacers. In fact, each chitinous sheet that form the solid/void cylindri-
cal multilayer is itself structured in the tangential directions, but without affecting strongly the
optical properties: we will come back to this in a moment. Note, also, that the outer cylindrical
chitinous sheet is slightly thicker than the internal layers: 220 ±20 nm of bulk chitin-based
material.
Figure 2(d) shows several sheets of chitin, broken at different distances along the bristle’s
axis and protruding in such a way that their outer lateral surface becomes visible. This, and the
bridges between adjacent sheets, suggest that they are separated by an empty layer. Figure 2(e)
shows that, in the region of the ribs and cross-ribs, the same multilayer structure is also present,
near the surface and deeper, but that the orientation of the layers is more turbulent than in the
cylindrical part of the bristle.
The cylindrical multilayer is, as will be confirmed below, the origin of the blue coloration. In
planar - not cylindrical - geometry, similar or more complex layered structures have been shown
to be responsible for coloration of other arthropods (see specifically [8,9], reviews in [10–14]
and special multilayers in [15–17]). Iridescence is an important aspect of the optical properties
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Fig. 3. Sections of the blue setae, viewed with the transmission electron microscope (TEM).
The peripheral multilayer as seen (a) through a section at right angle from the seta axis
and (b) through a section nearly parallel to the axis. The measurement of the multilayer
period on these sections confirm the values obtained from scanning electron microscopy
images. The slight opacity of the lighter layers is interpreted as the presence of infiltrated
embedding medium between the darker cylindrical chitin sheets.
of these planar structure, which has its counterpart – slightly more complicated, see below, – in
the case of a curved, cylindrical, surface.
4.2. Bristle’s cross-section, as observed in Transmission Electron Microscopy
The SEM observations and data are confirmed by transmission electron microscope (TEM)
images. However,as we will now see, some details call for caution and a careful discussion.
Samples were prepared by embedding individual bristles in an epoxy resin, letting this resin
infiltrate the structure at a constant temperature of 35C for 48 hours, and polymerize at 60C
for 72 more hours. Then, slices (90 nm thick) were cut perpendicular to the bristle’s axis and
examined with a FEI Tecnai Transmission Electron Microscope (TEM). Two typical images
are shown in Fig. 3. Figure 3(a) shows the structure of the multilayer in the plane of the bristle-
cross-section. Figure 3(b) gives a view of the same multilayer, along a section nearly parallel
to the bristle’s axis. The examination of these views confirms that: (1) The bristle is hollow;(2)
The internal hollow cylindrical volume is homogeneous; (3) A thin cylindrical multilayer (3 or
more bilayers) lies below the external surface; (4) The outer layer is thicker than those in the
multilayer.
The interesting part is the multilayer, which shows alternate clear and dark layers. There is
little doubt that the dark layers are made of the same chitinous material as the outer protective
cortex and the internal hollow homogeneous cylinder: the electron opacity is the same. In the
multilayer, the opacity of the “clear” layers separating the dark chitinous lamellae is not exactly
the same as the opacity of the resin outside the bristle. This may suggest that the outer resin is
different from the material imbedded between the dark sheets and the question of the nature of
the interstitial material in the structure rises. Since SEM shows unambiguously that the spaces
between chitin sheets are empty in the dry, non-embedded structure, we interpret the different
opacities by assuming that, in the transport process into the multilayer’s void spaces or dur-
ing the long polymerizing time, the infiltrated resin has gathered some absorbing material and
slightly changed its contrast with respect to the pure, outer-lying resin. This unwanted staining
is likely to be difficult to avoid when treating a complex structure that has been preserved dry.
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Fig. 4. (a) The bristle coloring structure, viewed through a window-like perforation on the
cylinder’s surface. The outer surface of the concentric layers is smooth except for random
perforations. However, the edges of the sheets are highly structured, laterally, suggesting
that the smooth surface hides a fine pattern of hard rods separated by air gaps. (b) An ide-
alized model of the photonic structure suggested by SEM and TEM, with a thick covering
layer, a multilayer build from winding chitin rods lying under a thin chitin sheet with bind-
ing bridges and a thick substrate. The structure is represented upside down (cover-layer
below) in order to reveal the sheet patterning. This model neglects disorder and the cylin-
drical curvature. (c) The parameters that tune the optical properties of this ideal structure
are as follows: his the thickness of the cover layer, ais the total multilayer period, wis
the thickness of the thin binding plate, bis the thickness of the chitin sheet, including the
binding plate, cis the width of the chitin bars. ris an artificial lateral period for representing
the pattern of rods in a square lattice symmetry.
4.3. Three-dimensional structure of the concentric sheets
In order to make more precise the structure of the chitin sheets, new samples were prepared for
scanning electron microscopy by gluing individual colored bristles flat on conductive carbon
adhesive tab on a metallic sample holder. A sharp blade was then used to puncture the bristles
at many different places, providing many chances to damage the multilayer by removing outer
sheets. A typical result is shown in Fig. 4(a). Several sheet surfaces are now apparent and the
image shows that the sheets are themselves structured. The chitin sheet is built from flat, wind-
ing chitin bars that are separated by a roughly constant gap, hence the pattern observed on the
sheet. The coherence and rigidity of this structure, reminiscent of those produced by spinodal
phase separation [18], is enhanced by a thin, continuous lamella that attaches to one side of
the flat chitin-bar assembly. When viewed from the outside of the bristle, this flat, cylindrical
thin film is seen first, hiding the chitin-bar pattern, except at places where the supporting film is
naturally or accidentally perforated. Again, from such images, it is clear that the dry structure
has solid/air internal surfaces, and not solid/solid interfaces.
This photonic structure, formed from winded chitin rods on a thin continuous sheet, is only
partially ordered: the distance between concentric sheets, the size of the cross-section of the
chitin-rods and the gap that separates them are nearly constant. On a larger scale, the chitin bar
pattern and the location of the bridges between the sheets are much more disordered. A possible
(flattened and ordered) idealization of the whole structure is shown in Fig. 4(b). This structure is
much more regular than the actual natural photonic structure, but, beyond accommodating the
observed multilayer’s characteristics, it introduces the structure of winded chitin bars on a very
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Fig. 5. Calculated reflection spectrum (solid line), based on the morphological data ob-
tained by electron microscopy. The structure is the three-dimensional model described in
Fig. 4(c). The dashed curve corresponds to the same calculation with all three-dimensional
features – bridges between the sheets and the groove between the chitin bars – suppressed.
