Download Reflections On Golden Scarabs

Reflections
On Golden Scarabs
Donald B. Thomas, Ainsley Seago, and David C. Robacker
“All that we see or seem, is but a dream
within a dream”
-Edgar Allen Poe
I
n Edgar Allen Poe’s The Gold Bug, the protagonist describes a species of “scarabaeus,” from
South Carolina: “It is of a brilliant gold color
– about the size of a hickory nut – with two jet
black spots near one extremity and another longer
one at the other.”
In Poe’s story the golden beetle is dropped to
the ground through the eyesocket of a human skull.
By digging at the spot where the beetle landed, a
treasure of real gold coins is found. Students of
Poe’s writing suggest that the passage of a golden
beetle through a human brain case is an allegory
for the acquisition of knowledge. Interestingly, such
a beetle, Pelidnota punctata (L.), is found in South
Carolina and may have served as the inspiration
for Poe’s story, though it is more shiny yellow than
gold. Truly metallic gold-colored scarab beetles are
found in the genus Chrysina (Fig. 1), whose very
name is derived from chrysos, the Greek word for
gold. The first society of entomologists, founded in
London in 1745, was called the Aurelian Society
from the Latin word for gold, aureolus. Chrysina
beetles are also known as “jewel” scarabs (Cave
and Hawks 2001), because in addition to the gold
and silver species, many—actually, most—are
emerald green (Fig. 2), with some spectacular species having combinations of green, gold and silver.
They are prized among collectors to the point of
an inordinate fondness, some would say obsession,
known among aficionados as the “green fever.”
Entomologists are truly treasure hunters!
Apart from their extraordinary beauty (and
market value), Chrysina beetles are of scientific
interest as exemplars of the role of color in evolutionary adaptation and radiation. The genus is a
diverse, and thus evolutionarily successful group,
224
Fig. 1. Chrysina aurigans (Rothschild & Jordan) is
normally a shiny gold beetle, but this variant shows
a red blush, perhaps attributable to an irregular
distribution of uric acid in the exocuticle.
Fig. 2. Chrysina gorda Delgado, named for the
Sierra Gorda in Queretaro, Mexico.
American Entomologist • Winter 2007
F
emer
varia
with over one hundred species known (Hawks
2001) and new species still being discovered (Fig.
3). Although they are sometimes thought of as
rare beetles, they are more properly regarded as
restricted in distribution. Essentially, all of the
montane forests from Arizona to Ecuador have
one or more species, with each individual species
typically restricted to a few and sometimes only
one mountain range (Moron 1990). A curiosity is
that among the green species, one not infrequently
encounters an individual color variant, and even
more curiously, these “sports” are almost always
red (Fig. 4). Moreover, some species have distinct
color morphs. Chrysina purulhensis (Monzon &
Warner), originally discovered in the cloud forests
around Purulha, Guatemala, is a lovely lavender
hue. But the population in the Maya Mountains
of Belize has about equal numbers of lavender
and emerald green individuals (Fig. 5), leading us
to wonder if there is some adaptive purpose driving the frequency of these color variants, or if it
is a character that is subject to a higher mutation
rate.
In the eye of the beholder
Like rainbows and mirages, color is an optical
illusion. As appropriately suggested by Poe in his
poem, “A dream within a dream,” color is a figment
of our imagination, or more exactly, the psycho-
Fig. 3. Chrysina alticola (of authors, not Bates), (above) a typical
rald green form and, (below) a red variant. The puzzle is why the
ants are almost always red and seldom (or never) yellow or blue.
American Entomologist • Volume 53, Number 4
Fig. 4. The two color
morphs of Chrysina
purulhensis (Monzon
& Warner) from the
Maya Mountains of
Belize.
logical imagery formed by our brains when sensing
the visible light phenomena detected by our eyes.
