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2012 NORTHEASTERN NATURALIST 19(2):345–352
A Heretofore Unreported Instant Color Change In a Beetle,
Nicrophorus tomentosus Weber (Coleoptera: Silphidae)
Abstract - The burying beetle Nicrophorus tomentosus (Silphidae) (Tomentose Burying
Beetle) achieves an instant color change from a strikingly black and orange animal to a
largely yellow one. This transformation achieves a mimicry of several species of bumblebees
when they are in flight, and it is accomplished by twisting the elytra to expose their
yellow undersides while simultaneously hiding the bright orange and black upper sides.
The overall effect is an apparent combination of both Muellerian and Batesian mimicry
Most burying beetles, Nicrophorus spp., are crepuscular or nocturnal and
spend the daylight hours in hiding. They feed on small animal carcasses that they
find above ground, often transport them a short distance, and then bury them.
They have evolved a complicated bi-parental care of their larvae (Milne and
Milne 1976, Trumbo 1990) where a mated pair cooperate (Fetherston et al.1990,
Trumbo 1991) in the feeding of and communication with their young. Most are
solidly black but with bright orange markings on their elytra. The coloration
of Nicrophorus, like that of other beetles, derives mainly from the wing covers
(elytra) which evolved from a front pair of wings (Dudley 2000) that now either
serve no, or only a slightly active role, in flight (Schneider 1975).
Beetle elytra are often strikingly colored, and the colors serve a variety of
adaptive functions such as in mimicry (Cott 1940, Ruxton et al. 2004). The
colors are achieved not only by pigments but also, as in some other insects
(Anderson and Richards 1942, Stavenga et al. 2011) and in some iridescently
colored birds (Prum et al. 2006), due to micro-anatomical features of the
integument. Such “structural” colors, based on diffraction gratings and multilayer
interference, may change reversibly in some beetles due to absorption
of moisture that alters the thickness of the transparent films and thus changes
the reflectance spectrum (Hinton and Jarman 1972). Other animals can change
colors by chromataphore movement in their skin, such as in some lizards, amphibians,
fish and octopi, but the hard exoskeleton of insects precludes these
rapid and spectacular physiological color changes (Bagnara and Hadley 1973).
Here I report the instantaneous change in appearance to more closely resemble
a bumblebee (Bombus) of the diurnally active N. tomentosus Weber (Tomentose
Burying Beetle) found in North America east of the Rocky Mountains. Burying
beetles of some species have been identified as convincing mimics of bumblebees
in flight (Fisher and Tucherman 1986, Lane and Rothschild 1965, Milne and
*Department of Biology, University of Vermont, Burlington, VT 05405; bernd.heinrich@
346 Northeastern Naturalist Vol. 19, No. 2
Figure 1. Photograph of a Nicrophorus tomentosus at a Blarina brevicaudus Say (Shorttail
Figure 2. Photograph of a dorsal view of a
Nicrophorus tomentosus in free flight as it
would commonly be perceived through human
eyes if it were encountered in the field—a fuzzy
Milne 1944), and that mimicry has been attributed primarily on the basis of their
flight tone, flight patterns, activity times, and body size.
I here examine color change and the role of color in N. tomentosus. As with other
Nicrophorus, this species has bright orange stripes on its elytra (Majka 2011), which
are prominent against a black background color (Fig.1). However, N. tomentosus additionally
has, like many bumblebees but unlike other silphids, a yellow pubescence
on the top and undersides of its thorax. Curiously, beetles of this species appear
yellow in the instant that they take flight, and the human eye then no longer detects
orange as would be expected from the bright orange on their elytra. However, flight
is fast and often erratic, and the orange might be easy to overlook. Nevertheless, (although
fuzzy) even photographs of beetles in flight in the field (Fig. 2) consistently
showed no orange. Instead, the entire dorsum of the beetle appeared to be lemon yellow,
and this was invariably the case in hundreds of sightings.