In this case, the spectrum is exactly as one would expect from a one-dimensional multilayer.
The red shift is consistent with the increase in the average refractive index.
thin continuous support. The size of the parameters which define this structure are extracted
from the scanning and transmission electron microscopy data and summarized in Fig. 4(c).
This three-dimensional structure will actually serves us to show that an even simpler structure
– the cylindrical one-dimensional Bragg mirror – is, here, appropriate to model the reflection
properties of the tarantula’s bristles.
5. Reflection from a colored bristle
The cylindrical structure is actually a multi-scale system: the bristle diameter (10
µ
m) de-
fines a scale where optical phenomena are driven by incoherent ray-tracing optics, while, at the
scale of the multilayer period (150 nm), interference plays a major role. In the present work,
we examine both scales independently.
5.1. Local reflecting structure
The ideal structure shown in Fig. 4(c) summarizes the observed shape and parameters that
control the optical properties of the local reflector found on the spider’s cylindrical bristles. The
values of the parameters have been be extracted from the TEM and SEM images. Though these
values may vary by as much as ±10% on different images, the following values can be safely
accepted: h=220 nm is the thickness of the cover layer, a=146 nm is the total multilayer
period, w=26 nm is the thickness of the thin binding plate, b=100 nm is the thickness of the
chitin sheet, including the binding plate, c=100 nm is the width of the chitin bars. r=1000 nm
is an artificial lateral period for representing the pattern of rods in a square lattice symmetry
replacing the isotropic irregular pattern. The exact value of the parameter ris deduced from the
length of straight sections of the rods, when these can be detected. The homogeneous substrate
layer has a thickness of about 2000 nm, but we will replace this with a semi-infinite medium.
As the 2000 nm layer absorbs essentially all the transmitted radiation, this simplification has
negligible consequences.
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The refractive index of the solid part of the structure is that of chitin, with absorbing melanin.
It is analogous to the dark chitin found in other arthropods. We use the value measured by Yosh-
ioka et al. [19]. This measured value accounts for both dispersion and absorption: it varies from
n=1.785+i0.097 at 400 nm to n=1.63+i0.012 at 700 nm. These values are higher than the
commonly accepted average value, n=1.56+i0.06 [20], obtained for the clear chitinous com-
pound in a Morpho butterfly scale. The real part of the refractive index may vary,in particular
with the contents of the absorbing pigments, as indicated by Kramers-Kronig relations.
The three-dimensional transfer-matrix technique [21] can be used to calculate the specular
reflectance from the structure shown in Fig. 4(b) (with 4 periods). The result, using a base set
of 64 plane waves (which means 64 propagating and evanescent diffraction orders) is shown as
a solid-line curve in Fig. 5. This calculation leads to a reflection band centered on the dominant
wavelength 451 nm. Removing the bridges between the sheets and the groove between the
chitin bars does not change the spectrum very significantly, as the dashed curve in Fig. 5 shows.
The main reason for this agreement is that the three-dimensional features actually have little
effect on the scattering of light: the bridges are small and weak scatterers, the gaps between the
rods are too close to each other, compared to wavelength, to produce diffraction of visible
waves. Neglecting the gaps between chitin rods, the bridges between chitin sheets and the
overall cylindrical curvature, the coloring structure is a one-dimensional multilayer, or Bragg
mirror.
This analysis shows that a one-dimensional Bragg mirror structure is highly justified and,
from now on, we will use this much simpler, one-dimensional, representation of the structure.
Note that, in the context of a one-dimensional periodic multilayer, the reflected color can
easily be predicted: the repetition of a bilayer, assembling a layer with a thickness d1=b=
100 nm (refractive index averaged over the visible spectral range n1=1.73) and an other one
with a thickness d2=ab=46 nm (refractive index of air n2=1) leads to the following
dominant reflected wavelength
λ
=2an
m(1)
(see for instance Land [8] or Kinoshita et al. [22], who refer to the pioneering work of Lord
Rayleigh in 1917 [23] (a simpler derivation, solely based on translational symmetry arguments,
has been given by Vigneron et al. [21, 24]). In the above equation, mis an integer (at least equal
to 1) that selects the band gap acting in the interesting range of wavelengths. Most often, in
natural photonic structures, the value m=1 is adequate, but exceptions have been found. The
average refractive index mentioned by most authors is [22]
n=n1d1+n2d2
d1+d2(2)
where, here, d1+d2=a. Another way of approaching nis to average the dielectric constant
ε
[21, 24] and then deduce the average refractive index, but for moderate refractive indexes,
both procedures lead to very similar results. Here, from Eq. (2), n=1.50 (
ε
=1.54) so
that the dominant reflected wavelength is
λ
=438 nm (from
ε
: 450 nm) – very close to the
maximum reflection wavelength on the spectra in Fig. 5. This wavelength indicates a purplish
blue color [25]. This very simple approach (1) assumes an infinite stack, (2) neglects the special
outer layer and (3) neglects absorption by dark pigments. More importantly, the cylindrical
curvature of the multilayer is not yet accounted for. This is the object of the next section.
5.2. Scattering from a cylindrical reflecting structure
The scattering by structured and unstructured cylinders have been considered in the context
of optics and radio-wave propagation [26–28]. It is interesting to note that one of the first
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Fig. 6. The model’s geometry. The cylindrical fiber is set along the zaxis (which is also
the cylinder’s axis) and the incident beam (along~
kin) lies in the plane containing the xaxis.
Due to conservation laws, the scattered beam (along~
kout) lies on the surface of a cone and
makes the same angle to the xy plane as the incident beam. Once the incidence angle
θ
is set, the possible emergent directions are described by a single parameter, the azimuthal
angle
α
.
applications of the theory of cylindrical light scattering was dealing with spider web fibers [29].
As seen in the past history of this problem, the full vectorial treatment of the scattering by a
cylinder is a rather cumbersome calculation, which requires the solution of Maxwell’s equations
for cylindrical coordinates. This calculation is interesting and important because it is amenable
to the quantitative numerical evaluation of the directional scattering spectra. In the present case,
we only need to provide a short account of this question, relevant to the present study.