Our brains perceive different wavelengths of light
as color. Among mammals, only the tree dwellers—squirrels, tree shrews and primates (including
the naked ape)—perceive trichromatic color. The
retina on the inner surface of the lumen of our
eyes has cone-shaped sensors of three types, which
respectively detect (are stimulated by) wavelengths
of light that correspond to the colors green, blue,
and red. The varying proportion of stimulation
among the respective types of cone induces our
impression of the full spectrum of colors. If all three
are stimulated in equal proportion, we perceive the
color white. If none of the three are stimulated, we
perceive the color black. In the absence of cones,
we would see the world as most mammals or colorblind people do: in shades of gray. Interestingly,
birds have four types of cones: three responding
to the green, red, and blue wavelengths, but also
another that detects wavelengths in the ultraviolet
part of the non-visible spectrum—that is, non-visible to us. Who knows what color the bird brain
images with this cone; perhaps a color that we
can’t imagine! Or it may be that birds see the same
spectrum as we do, but offset such that our blue
is the bird’s green.
Insects also perceive color, using analogs of
our cones, specialized types of visual cells in their
ommatidia (Carlson & Chi 1979). But whereas
humans have trichromatic vision, insects have
tetrachromatic or even pentachromatic vision,
depending on the species (Srinivasarao 1999).
Furthermore, and perhaps because of the size
limitation of the facets in their compound eyes,
insect vision is shifted to the shorter wavelengths
of light, the ultraviolet part of the spectrum. Even
more astounding, their eyes and brains are able
225
Fig. 6. Chrysina xalisteca (Moron), is green when
viewed head-on but exhibits a blue sheen over its high
curvature areas when viewed at an oblique angle.
This is an optical effect known as Tyndall scattering.
It is caused by impurities in the epicuticle and the fact
that blue light (shorter wavelengths) is more easily
scattered by particulates. Note also the contrasting
black legs.
Fig. 5. An undescribed
species with purple legs
from Nayarit in western
Mexico..
to perceive polarized light whereas ours do not
(Wehner 1976). Objects reflect light of different wavelengths
from their surface, producing the optical quality
that we perceive as color. An insect appears green
to us if it reflects light of a wavelength around 550
nm. Most insects are black or brown because of
a pigment called melanin that is embedded in the
integument. Melanin absorbs light, and thus such
insects appear black to us. Native chitin, the basic
material of which insect integument is made, is
colorless. It is as transparent as cellophane, a material chemically related to chitin. If it weren’t for the
scales (modified hairs), a butterfly’s wings would
be transparent, and some butterflies, such as the
ithomiine nymphalids, do have clear wings. Light is
transmitted through the chitin, not reflected or absorbed, and is thus perceived by us as colorless.
The how and why of color
The ability to see color increases our visual acuity—a very handy sensory capability. We can spot
a red apple in a tree of leafy green, for example.
Using color vision, an insect can spot objects like
flowers more easily, but similarly, birds can use
color to spot insects. Clearly, there is an evolutionary advantage in terms of an animal’s capacity to
search the environment for food, mates, and natural enemies aided by color vision. But there is also
an evolutionary disadvantage if an insect reflects
light of a different wavelength from its immediate
environment. Presumably the reason why so many
insects are green or brown is for camouflage; they
can match the color of the leaves they feed on or
the branches they perch on.
Many insects have color pigments embedded
in their integument. Pigments produce color by
selectively reflecting a short range of wavelengths
226
from the incoming sunlight and absorbing the
rest. The commonest green pigment in insects is
insectiverdin, a compound chromo-protein which
includes blue biliverdin and yellow carotene.
Insects acquire carotenes and flavinoids in their
diet by eating plants and metabolizing them into
the pigments which they sequester in their cuticle.
Chrysina beetles never evolved the ability to make
insectiverdin mainly because their larvae do not
feed on the parts of the vegetation where they
could acquire the right components (preferring a
repast of rotten log). The green color in Chrysina
and almost all other beetles is structural color
(Crowson 1981).
