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ON RESEARCH AND ENTOMOLOGICAL EDUCATION, AND A DIFFERENT
LIGHT IN THE LIVES OF FIREFLIES (COLEOPTERA: LAMPYRIDAE; PYRACTOMENA)
James E. Lloyd
Abstract
Research at institutions of higher education could
be restored to at least a shadow of its original role through
publication in a manner appropriate for immediate classroom use,
with questions that pique and direct the interests and activities
of students. Studies on basic natural history may be good candidates
for such publication and an example is drawn from fireflies: Two
woodland species show directional orientation in their pupation
sites on the trunks of trees; one uses southerly exposure and
the other occurs on the north side of smaller trees, and much
lower on the trunks. These contrasting positions have different
thermal consequences, as demonstrated with a physical model, which
possibly have a role in reducing interspecific sexual contact
or prey competition.
Key Words: fireflies, behavior, life history, orientation,
ecology
Resumen
La investigación en instituciones de educación
avanzada podría ser restaurada parcialmente a su rol original
a traves de publicaciones, de manera tal que las mismas puedan
ser usadas para enseñar, con preguntas que atraigan el
interés de estudiantes y que se relacionen con sus actividades.
Los estudios de historia natural básica pueden ser buenos
candidatos para ese tipo de publicaciones, y un ejemplo del mismo
se puede obtener con luciérnagas: Dos especies de luciérnagas
muestran diferencias en la ubicación de sus pupas en los
troncos de los árboles; una especie las ubica expuestas
hacia el sur y la otra usa el lado norte de árboles mas
pequeños y en la zona mas baja del tronco. Estas posiciones
contrastantes tienen diferentes consecuencias térmicas,
como se demuestra con un modelo físico, las cuales podrían
tener un papel en reducir el contacto sexual o la competencia
por alimento entre las dos especies.
In times past it went without question that the connection
between research and teaching was that professors who did basic
research maintained their intellectual interest in scholarship
and passed on to their students an inquisitive attitude and love
of the pursuit of knowledge as the essence of life and a life-sustaining
spirit. Students thus became living repositories of what was then
acknowledged to be a civilizing Ideal of western culture. An academician
of the time translated the expression “publish or perish”
as meaning that if he did not publish he had mentally perished,
and in doing so was failing in his professional responsibilities
to his students and his civilization. Over the past 30 years this
fundamental understanding and connection has been eroded and forgotten,
and a great deal of what is now done as “scholarly publication”
has little direct bearing on a “civilizing education.”
The essence of scholarly research is discovery and
originality. In my experience, good students find it more interesting
to actively participate in doing something that relates to discovery
than to see someone else do it on TV. It is worth exploring to
determine whether some primary publications in science could be
written directly for the classroom, rather than for the narrow
and generally disinterested “readership” of a scientific
journal, even leaving some obvious refinements for students to
manage. Original research papers could be used as texts, and beginning
students have direct contact with researchers themselves-who could
speak directly to them in their papers, and then perhaps personally
through the internet, thus achieving a quasi-oral tradition of
wide dimensions! Students would use an original publication as
a source of information and to stimulate their imaginations for
initiating their own school-time and life-time pass-time research.
What once might have been a scarcely read, esoteric and expensive
“contribution to . . .” could be an informative introduction
and background with suggestions and questions for personal projects
and class discussion. Though it pains me to admit it, fans of
electronic publication may be the first to see the desirability
and simplicity of doing this.
There is another twist to this notion. Since I have
chased fireflies for about a third of a century, I am often asked
by citizens and reporters, by letter and phone, “what is
happening to the fireflies, I don’t see them anymore?” Only
people who once knew and pursued fireflies can ask such a question,
because those who have never known them cannot miss them. Similarly,
might not students who learn by reading and doing original research
and see it in connection with their personal education, understand
and care more about what we have long considered to be the intellectual
values and strengths of an enlightened civilization? The irony,
the flip side of this is that here I address this notion to many
who have never seen a firefly.
Obviously, some research subjects lend themselves
to such instruction better than others, because of technical complexity
and expense, but there are many available sources of inspiration.