The approach is inherently multiscale, considering that the radius of the cylinder is large
compared with the incident radiation wavelength. This is clearly the case here, where the diam-
eter of the seta is of the order of 10
µ
m, while the optical layers found in the cortex are only
about 150 nm thick. This allows to treat the global illumination of the bristle from the point of
view of geometrical optics, while the scattering on the light ray on the cylinder surface is treated
with the prescriptions of wave optics. There, all the details introduced by Maxwell’s equation
on a multilayer optical filter are accounted for. This type of two-scales approach was used, for
instance, when discussing the iridescence of the shield bug Calidea panaethiopica [30] and,
since then, in a number of other occasions.
Figure 6 shows the geometry of the scattering problem. An infinitely long cylinder is set up-
right, along the zaxis. Its surface with air is structured by the presence of a multilayer, while the
volume, below the multilayer, is a homogeneous, strongly absorbing, material. The incidence
plane contains the incidence beam (with wavevector~
kin and the cylindrical fiber’s axis. This
plane defines the xcartesian axis, so that the incident wave vector has no ycomponent.
The output wave vector~
kout is constrained by two invariance prescriptions:
1. The frequency of the incident wave
ω
is conserved, because the medium dielectric re-
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sponse has no time dependance. This means that the norm of the output wavevector,
kout =
ω
/cis identical to that defined by the input wavevector. The end of the output
wave vector~
kout is found on the surface of a sphere of radius
ω
/c, centered on the origin.
2. The zcomponentkzof the wave vector is also conserved because the propagating medium
has full translational invariance in the direction of the cylinder axis. The zcomponent of
the output wave vector must be identical to the zcomponent of the input wave vector.
Following this, the output wave vector must point somewhere on the horizontal plane
(parallel to xy) at z=kz,in.
This plane intersects the sphere of constant frequency along an horizontal circle centered on
the cylinder axis. This means that, when a beam of light strikes a cylindrical fiber from any
incidence, the scattered light escapes along the surface of a cone with the same axis as the
cylinder. The angle from the fibre axis to~
kout is the same as the angle from the fibre axis to~
kin.
This statistical redirection of an incident beam about the axis of the fiber, in the cone described
here, was recognized, some years ago, as a new mechanism for guiding waves in biological
fibrous materials, such as dentin [31].
This result – namely the distribution of the scattered waves on the periphery of a cone of
known opening angle – results only from exact conservation laws, so that it is valid for any ap-
plicable fiber diameter and light wavelength. It is a robust result in the context of wave optics,
as well as ray-tracing optics. For fiber radii significantly larger than the incident light wave-
length, there is an instructive interpretation of this result (from ray-tracing optics) which helps
with understanding the expected optical behavior of the fiber.
The incident wave vector has cartesian coordinates
~
kin =
ω
c(cos
θ
,0,sin
θ
)(3)
where the angle
θ
is the angle between the incident beam and the xy plane, at a right angle to
the fiber axis (see again Fig. 6).
θ
is also the angle between any outgoing wave vector~
kout and
Fig. 7. The projection of the elements of Fig. 6 in the xyplane, showing the direction of
the vector ~
Nbisecting the angle between ~
kin and~
kout. This shows that the scattering by
the fiber, under the invariance conditions which conserve the norm and the zcomponent of
the wave vector, can be seen as a reflection on the fiber surface, from the point where this
surface meets the normal ~
N.
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the xyplane. There is only one parameter which fixes the direction of the outgoing wave~
kout,
namely its azimuthal angle
α
, measured from the xaxis:
~
kout =
ω
c(cos
θ
cos
α
,cos
θ
sin
α
,sin
θ
)(4)
The whole circle is described for 0
α
<2
π
. In particular,
α
=0 corresponds to ~
kout =~
kin
and a “straight through” ray trajectory. The “longitudinal” reflection on the fiber axis, in the
incident xz plane, gives an outgoing wave vector with
α
=
π
.
The angle 2
δ
between the incident beam and the emergent beam can be calculated from the
scalar product
cos2
δ
=(~
kin)·~
kout
(
ω
c)2.(5)
This leads to the following value of half the angle between the “in” and “out” beams,
cos
δ
=cos
θ
sin
α
2(6)
Now, the vectors~
kin and~
kout, together, define a plane that does not (in general) contain the
fiber axis. If we look for the vector that bisects the angle between (~
kin)and~
kout, we find the
vector ~
Nwhich lies in the xy plane:
~
N= (sin
α
2,cos
α
2,0)(7)
This direction is visualized in Fig. 7, which shows the relevant vectors projected onto the xy
plane. The vector ~
Nis always normal to the fiber axis (so in the xy plane). It is constructed
as the perpendicular to the fiber radius at an angle
α
/2 from the xaxis. This normal is in
the plane defined by the “in” and “out” beams and these beams both make the same angle
δ
with this normal. The scattering of the incident beam on the fiber, from~
kin to~
kout, can then be
interpreted, in three dimensions, as a reflection of a light ray which hits the cylinder surface at
the intersection point with the direction of the normal ~
N.
This result allows to determine the color scattered along the cone surface, at the azimuth
α
,
when the cylinder surface is covered by a cylindrical Bragg mirror. The angle of incidence
δ
for the reflection is given as a function of the azimuth by Eq. (6) and the dominant wavelength
in this direction is approximately (i.e. under the assumption – as justified above – of a “local”
infinite planar multilayer [24,32]
λ
=2aq¯
ε
1+sin2(
α
2)cos2
θ
m(8)
Figure 8 shows the iridescence produced by the absorbing Bragg mirror described above,
curved to form a cylinder and evaluated according to Eq. (8). We note that the curves indicate
an iridescence that locate the dominant wavelength between blue (450 nm) and near-ultraviolet
(342 nm). The maximal iridescence richness [33] is obtained when
α
=180, a geometry for a
longitudinal reflection on the fiber surface, when the incident beam, the reflected beam and the
fiber axis are all in the same plane. The other extreme case,
α
=0, corresponds to a straight,
tangential, transit of the light, in the xz plane, without any direction change. In this situation, the
local incidence angle on the surface approaches 90, with an impact parameter (distance to the
fiber’s axis) equal to the fiber radius, so that the reflected wavelength experiences a maximal
shift to the ultraviolet. This grazing reflection occurs for any value of the incidence on the fiber
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Fig. 8. Dominant fundamental scattered wavelength, for various values of the incidence an-
gle
θ
, measured from the xy plane, normal to the fiber, and for various azimuthal directions
α
of the scattered beam in the outgoing wave cone. The iridescence (change of color with
viewing direction is maximal for a full reflection in the longitudinal direction of the fiber
(
α
=180) and minimal for a tangential, grazing transit (
α
=0). For parameters matching
those of the multilayer found on Pamphobeteus antinous, the scattered wavelengths span a
blue-to-ultraviolet range.