The cuticle of insects consists of three layers:
from the outside in, these are the epicuticle, the
exocuticle, and the endocuticle. The epicuticle is
a waxy lipid and cuticulin layer which functions
mainly in water balance, keeping the insect from
dessicating. This layer is typically thin and transpar-
Fig. 7. Schematic representation of a multilayer
reflector, a stack of chitin layers, some with urate
crystals incorporated to enhance reflection.
American Entomologist • Winter 2007
ent. It also has some optical properties that scatter
light, contributing to the shiny appearance of some
beetles (Fig. 6). The endocuticle usually consists of
sclerotized chitin; chitin tanned with polyphenols
and protein residues. The endocuticle is a thick
layer functioning as the structural exoskeleton, but
also contributes some optical properties by acting
as an absorber of residual light transmitted through
the cuticle above. Between the epicuticle and the
endocuticle is the exocuticle, which produces the
structural color in beetles.
In Chrysina the exocuticle is lamellar, consisting
of many stacked layers of chitin. The layers alternate between thick sheets of transparent chitin and
thinner sheets of chitin containing small quantities
of uric acid (Fig. 7). Uric acid is the primary waste
product from protein metabolism in insects and in
its native state has a white color (yes, it’s the white
stuff in bird poop!). The moiety incorporated into
the chitin is probably the urate crystal form. The
presence of the urate crystals increases the reflectivity by a factor of 20-fold (Caveney 1971). All
of these chitin layers transmit light, that is, they
allow most of the direct light to pass through to the
layers below. But the layers with uric acid reflect
some of the angular light back towards the surface.
This arrangement is called a multilayer (or quarterwave stack) reflector. The incident light, that is,
the incoming sunlight, is “white” light consisting
of many wavelengths. As the incident light passes
through the chitin layers, it is refracted: the light
beams arriving at an angle to the cuticle surface
are bent by the selective slowing of the beam as it
passes through one layer to the next. Refraction
is the phenomenon that causes a pencil to appear
bent in a glass of water. It is the ray of light that is
bent, not the pencil.
A consequence of refraction is that the incident
light also changes its color due to dispersion. An
everyday example is the color of the sun. The sun
appears white when it is directly overhead and
refraction is minimal. But later in the afternoon,
the sun appears yellow because the light now slows
as it passes through the atmosphere at an angle
relative to the interface, dispersing the shorter
wavelengths at a higher angle and leaving the
longer wavelengths of the light to reach our eyes.
This stimulates the red receptors in our eyes more
and the blue receptors less, causing our brains to
perceive the light as yellow.
The layers in the beetle cuticle work much the
same way. In this case, natural selection has favored
an optical thickness of the layers such that the reflected light is in the green wavelength. In order to
reflect bright green light (constructive interference),
the optical thickness of each chitin layer must be
close to 150 nm, the wavelength divided by four,
hence the term “quarter-wave.” Optical thickness is
a combination of the physical thickness of the layer
and the refractive index of its constituent material.
Native transparent chitin has a refractive index
about the same as window glass. By incorporating
uric acid crystals into alternating chitin layers, the
refractive index increases, and thus these layers
American Entomologist • Volume 53, Number 4
have to be thinner so that their optical thickness
is the same as the layers without the uric acid. By
having all of the layers the same optical thickness,
all of the layers reflect the same color, which in
this case is green. The structural green of Chrysina
beetles is much more intense than the pigment green
of insects like grasshoppers because in pigmented
insects, only a small portion of the total incoming
light is reflected, but because of the constructive
interference at each layer in the beetle cuticle, most
of the incoming light is reflected (Fig. 8).
Some Chrysina beetles are metallic silver
in color. It takes only a simple morphological
modification to produce this optical effect. A
slight change in the thickness of the chitin layers
would correspondingly increase or decrease the
wavelength of the reflected light, so if the layers
were thicker, the reflected light would be shifted
toward the red end of the spectrum, or if thinner,
the color would be shifted to the blue end of the
spectrum. This perhaps explains the red color variant among the green species, but leaves us without
a clear explanation of why blue or yellow variants
are seldom found.