2012 B. Heinrich 347
Figure 3. Diagrammatic representation of a (right) wing cover (elytron) during the sequence
(1 to 6) in the twist and flip maneuver that hides the colorful dorsal coloration and
exposes the yellow underside during flight. X represents the outside edge of the elytron,
and • shows the inner edge at the terminal tip of the elytron.
Figure 4. A Nicrophorus tomentosus at the moment of take-off as it raises its elytra at
the same moment that it extends its wings from their folded position beneath the elytra
(Photograph courtesy of Stephen T. Trumbo.)
348 Northeastern Naturalist Vol. 19, No. 2
Results and Discussion
Unlike in most other beetles (where lifting the elytra causes them to flip
into a new stable position laterally to the sides), when I slightly lifted the
elytra of live or freshly killed nicrophorines (three species: N. tomentosus,
N. orbicollis Say, and N. sayi Laporte) from their locked positions, they always
became twisted at the hinge where they are attached to the thorax. Instead of
moving straight out to the side, their outer edges moved upward, and the whole
elytron then moved back.
As in other beetles, elytron movement is in part linked to wing extension.
After the nicrophorine elytron was up, the wing could extend and move forward
and back. Extending (pulling) the wing to the side caused the elytron to
flip up, thus wing opening was linked to elytron opening and twisting. Return
of the elytron to its original (dorsal side up) position caused the wing to lock
in with the elytral movement and return back over the abdomen. When the
elytra were pushed inward to cover the abdomen, they did not assume their
previous position, as is the case with other, non-nicrophorine beetles. Instead,
they continued their rotation until their formerly inner edges were to the outside
and the formerly outer edge inward (Fig. 3). In this manner, the elytra
were thus reversed over the abdomen, with the result being that Nicrophorus
expose their lemon-colored elytral undersides instead of their dorsal surface
This mechanical operation, which to my knowledge occurs in no beetles other
than Nicrophorus, accounts for the otherwise black and orange beetle always appearing
to be yellow the moment it takes flight.
In a laboratory study of the role of beetle elytra during fixed flight on 16 different
species, Schneider (1975) described one type, as exemplified by the June
bugs (Phyllophaga) and rhinoceros beetles (Melolonthus) (both Scarabaeidae),
where the elytra moved along with the wings (though through a much lower
angle) and thus may serve a small but active function in flight. In tiger beetles
(Cicindela) (Carabidae), the elytra were extended laterally but remained motionless
in flight. In rose chafers (Cetonia) (Scarabaeidae), the elytra remain closed
over the back, and in Nicrophorus, the elytra were not only held motionless and
roof-like over the back in flight as in Cetonia, but their outer edges folded up
in the long axis over the abdomen (“Die Elytra werden um die Langsachse nach
ober geklappt und stehen dachartig über den Abdomen ohne mitzuschwingen”).
Schneider (1975) was unable to demonstrate any effects on lift of the elytral positions
taken during flight, but posited instead that the elytral posture may have a
possible role in steering. No other possible function for the unique elytral flight
position for Nicrophorus was offered. The origin of the elytral flip mechanism is
unknown, but could it aid in mimicry?
Although the above elytral flip and twist mechanism, which is mechanically
coupled with the wing extension, can be achieved in dead animals, it appears
in N. tomentosus only during flight, when the elytra are relatively closely
2012 B. Heinrich 349
depressed over and onto the abdomen. The mechanism occurs as the wings are
extended, i.e., the moment before and during flight. High-speed photography
of N. tomentosus beetles during the split-second of wing opening or closing
strongly reinforces the inference that it functions in bumblebee mimicry
(Fig. 4). Most nicrophorid beetles are nocturnal or crepuscular, but N. tomentosus
is unusual in flying in the daytime.