Fig. 9. Polar plot of the cylindrical Bragg mirror iridescence. The outgoing wave direction
is determined by the angle
θ
to the xy plane (the same as for the incident wave) and the
azimuthal angle
α
. The black area corresponds to the direction where, for humans, no col-
oration occurs, because the scattered wavelength is ultraviolet, below 380 nm. The lighter
shades area indicate visible light, blue in the central region and violet near the border. The
plot is based on Eq. (8).
(the angle
θ
), so that the reflected wavelength is constant, the spectral richness vanishes, and
the wavelengths become degenerated.
Another important remark relating to the result brought by this model is that the human
access to colors is restricted to wavelengths largerthan 380 nm, so that the range of visibility is
limited, for both angles
θ
and
α
. Figure 9 essentially provides the same information as Fig. 8
and Eq. (8), but indicates (in black) (
α
,
θ
) areas of the polar plot where the emission is invisible
to humans, because it is shifted into the ultraviolet range.
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6. Spectrophotometric measurements
The above theoretical considerations suggest optical experiments. Spectrophotometric hemi-
spheric measurements were first performed with a double-beam spectrophotometer Perkin
Elmer Lambda 750S. This apparatus is equipped with an integrating sphere and pre-aligned
tungsten-halogen and deuterium sources, covering the UV-visible and the near-infrared spec-
tral regions. The measured reflectance is normalized to the diffuse reflection of a polytetraflu-
oroethylene calibrated standard. For this experiment, the blue side of a whole femur from the
spider’s leg was exposed to a parallel light beam under near-normal incidence. The spot was a
rectangle 1 cm high (along the femur long dimension) and 0.5 cm wide (within the width of the
femur). The measurement included diffuse scattering from all kinds of bristles on the probed
surface, as well as from the flat integument that supports these bristles.
Figure 10 shows the result of this hemispheric reflectance measurement for an incident beam
normal to the cuticle. This reflection shows a broad enhancement close to 430 nm, with a
bandwidth approximately equal to 100 nm (FWHM). This band induces a perception of purplish
blue, not so far from the end of the human visible spectrum. The reflectance also increases
towards the red. This broad reddish contribution can also be seen on non-iridescent parts of
the spider’s body, so that it is interpreted as originating from the scattering properties of dark
pigments. Many of the pigments on the spider’s cuticle, indeed, tend to absorb short-wavelength
radiation and to scatter long wavelengths, in the red and infrared.
An Avaspec 2048/2 fiber-optic spectrometer was used for fixed-angle specular measure-
ments. The measurement chain was equipped with a combined equilibrated halogen-deuterium
source covering 250 nm - 1100 nm, slightly exceeding the human visible spectral range. In
these measurements, the intensity is also systematically compared with the intensity of light
scattered by a standard, diffusive white, polytetrafluoroethylene reference surface, under iden-
tical angular configurations. With this normalization, the reflected intensity is usually referred
to as a “reflection factor,” expressed in %. This quantity is not bound to be less than 100%. The
optic fiber that brings the illumination in, and the fiber that takes the reflected signal out, are
equipped with ferules 1.5 mm in diameter, so that the measurement takes place in a circular
area of 2 mm, again incoherently collecting the light from several blue bristles (approximately
parallel to each other) and from the flat integument, deeper.
The structural origin of the blue coloration leads to the phenomenon of iridescence, which
Fig. 10. Measured hemispheric reflectance spectrum of a blue area on the spider’s cuticle,
under near normal incidence. The distribution of reflected wavelengths peak near 430 nm,
due to iridescence. This spectrum integrates contributions from all outgoing directions.
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Fig. 11. Iridescence of the “blue” setae of the spider Pamphobeteus antinous. The wave-
length selected by the cylindrical Bragg mirror experience a blue shift when the incidence
angle (measured from the normal to the fiber) is increased. The spectra reported are ob-
tained in a longitudinal specular reflection geometry
α
=180. Solid line:
θ
=15; dashed:
θ
=30; dot-dash:
θ
=45; dot-dot-dot-dash:
θ
=60; dotted:
θ
=75.
– here – depends on both the incidence (
θ
) and the outgoing azimuthal angles (
α
). Figure 11
shows the spectra measured for the special direction
α
=180, for which the iridescence rich-
ness is maximal.
These measurements confirm that the dominant reflected wavelength shifts to shorter wave-
lengths as the incidence angle is increased. The color changes from purplish blue to deep violet
and ultraviolet. However, the intensity and saturation of the reflected light fades away as the
wavelength approaches ultraviolet. Since a multilayer response would not explain this fast de-
crease in intensity, we suggest that short-wavelength absorption takes place in the chitinous
materials that constitutes the optical structure. In the present case, the black region in Fig. 9,
due to a shift to invisible radiation, is also a region of low scattering intensity. The limitation
of the reflection does not imply a correlated absence of visual sensitivity: spiders do perceive
ultraviolet radiation [34].
The spectral bandwidth of the spectra shown in Fig. 11 is much larger than could be ex-
pected from the gap width implied by the relatively weak refractive index differences that are
characteristic of natural structures. The reason for the broadening could have been disorder, but
this turns out not to be the case: the axes of the setae fluctuate about the long direction of the
spider’s femur, but not by much more than about 10and optical microscopy inspection shows
that most of the setae present their cylindrical side outwards, so that the hidden irregularities
do not influence the optical response very significantly. The coloring function of the cylindrical
Bragg mirrors remains rather robust in presence of this kind of disorder. In fact, the origin of
the broadening can be better understood by considering the very limited number of layers and
the strong absorption by the chitinous compound that constitutes the bristle.
A spectrum calculation, based on the above model – a “local” multilayer consisting of a
220 nm black chitin cover layer supporting four bilayers (46 nm air and 100 nm black chitin)
on a thick dark chitin substrate and an incidence angle
δ
given by Eq. (6) as a function of
θ
and
α
– confirms the location and blue shift of the “purplish blue” iridescence and the origin of the
broadening. The spectra in Fig. 12, computed for
α
=180 (this corresponds to the measure-
ments setup for Fig. 11) are presented for the same five values of the incidence angle
θ
. The
bandwidth and the iridescence of the calculated coloration band are consistent with the values
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Fig. 12. Calculation of the unpolarized reflectance spectrum of a perfect multilayer mirror,
avoiding all effects of disorder, for a longitudinal specular reflection geometry, under con-
ditions similar to those in Fig. 11. Solid line:
θ
=15; dashed:
θ
=30; dot-dash:
θ
=45;
dot-dot-dot-dash:
θ
=60; dotted:
θ
=75. The structure is assumed flat, with a 220 nm
black chitin cover layer above four bilayers (46 nm air and 100 nm black chitin) and a thick
black chitin substrate. Black chitin is absorbent and dispersive [19]. Note that the calcu-
lated reflectivity is normalized by the incident intensity, while the reflection factor in Fig.