As John Sivinski has pointed out, one unfailing repository of
observations and ideas worth developing are the anecdotes, sketches,
and speculations that insect naturalists accumulate. From my search
for new sources and angles, I would add that many taxonomists
especially know what is lost to lab-bound and urban biologists,
because of their solitary hours of collecting and observing their
quarry in the field, which are as basic field investigations,
typically followed by solo hours of contemplation as they curate
their specimens. I have found that much of what can be done with
firefly taxonomy and behavior can be used almost immediately in
the classroom. It should be as a personal goal and measure of
scholarly accomplishment and fulfillment to see the development
of some significant area of insect research begun and developed
by undergraduate students in a teaching/research connection. Think
of the satisfaction that graduates would enjoy when they subsequently
saw their own studies used in a general entomology text.
For several years I have taught a general biology
course entitled Biology and Natural History With Fireflies in
which every class meeting is a field trip or lab and involves
some research-related activity. Instead of giving oral lectures,
I write the students letters; instead of laboratory and field
exercises with recipes and empty lines to write on, I give them
a background text on a subject, the material and equipment they
may want to use, and directions so they can do some things they
will find interesting. English, religion, architecture, microbiology,
German literature, journalism, pre med., and animal science majors,
to mention a few of the represented fields, experience first hand
the basics of biological research, including the design of empirical
studies and the gathering of data, the use of statistical analysis,
and the value of models and theoretical perspective. During class
meetings students are only required to be focused and interested,
and try to accomplish what they recognize with increasing skill
as sound biology.
As an example, the “Letter” below provides
the introduction and background for a number of field studies
that students can make in winter in a flood plain forest in Gainesville,
about two miles from the indoor classroom. The Letter is modified
for use here. Scientifically, this Letter is the first publication
of the outlines of a seemingly simple but perhaps very complex
element of firefly biology. The Letter omits statistical descriptions
and analyses, which are a field/lab experience themselves, but
illustrates the observations and raises questions that students
anywhere in the geographic range of the species can discuss and
independently or jointly pursue in the lab and woods (Fig. 1).
More than this, when students begin to address specific questions
about this apparently simple behavior of mere beetle larvae, they
discover that it is potentially so complex that it may never be
completely understood, and for them this itself is encouragement
to continue, to enjoy the study, and sometimes to see such biology
as also of the arts and humanities.
Letter XIII: A Different Light in the Lives of Fireflies
Dear Fireflyers, When fireflies and light are mentioned
in the same breath, one reflexively thinks bioluminescence, and
of the use that fireflies and taxonomists have made of pulses
of living light for species recognition, that behavioral ecologists
have made of firefly flashes for studying mate competition and
mate choice, and finally, of the use that biochemists, cell biologists,
and physicians now make of bioluminescence chemistry for enzyme
analysis, cell physiology, exobiology (extraterrestrial life searches),
and medical diagnoses. Our knowledge of firefly flash communication
in nature began with the incidental observations of a chemist,
Frank McDermott, who went to the field to observe fireflies out
of an interest in the mechanism of their luminosity, but stayed
to discover that some lightningbug species can be distinguished
by their flashed mating signals. What I will tell here began with
a taxonomist’s interest in getting a photograph, and became an
enigma in the realm of what some might call environmental physiology.
It is about a connection that some fireflies have with light other
than through their remarkable ability to generate it.
The larvae of one species may use sunlight to hasten
or perhaps, maybe, even to manipulate their pupal duration and
adult eclosion time (“date”). Pyractomena fireflies,
and perhaps all of the fireflies in their tribe (Cratomorphini),
unlike other lampyrids that do it in hidden chambers underground,
climb up on vegetation to pupate. Aerial pupation was reported
by Francis Williams near the beginning of the passing century
and observed in some detail by Lawrent Buschman, who examined
this behavior in the marsh-inhabiting species Pyractomena lucifera
(Melsheimer). Aerial pupation would seem to be a reasonable adaptation
for larvae that live on emergent vegetation over water and hunt
the aquatic snails below, or that could have their habitat submerged
by the flood water of a creek or river spilled out of its banks
onto adjacent flood plain. Pyractomena borealis (Randall) pupae
hang on tree trunks, by means of laterally projecting points that
extend into their cast larval skins they previously glued to the
trunk by the tail-end. At eclosion, the pearly-white, teneral
adults walk a few centimeters leaving behind the larval and pupal
skins and dangling tracheal linings, and remain motionless until
their cuticle has tanned. Sometimes adult males are found waiting
next to or on top of pupae (female only?; Fig. 2).