When burying beetles are disturbed by a potential predator at a carcass, they
bury themselves quickly, or feign death. While searching for carcasses, however,
they must fly and as a consequence become conspicuous targets to predators such
Birds avoid capturing bumblebees after learning to recognize them (Evans
and Waldbauer 1982), and a diversity of flies gain protection by mimicking them
(Brower et al. 1960) in often, to our eyes, precise Batesian mimicry (Wickler
1958). The mimicry is so close that few besides entomologists may differentiate
some flies, especially the bumblebee-mimicking syrphids, asilids, and oestrids,
from bumblebees. That a brightly-colored, orange-and-black beetle can almost
instantly change in appearance to that of a bumblebee provides a compelling
example of visual mimicry, which complements the aural mimicry of the bees’
flight tone (Fisher and Tucherman 1986, Lane and Rothschild 1965), as well as
matching their seasonal and diurnal activity cycle.
In the most recent and most detailed publication of the mimicry of carrion
beetles (Fisher and Tucherman 1986), it was proposed that because of its “bright
yellow thoracic pile and orange-red elytra” the models of the N. tomentosus
mimicry in Ontario are the bumblebees Bombus ternarius Say and B. rufocinctus
Cresson. However, since these two bumblebee species are the most common
northeastern species with orange on the abdomen, they could only serve as models
if it is assumed that N. tomentosus follows the conventional beetle pattern and
lacks the mechanism here elucidated; the beetles’ bright orange elytral stripes are
not likely to have arisen to serve in the proposed mimicry because they are made
to be invisible precisely during flight, the only time a nicrophorine resembles
a bumblebee. While death-feigning nicrophorines have also been suggested to
mimic bumblebees, the resemblance to a dead insect is then great but that to a bee
is not, and the efficacy of mimicking a dead bee is also of questionable value. I
propose here instead that N. tomentosus does not specifically mimic orange-red
bumblebees, but instead hides its red, and creates a phenotype that mimics seven
local yellow-and-black Bombus spp.
Most of the about 46 species of North American bumblebee species (Heinrich
2004) have black bodies marked with yellow pile. Although seven of these
species also have varying amounts of orange setae over the gaster, that color
is always bordered by yellow and is never in sharp contrast to a black band
as in nicrophorines. Unlike other local nicrophorines, N. tomentosus is a lateseason
flyer (July to September in northeastern America [Majka 2011]), which is
precisely when the worker numbers of up to seven black-and-yellow Bombus species
(B. affini Cresson, B. vagans Smith, B. bimaculatus Cresson, B. sandersoni
350 Northeastern Naturalist Vol. 19, No. 2
Franklin, B. impatiens Cresson, B. perplexus Cresson, and B. griseocollis Degeer)
peak. All these species have nearly identical color patterns and are difficult
to differentiate (see color plate in Heinrich 1979).
Bumblebees form apparent Muellerian mimicry rings (Plowright and Owen
1980). Since the workers of all bumblebees are distasteful and they sting, I
conclude that N. tomentosus may, aided by its elytral flip to show bright yellow
undersides, be a credible Batesian mimic, especially of the Muellerian mimicry
ring of yellow bumblebees (Fig. 5). However, the orange markings of all the
nicrophorids suggest a warning function, and future experimental study is needed
to find out if N. tomentosis is indeed attacked less during flight because of the
coloration it achieves by the elytral flip..
Although the elytral reversal convincingly serves N. tomentosus to become a
bumblebee mimic, it raises many questions about the extent, diversity, and origin
of both the unique mechanism and its possible application in the other 67 nicrophorine
species. A comparative taxonomic study of their seasonal, diel activity,
and geographic distributions, in conjunction with the ventral elytral colors and
the amount of elytral twist, should provide the answers.
Figure 5. Sketch of the approximately one-second-interval sequence of a N. tomentosus
walking (lower right) to flight (top) and back to perching position (right). The bumblebee
(center) has the approximate color pattern of seven locally sympatric species (see text ).
2012 B. Heinrich 351
I thank John C. Abbott, Alfred Newton, Stephen T. Trumbo, Derek S. Sikes, Doekele
Stavenga, David L. Wagner, and an anonymous reviewer for alerting me to valuable
literature, stimulating discussions, and generous help in orienting me into the fascinating
biology of Nicrophorus beetles.
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