11 is normalized by the diffuse reflection from a white PTFE standard.
observed in the measured reflection factor. This suggests that the consideration of disordered
effects should not add much to the spectral width, due to the limited thickness and due to the
role of its contents in absorbing melanin. Note, however some discrepancies exist: the rise in
the reflectance for long wavelength and the weakening of the constant background reflection at
large angles are not reproduced satisfactorily by the model. These features may be controlled
by unidentified pigments that are not accounted for in the simulations.
7. Conclusion
The setae which cover the dorsal side of the femurs and other parts of the cuticle of a mature
male tarantula Pamphobeteus antinous are colored by a cylindrical Bragg mirror.
In nature, a few bristles and other kind of fibers have been observed to be brightly col-
ored. Certain Polychaete worms, for instance [35,36], have evolved bristles structured as two-
dimensional photonic crystals. The marine worm Aphrodita aculeata has rigid spines and flex-
ible bristles shaped as hollow tubes, with walls containing a two-dimensional triangular array
of straight, parallel, microchannels. These bristles are produced by the secretion of chitin and
protein from the base of cylindrical “microvilli”; the microvilli then leave the empty microchan-
nels, as new material is extruded from their sides. The way the structure shown in the present
study is produced may be quite different, as the bristles show many similarities with standard
arthropod scales, some of which assume the shape of cylindrical rods [37, 38]. The highly
structured part of the setae is, indeed, strongly reminiscent of a standard ridge-crossrib struc-
ture. An examination of how complex structures can vary between close species can be found
in Ghiradella and Butler [39] and a review of the formation of this type of complex structural
appendages can be found in Ghiradella [7]. Ideas on how living organisms actually produce
these complex photonic structures are very important, as the structural coloration of fibers may
avoid many of the difficulties encountered in the deposition of stain on textile fibers.
Meanwhile, from a biomimetic point of view, we can see potential for artificial ways of
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obtaining fibers coated with a Bragg mirror that produces a structural coloration. Efforts have
already been made towards that end, mostly for planar surfaces [14, 40,41] or more elaborate
hybrid structures [42].
Designing structurally colored fibers is a very active field, as fibers with coloration are
useful in fabrics for clothes, papers, paints, etc... Structurally colored fibers using polymers
(polyethyleneterephtalate with refractive index 1.63 and nylon-6 with refractive index 1.53)
have been described by Hiroshi Tabata [43], who also consider some of their potential appli-
cations. The type of fibers described there are shaped as a long and narrow strip: a strongly
anisotropic geometry. The cylindrical invariance discussed here may present advantages when
considering knitting fibers to produce fabrics with large surfaces. Closer, but still not identical
to the structure inspired by Pamphobeteusantinous is the recent demonstration, by Finlayson et
al. [44] of an extruded polymeric opal fiber that is knittable and shows strong structural colors,
that are tunable by applying mechanical tension.
Structurally colored fibres owe their importance, partly, to difficulties with the use of pig-
ments. Although pigments have been used for many centuries to dye fabrics, this is a partic-
ularly polluting process because of the requirement for preconditioning the fibres and their
limited capacity for absorbing dyes. This has long been known, for instance for vegetable dyes
on wool or, more recently, for synthetic dyes on synthetic fabrics. Using structural coloration
on textile materials – as in the colored fibers used by the tarantula Pamphobeteus antinous
may open an important way of developing textile colors, with the supplementary advantage of
offering a wide range of new visual effects, due to iridescence and a tunable level of disorder.
Acknowledgments
P. S. acknowledges the Wallonia-Brussels Federation for financial support. A. B. was supported
as a PhD student by the Belgian Fund for Industrial and Agricultural Research (FRIA). J.-F. C.
is supported by the F.R.S.-FNRS (BELGIUM) as Research Associate. The project was partly
funded by the Action de Recherche Concert´ee (ARC) Grant No. 10/15-033 from the French
Community of Belgium. The authors also acknowledge using resources from the Interuniversity
Scientific Computing Facility located at the University of Namur, Belgium, which is supported
by the F.R.S.-FNRS under convention No. 2.4617.07.
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... The iconic "Christmas tree" structures from the blue Morpho butterflies ( Fig. 1.3F) can be considered as some derived varieties of multilayer structures too (108). Multilayer structures have been described in many species of spiders ( Fig. 1.2B&C) (69,70,109), and once thought to be the only kind of color production mechanisms in tarantulas (110) before the amorphous spongy structures were found (102). A unique color production structure involving multilayers was recently described in the blue iridescent setae on the opisthosoma of peacock spider Maratus splendens (111). ...
... Thus, distinguishing between the effects of sexual 5.2). Although previous work revealed that these colors are structural in origin and likely produced via multilayer interference effects (109,110), the evolutionary diversification of both the nanostructures and the colors they produced has not been addressed. By using an integrative approach, combining reflectance spectroscopy, electron microscopy and theoretical modeling, we here examine the evolution of coloration in the absence of intraspecific visual color signaling using tarantulas as a model system. ...
... The nanostructures were composed of two alternating materials with high and low electron densities in TEM micrographs (Fig. 5.4 and Fig. 5.5). According to previous reports, the high-electron density material is a chitin-protein composite and the low density material is air (109,110). We verified this by a refractive index (n r ) matching test, in which color disappeared when hairs were submerged in quinoline liquid (n r = 1.63) (Fig. 5.6). ...