In silver beetles, instead of having all of the
layers of the same thickness, the layers successively
decrease in thickness proceeding from outside in
towards the endocuticle. This is called a “chirped”
reflector. Instead of reflecting only one color, the
cuticle can now reflect the full array of colors. This
would produce a white color if the reflected light
were scattered (certain weevils produce the most
optically brilliant whiteness known, according to
Vukusic et al. 2007). But because the reflected light
is directional and coherent and not scattered, an
effect called “specular” reflection is produced. The
result is a mirror finish, like the reflection of light
off the surface of a pond. Just as one can see the
sky in a reflecting pool of water, one can see one’s
Fig. 8. In a
quarter wave
stack, constructive
interference at each
layer reflects light of
a wavelength four
times the optical
thickness. Structural
colors are more
intense because a
greater proportion
of the incident
light is reflected
compared to the
light reflected by
pigments.
227
Fig. 9. Chrysina
chrysargyrea
(Salle) has a
metallic silver
color because of
a regular “chirped”
stack reflector, as
opposed to the
irregular, chaotic
stack in the scales
of silver fish
(vertebrates, not
thysanura).
own face in the elytra of a beetle like Chrysina
chrysargyrea (Salle) (Fig. 9), and that is why we
are careful not to be naked when photographing
silver Chrysina beetles. Another simple modification produces gold instead of silver. If the thinner
chitin layers are absent, there is no reflection of
the blue light component, and just as in the case
with the sun low on the horizon, the spectral shift
in the reflected light is towards the yellow. As a
consequence, instead of a silver beetle, the specular
reflection is golden-hued (Parker et al. 1998).
Why shine?
While it is understandable that a beetle would
have an evolutionary advantage to being green,
why would a beetle be metallic silver or gold? Hinton (1973) suggested that the bright glare from the
surface might temporarily blind a potential predator, enabling the beetle to escape. Alternatively,
it is thought that a mirror finish enables a beetle
to match whatever environment it finds itself in.
Thus, hiding among the leaves, its shiny surface
would mirror the green color of its environment,
but during the dry season, it could also hide among
the same leaves when they are brown. As elegant
as this explanation is, it is not entirely satisfactory.
One might predict that the silver and gold beetles
should be most prevalent in seasonal environments,
such as the tropical deciduous forests. In fact, all of
the gold and silver species known are found in evergreen tropical cloud forests. Even the green species
present yet another puzzle. If the bodies of Chrysina
beetles are green to blend with their environment,
why do so many of the species have contrastingly
colored legs and tarsi (Fig. 10)? One suggestion is
that the striking appendage color may function in
conspecific mate recognition. Typically, Chrysina
species occur in sympatric species assemblages and
the need to distinguish one another may be driving
character displacement in color pattern. 228
One of the most spectacular Chrysina species
is C. gloriosa (LeConte), a beetle which combines
a green dorsum with irregular stripes of silver.
The host plant of C. gloriosa is the juniper tree,
Juniperus monosperma (Engelmann). The foliage
of this tree is green with white flecks and many of
the insects that specialize on juniper, such as the
stinkbug Banasa euchlora Stål and the larvae of the
inchworm Semiothisa, are also green with white
spots and stripes. So this striking beetle, gaudy in
hand, is quite well camouflaged when tucked into
the foliage of its host plant (Fig. 11).
Some of the metallic Chrysina beetles, such
as C. resplendens (Boucard) (Fig. 12) are brassy
in color rather than a pure gold or silver. This
species reveals some remarkable aspects of color
Fig. 10. Chrysina macropus Francillon has a green
body but striking red legs and blue tarsi. The male
specimen in the photo also exhibits the sexual
dimorphism of metafemoral enlargement.
American Entomologist • Winter 2007
Fig. 12. A brassy Chrysina resplendens (Boucard), a species found in Panama.