Thesis
Understanding the color production mechanisms is important to the advancement of understanding color evolution, ecology, adaptation, and functions. Therefore, investigating how spiders produce colors is a critical piece of the puzzle to fully understand spider biology. In this dissertation, I investigated how spiders produce colors through pigments and biogenic photonic structures using varies techniques. We discovered the presence of eumelanin and carotenoids in spiders, which are both common in nature but were previously thought to be absent in spiders. I also discovered melanosomes – a melanin-containing cellular organelle previously assumed to be a synapomorphy for vertebrates – in spiders. Pigments aside, I described many novel and unique biogenic photonic structures in spiders. The blue color for many tarantulas is produced via specialized setae with diverse photonic structures within. A phylogenic analysis on the blue traits iii showed that despite being a very specific (narrow band) blue color, the blue traits evolved independently at least 8 times. In other words, diverse photonic structures evolved convergently to produce a very specific blue color in tarantulas, suggesting an important visual function for yet to be determined receivers. Among these structures, a flower-shaped multilayer structure is of particular interests due to its ability to attenuate iridescence. This particular structure may inspire the design and fabrication of vibrant, durable colorants in the future. On the other hand, two particular species of peacock spiders showcase extremely angle sensitive iridescence, which produces all the colors within the human visible spectrum with the slightest movement. We determined that this rainbow-iridescent optical effect was produced by unique airfoil-shaped setae with surface nanogratings. These setae also possess an unusual high wavelength resolving capability that may contribute to the design and fabrication of miniature optical components to further advance human light-based technologies.
... While sensitivity to different wavelengths doesn't necessarily mean that tarantulas can actually discriminate between them, by itself it offers no reason to rule out colour perception a priori. If they can indeed perceive colours, then those colours could plausibly function in sexual signalling, and Simonis et al. (2013) noted that iridescent blues and violets were only observed on the males of Pamphobeteus antinousa phenomenon which has been documented in several tarantula subfamilies (Schmidt, 2003;Schultz & Schultz, 2009;Teyssié, 2015). This led them to consider that such colours might function in sexual signalling. ...
... However, they overlook how males of several species (e.g. Pamphobeteus, as noted in Simonis et al., 2013) gain iridescent sheens upon maturity, which lends support to a sexual function. It must be noted, however, that there is tremendous variation in how animals perceive colours (Osorio & Vorobyev, 2008), and we almost certainly do not perceive colours in the same way as tarantulas. ...
... Pamphobeteus antinous male with vivid iridescent blue bristle ornamentation on the legs and pedipalps, Simonis et al. (2013) proposed that such colouration might play a role in sexual signalling. ...
Thesis
Theraphosidae, commonly known as tarantulas, represent some of the most charismatic spiders on the planet. With almost 1000 described species, they have colonized the subtropics of every continent and have adapted to fill many of ecological niches. I address gaps in our understanding of the tarantula phylogeny with a view towards understanding the evolutionary patterns involved in generating the great diversity of tarantulas we see today. My thesis is hence focused into three main parts: (i) constructing the first robust subfamily-level phylogeny for tarantulas, (ii) time-calibrating this phylogeny to offer insights into their widespread distributions in tandem with biogeographic data, and (iii) estimating the ancestral histories of greenness, blueness, and other life history traits, with correlative tests to determine the functions of colouration in theraphosids.
... [30] The nano-multilayer structure is composed of a high-refractive-index chitin-protein layer and an air layer alternately. [31] The protrusion structure largely destroys the long-range order of the color-generating unit, giving spiders an angle-independent color. [32] Referring to the spider's control scheme of its color, we can introduce wrinkles into flexible photonic material to solve the problem of angle correlation. ...
Article
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Color is an excellent carrier of information between organisms. Photonic elastomers that convert abstract mechanical information into intuitive color information have shown great potential in various application fields. However, angular correlation of structural colors and monotonous color conversion mode affect the accuracy of material information reading and the applicability in diverse application scenarios. Here, inspired by a kind of bright blue spider‐ Poecilotheria metallica, a new wrinkled photonic elastomer structure is reported. Through wrinkling stretchable 1D photonic crystals (1DPCs), photonic elastomers with both one‐directional and omni‐directional angle‐independent brilliant structural colors are realized. It is worth noting that with the hybrid 1DPC and polydimethylsiloxane substrate support, the wrinkled photonic structure and structural color remain stable after 1000 strain cycles, and mechanochromic sensitivity can reach 3.25 nm/%, indicating strong structural stability and sensitive deformation performance under stress. More importantly, by comprehensively manipulating micro‐wrinkle structure and lattice spacing of the photonic films, the structural color can achieve delayed discoloration and invisible‐visible reversible switching performance only through a single strain direction. The proposed wrinkled photonic elastomers have a broad application value in the fields of visual strain sensing, wearable devices, and information encryption. Furthermore, it provides a new strategy for color regulation of photonic materials.
... They produce structural colours by a variety of optical mechanisms. These include multi-layered cuticular structures [79,80], diffraction gratings [81][82][83], cylindrical Bragg mirrors [84], and combinations thereof. The result is iridescent and reflective structures with a wide spectral range including ultraviolet, blues and greens and even yellow, which is otherwise typically achieved by pigmentation [81]. ...
Article
Engineered systems are typically based on a large variety of materials differing in composition and processing to provide the desired functionality. Nature, however, has evolved materials that are used for a wide range of functional challenges with minimal compositional changes. The exoskeletal cuticle of spiders, as well as of other arthropods such as insects and crustaceans, is based on a combination of chitin, protein, water and small amounts of organic cross-linkers or minerals. Spiders use it to obtain mechanical support structures and lever systems for locomotion, protection from adverse environmental influences, tools for piercing, cutting and interlocking, auxiliary structures for the transmission and filtering of sensory information, structural colours, transparent lenses for light manipulation and more. This paper illustrates the ‘design space’ of a single type of composite with varying internal architecture and its remarkable capability to serve a diversity of functions. This article is part of the theme issue ‘Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 1)’.
... Haplopelma, Xenesthis [2,19], Psalmopoeus [20], Aphonopelma [21] and many Aviculariinae [22]). Penultimate males frequently display vivid metallic colorations, and having examined a mature Pamphobeteus antinous male with vivid iridescent blue bristle ornamentation on the legs and pedipalps, Simonis et al. [23] proposed that such coloration might play a role in sexual signalling. These observations taken together suggest it is premature to preclude a role for sexual selection in the origin and evolution of blue tarantula coloration. ...
Article
Tarantulas paradoxically exhibit a diverse palette of vivid coloration despite their crepuscular to nocturnal habits. The evolutionary origin and maintenance of these colours remains mysterious. In this study, we reconstructed the ancestral states of both blue and green coloration in tarantula setae, and tested how these colours correlate with presence of stridulation, urtication and arboreality. Green coloration has probably evolved at least eight times, and blue coloration is probably an ancestral condition that appears to be lost more frequently than gained. While our results indicate that neither colour correlates with the presence of stridulation or urtication, the evolution of green coloration appears to depend upon the presence of arboreality, suggesting that it ptobably originated for and functions in crypsis through substrate matching among leaves. We also constructed a network of opsin homologues across tarantula transcriptomes. Despite their crepuscular tendencies, tarantulas express a considerable diversity of opsin genes—a finding that contradicts current consensus that tarantulas have poor colour vision on the basis of low opsin diversity. Overall, our findings raise the possibility that blue coloration could have ultimately evolved via sexual selection and perhaps proximately be used in mate choice or predation avoidance due to possible sex differences in mate-searching.
... Spiders employ a rich variety of structural coloration mechanisms, ranging from common multilayered structures, 10,11,13 to coaxial Bragg mirrors, 18,19 to the nanogratings observed in the peacock spiders as well as in other spiders. 20 The investigated peacock spiders feature a common coloration motif in the form of an ultra-dense diffraction grating (Fig. 2). ...
Article
Full-text available
Controlling light through photonic nanostructures is important for everyday optical components, from spectrometers to data storage and readout. In nature, nanostructured materials produce wavelength-dependent colors that are key for visual communication across animals. Here, we investigate two Australian peacock spiders, which court females in complex dances with either iridescent color patterns (Maratus robinsoni) or an approximately angle-independent blue coloration (M. nigromaculatus). Using light microscopy, FIB-SEM imaging, imaging scatterometry, and optical modeling, we show that both color displays originate from nanogratings on structured 3D surfaces. The difference in angle-dependency of the coloration results from a combination of the local scale shape and the nanograting period. The iridescence of M. robinsoni arises from ordered gratings on locally flat substrates, while the more stable blue colors of M. nigromaculatus originate from ultra-dense, curved gratings with multiscale disorder. Our results shed light on the design principle of the peacock spiders’ scales and could inspire novel dispersive components, e.g. used in spectroscopic applications.
... Iridescence has been proposed to serve many important visual functions (e.g., conspecific recognition, mate choice, crypsis, aposematisms) and non-coloration functions (e.g., thermoregulation, photoprotection, mechanical strengthening, friction reduction, water repellency) (Doucet & Meadows 2009), but may also be a non-adaptive byproduct (Doucet & Meadows 2009;Seago et al. 2009;Barthelat 2010;van der Kooi et al. 2014). Multilayer structures occur in many species of spiders (Fig. 2B, C) Ingram et al. 2009Ingram et al. , 2011Simonis et al. 2013), and were once thought to be the only structural color production mechanism in tarantulas (Foelix et al. 2013) before amorphous spongy structures were found (Hsiung et al. 2015b). ...
Article
Full-text available
Spiders were once thought to have limited color production palettes, and even to lack melanin-one of the most ubiquitous biological pigments. Recent discoveries upend that view and show that the color production mechanisms in spiders are as elaborate as some of the more classically colorful groups of animals, such as birds, butterflies, and beetles. Here we summarize how colors are produced by spiders, identify gaps in our knowledge, show how researchers investigating color in different groups of animals can learn from each other, and suggest future opportunities for spider color research. Our understanding of color production mechanisms in other colorful groups of animals can be used as guidelines for discovering existing mechanisms previously unknown in spiders and vice versa. For example, spider species with colors potentially produced by the same kind of photonic structures previously described in white beetles, and in the blue/green scales of fishes and lizards are suggested here. In addition, novel principles first found in spiders that modify the iridescence of structural colors via the interaction of structural features across different length scales (i.e., micro-nano) may also be found in other colorful groups in the future. This review summarizes the state-of-the-art understanding regarding the proximate color production mechanisms in spiders, suggests a few future research directions that are likely to be fruitful, and facilitates the advancements in related fields, including the ecology, evolution, and functions of spider coloration.
... C ontrolling light through photonic micro-and nanostructures can transform human technology, including communications, sensing, security, and computing [1][2][3] . Biogenic photonic nanostructures have high translational potential 4 and reveal a diverse array of structural colour production mechanisms in plants and animals, including spiders [5][6][7][8][9][10][11][12] . In particular, some Australian peacock spiders can display extremely angle-dependent full-spectrum iridescence with high purity 13 . ...
Article
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Colour produced by wavelength-dependent light scattering is a key component of visual communication in nature and acts particularly strongly in visual signalling by structurally-coloured animals during courtship. Two miniature peacock spiders (Maratus robinsoni and M. chrysomelas) court females using tiny structured scales (~ 40 × 10 μm2) that reflect the full visual spectrum. Using TEM and optical modelling, we show that the spiders’ scales have 2D nanogratings on microscale 3D convex surfaces with at least twice the resolving power of a conventional 2D diffraction grating of the same period. Whereas the long optical path lengths required for light-dispersive components to resolve individual wavelengths constrain current spectrometers to bulky sizes, our nano-3D printed prototypes demonstrate that the design principle of the peacock spiders’ scales could inspire novel, miniature light-dispersive components.
... Pigmentary colours are independent of viewing angle (non-iridescent), while structural colours are usually angle dependent (iridescent). The nanomorphology of structural colours has been investigated recently for some spiders (Foelix et al., 2013;Hsiung et al., 2015b;Ingram et al., 2011;Land et al., 2007;Simonis et al., 2013;Stavenga et al., 2016). However, our knowledge of spider pigment biochemistry has remained stagnant for almost 30 years (Holl, 1987, but see Hsiung et al., 2015a). ...
Article
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Elucidating the mechanisms of colour production in organisms is important for understanding how selection acts upon a variety of behaviours. Spiders provide many spectacular examples of colours used in courtship, predation, defence and thermoregulation, but are thought to lack many types of pigments common in other animals. Ommochromes, bilins and eumelanin have been identified in spiders, but not carotenoids or melanosomes. Here, we combined optical microscopy, refractive index matching, confocal Raman microspectroscopy and electron microscopy to investigate the basis of several types of colourful patches in spiders. We obtained four major results. First, we show that spiders use carotenoids to produce yellow, suggesting that such colours may be used for condition-dependent courtship signalling. Second, we established the Raman signature spectrum for ommochromes, facilitating the identification of ommochromes in a variety of organisms in the future. Third, we describe a potential new pigmentary–structural colour interaction that is unusual because of the use of long wavelength structural colour in combination with a slightly shorter wavelength pigment in the production of red. Finally, we present the first evidence for the presence of melanosomes in arthropods, using both scanning and transmission electron microscopy, overturning the assumption that melanosomes are a synapomorphy of vertebrates. Our research shows that spiders have a much richer colour production palette than previously thought, and this has implications for colour diversification and function in spiders and other arthropods.