Fig. 11. Chrysina gloriosa (LeConte) blending with the
foliage of its host plant, juniper.
in Chrysina. Instead of having a single stacked
reflector, C. resplendens has three. Chitin is a
long-chain molecule, much like cellulose. Each
layer of chitin is laid down as a sheet, and within
this sheet the chitin molecules are parallel to one
another. It thus has the ability to act as a polarizer
of light (materials with long chain molecules are
used to make polarized sunglasses). But even more
remarkably, as the sheets of chitin are laid down,
each is rotated slightly relative to the layer below
it (Fig. 13). Hence, the chitin stack has a period,
that is, the layers successively rotate the full 360º
and continue through another turn in a helicoid
arrangement. Optimal reflectivity is achieved when
the periodicity, the pitch of the helix, matches the
wavelength of the light. Consequently, the light
reflected by Chrysina beetles is not just polarized,
it is circularly polarized, a characteristic of the
light produced by liquid crystals in the LED screen
in laptop monitors (Neville and Caveney 1969).
The sense of the rotation is always anticlockwise
(looking down through the beetle), producing
reflected light that is left circularly polarized. One
consequence is that if one looks at a Chrysina beetle
through a circular analyzer (a polarized filter and
quarter wave plate), the color is extinguished (Fig.
14), whereas a grasshopper with insectiverdin, or
other non-ruteline scarabs such as Phanaeus or
Euphoria, will still be green (Kattawar 1994).
In Chrysina resplendens, two of the stacked
layers are helicoid in arrangement. But between
the two helicoid stacks is a stack of layers that
all lie in the same direction (Caveney 1971). This
unidirectional stack acts as a retardation plate.
Whereas in other Chrysina species the reflected
light from the cuticle is only left circularly polarAmerican Entomologist • Volume 53, Number 4
ized, the retardation plate reverses the sense of the
light reflected from the underlying helicoid stack,
such that this beetle reflects both left and right
circularly polarized light. Consequently, C. resplendens is able to reflect more of the incident polarized
light than do the other species. The top stack of C.
resplendens reflects primarily green wavelengths,
while the lower stack reflects primarily in the orange wavelengths. The combination produces the
brassy color. Hence, although C. resplendens does
not appear to be as shiny to us humans as the gold
or silver species, it is probably even shinier to other
insects, because unlike us, insects have P-vision:
they can see polarized light. It has thus been suggested that the optical property of the integument
could function in intraspecific signaling (Vulinec
1997). The enhanced intensity in the reflection of
polarized light might be a display that is important
in mate recognition and stimulation, while not even
visible to vertebrate predators. One can’t help but
wonder what other optical effects occur in nature
Fig. 13. The helicoidal arrangement of the chitin layers results
in the reflection of circularly polarized light.
229
Fig. 14. A green Chrysina beetle viewed through a circular analyzer (a quarter wave plate and polarizing filter).
Filtering of the polarized light depends on the orientation of the long-chain molecules in the filter; hence, a
rotation of the filter by 90 degrees extinguishes the effect.
that are beyond our visual capacity. Obviously,
there is more to these beetles than meets the eye.
Acknowledgements
Karen Robacker executed the photographs for
this article. David Hawks confirmed our species
determinations. Mary Liz Jameson provided helpful
comments on the manuscript.
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Donald B. Thomas is a USDA-ARS Research En-
tomologist with expertise in insect ecology and
quarantine issues concerning invasive species.
Apart from a sideline describing new species of
pentatomids, he is also President-Elect of the
Coleopterists Society. Ainsley Seago is currently
completing her graduate studies at the University
of California, Berkeley. She is a staphylinoid beetle
systematist who spends a disproportionate amount
of time investigating the structure and function of
iridescence mechanisms throughout Coleoptera.
David C. Robacker is a Research Entomologist
with the USDA-ARS with expertise in chemical
ecology and behavior of Anastrepha fruit flies. He
collaborates on websites featuring the biodiversity
of tropical Lepidoptera and Chrysina beetles, requiring travels to remote areas of Mexico, Central
and South America, searching for new records and
undescribed species.
7
American Entomologist • Winter 2007