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Brilliant iridescent colouring in male butterflies enables long-range conspecific communication and it has long been accepted that microstructures, rather than pigments, are responsible for this coloration. Few studies, however, explicitly relate the intra-scale microstructures to overall butterfly visibility both in terms of reflected and transmitted intensities and viewing angles. Using a focused-laser technique, we investigated the absolute reflectivity and transmissivity associated with the single-scale microstructures of two species of Morpho butterfly and the mechanisms behind their remarkable: wide-angle visibility Measurements indicate that certain Morpho microstructures reflect up to 75% of the incident blue light over an angle range of greater than 100 degrees in one plane and 15 degrees in the other. We show that incorporation of a second layer of more transparent scales, above a layer of highly iridescent scales, leads to very strong diffraction, and we suggest this effect acts to increase further the angle range over which incident light is reflected. Measurements using index-matching techniques yield the complex refractive index of the cuticle material comprising the single-scale microstructure to be n = (1.56 +/- 0.01) + (0.06 +/- 0.01)i. This figure is required for theoretical modelling of such microstructure systems.
Article
Colouration arises when (1) a white-light illumination beam is spectrally filtered by transmission through an optically responsive material and (2) a vision system with specific discrimination capability has analysed the remaining energy. The resulting optical spectrum can either be caused by absorption (the so-called ‘pigmentary’ colouration mechanism) or by light-wave interferences (the so-called ‘structural’ or ‘physical’ colouration mechanism). In this chapter, we will focus on the latter. The structural colouration effects arising from the presence of overlayers and gratings—in general assembled from chitin-based elements—are exposed and illustrated by typical examples arisen in animal colour displays. Photonic-crystal films, with or without long-range disorder will also be described, along with examples of appropriate living organisms that have evolved these structures.
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The arthropod integument is noted for its surface outgrowths, especially the microchaetes (hairs) and macrochaetes (bristles and scales). These start as hollow projections templated on cellular microvilli or on filopodia (microchaetes), or on larger projections from their parent epidermal cells (macrochaetes).Ultrastructurally, finished microchaetes are relatively simple, but macrochaetes are typically highly ornamented, some so much so that their surface architecture can produce structural colours. Similarly, macrochaete interiors may be filled with lattices or laminae that may also produce structural colours.Many of the patterns appear to be variations on relatively few themes, and control of this pattern formation by the epidermal cell appears to reside in particular with two cellular systems, the actin cytoskeleton and the smooth endoplasmic reticulum. This suggests that focused study of these two systems may yield real insight into pattern formation in general, especially in the arthropods and in other armoured organisms.
Article
Segestria florentina (Rossi 1790) (Segestriidae) displays iridescent green coloration on the paturons of the chelicerae. This was confirmed by reflectance measurements, which gave a spectral peak at 505 nm. Scanning electron microscopy did not identify cuticular surface scales or sculpturing, suggesting that the cause of the iridescence was subsurface. Transmission electron microscopy revealed 86 alternate dark and light layers in the exocuticle, the mean dimensions of which were 126 nm ± 28 nm and 88 nm ± 55 nm respectively. The identity of each layer was initially unclear. However, by using a combination of materials with different refractive indices in calculations of theoretical reflectance spectra, we concluded that they were most likely to be composed of chitin and air, since a peak of 480 nm was obtained, which most closely matched that which was recorded. The function of the green color is not clear, since S. florentina has relatively poor vision and relies predominantly on vibratory and acoustic signals. The study provides useful information relevant to research into the evolution of structural colors in spiders and, more generally, in nature.
Article
The most intense colours displayed in nature result from either multilayer reflectors or linear diffraction gratings. Here we investigate the spectacular iridescence of a spine (notoseta) from the sea mouse Aphrodita sp. (Polychaeta: Aphroditidae). The spine normally appears to be deep red in colour, but when light is incident perpendicular to the axis of the spine, different colours are seen as stripes running parallel to the axis of the spine; over a range of smaller incident angles, the complete visible spectrum is reflected with a reflectivity of 100% to the human eye. The simple structure responsible for this effect is a remarkable example of photonic engineering by a living organism.
Article
The scattering problem of obliquely incident waves on a multilayered elliptical cylinder is considered. The cylinder is assumed homogeneous and isotropic but 1ossy. Both polarization of the incident wave, the E wave, and the H wave are considered. In the theoretical analysis the number of layers are unlimited, but in the numerical analysis we limit the number of layers so we can avoid excessively long computer running time. The numerical results include the bistatic radar cross sections, the specular differential cross sections, and the efficiency factors for absorption, scattering, and extinction. The computed results show the dependence of the scattering on the size, shape, material, and the angle of the incident. Rayleigh [ 1881] first used the Maxwell' s equation [Maxwell, 1864] to solve the problem of scattering of an electromagnetic wave normally incident on a circular lossless dielectric cylinder by using the separation of variables method. Van de Hulst [1949] solved the problem of the scattering of a normally incident wave by a circular lossy dielectric cylinder, and Wait [1955] did the scattering of obliquely incident wave by a circular lossless dielectric cylinder theoretically and numerically by using the separation of variables method. For a multilayered circular lossless dielectric cylinder. Evans et al. [1964] gave first the complete solutions of the scattered wave incident normally, and many interesting numerical results were shown [Datta and Som, 1975]. Both used the separation of variables method. Bussey and Richmond [1975] did the same analysis for a lossy layered cylinder. Compared to the circular cylinder, the scattering problem of an elliptical cylinder was solved much later because of its complexity. Morse and Rubenstein [1938] obtained the exact solution and numerical results of the scattering problem by the elliptical conducting cylinder after a table of the Mathieu functions was
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The scattering efficiency and backward component of the scattered flux have been calculated for infinitely long, hollow cylinders with real indices of refraction of 1.5, 2.0, and 2.5; ratios of inner diameter to outer diameter of 0.0, 0.01, 0.10, 0.50, 0.90, and 0.99; and outer circumference to wavelength ratios of 0.12(0.02) 6.00. Values were obtained for the cases of incident radiation parallel polarized and perpendicularly polarized. A generalized computer subroutine has been made available for the calculation of the amplitude functions, as well as the above functions, for any arbitrary values of the parameters.