In the winter of 1982-83 I visited the flood plain
forest along Possum Creek in Gainesville to get photographs of
pupating Pyractomena borealis, whose adults I had seen flying
and flashing there in considerable numbers the previous March.
I found one, then several, then numbers of them, and it soon became
obvious that they did not occur randomly over the tree trunks.
Sometimes pupae occurred together, sometimes alongside vines or
in crevices, and occasionally below twig bases. They used trees
of several species and bark textures, usually anchoring themselves
between knee and basketball-rim height. I returned again and again
for more photographs, notes, and measurements of pupation locations.
Then, larvae and pupae of another woodland species, Pyractomena
limbicollis Green, began to appear up on trees and in many respects
this species was as a foil for P. borealis, providing a useful
and informative and certainly puzzling contrast.
P. borealis pupae show a surprising directional orientation
in their choice of pupation sites on the trees. In a sample of
240 pupae during three winters, the mean direction was southerly,
that is, about 180° true (= compass -2°; Fig. 3). But
sunlight is more than illumination and a suitable directional
cue for orientation-if indeed the larvae are using sunlight for
orientation-because it warms what it shines upon. By choosing
a pupation site at or near the south side of trees in January,
when ambient temperature may be low for many days and even drop
below freezing, P. borealis pupae raise their body temperature
during pupal development by several degrees, presumably decreasing
the duration of pupation. One potentially dangerous thermal consequence
of the sun-exposing behavior of P. borealis is that they must
be able to survive extreme temperature changes over a very short
period of time; on a clear and sunny winter day the temperature
of a dark-barked tree may reach over 90° F (32° C) at
three in the afternoon, and by midnight drop well below freezing
(32° F, 0° C). One wonders how they manage this!
Pupation up on trees has another conspicuous variable
that has thermal consequences. Were the adaptive significance
of aerial pupation merely the avoidance of rising flood water,
we might expect their vertical distribution on the trees to be
rather limited, with pupal distribution clumped around some height-perhaps
just above a residual high-water mark left by previous flooding,
possibly cueing upon chemical residues left by the water, or algal
growth encouraged by flood borne nutrients. Not so; the vertical
distribution has considerable spread (Fig. 4). Height may have
thermal significance because (1) in winter the ground below may
be a heat sink and have a tendency to hold lower-trunk temperatures
down, and (2) with increasing altitude there is less shading from
sunlight by the trunks, branches and leafless twigs of adjacent
trees. Obviously then, vertical as well as circumferential positioning
on a tree could potentially be used by larvae for manipulating
the timing of their metamorphoses. And, there are other possible
though more subtle influences on the thermal relations of these
pupae. For example, larvae use different species of trees, species
that vary in the smoothness of their bark and in the water content
of their wood, and these are probably not independent in their
effects.
The bark on beech trees is smooth and presents few
cliffs and side-directing channels; the bark on oak is rough,
with the crevices seemingly the equivalent of four story buildings
and presenting an obstacle course for short-legged, prostrate
larvae. I comparatively ranked the bark of each tree that larvae
selected for the energy and time I expected would be required
to climb over (up) them. Beech and sugarberry were typically toward
the least expensive end of the ranking, and red maple and oak
were at the most expensive end. In consideration of the difficulty
of climbing, one would expect that pupae might be found higher
on smooth than on rough trees, and perhaps there would be fewer
of them. This is what I observed. Trees with smoother bark had
more, and species with coarser bark had fewer pupae and they were
not as high on the trees (Fig. 5).
Because trunks of different tree species vary in
their water content, in sunshine a tree with more water will take
longer to warm up, and remain warm longer into a cooling winter
evening. Tree-water will also dampen temperature changes, preventing
rapid extremes-only two pupae were found on dead (dried out?)
trunks. Bark coarseness and thickness could have an influence
through the insulation it places between a hanging pupa and the
warm water held in the tissues of the trees. On the other hand,
rough bark and its crevices provide protective and perhaps thermally
amplified niches that provide dead air pockets and radiating walls.
Questions of water content and heat storage can be
explored with a simple physical model. I made artificial tree
trunks of plastic jugs used to store photographic darkroom chemicals,
and hung them in the sun on cool winter days. Each bottle had
a 1 cm clay sphere with a thermocouple inside, at each of four
directions (N, S, E, W); spheres were painted flat (i.e., not
enamel) black and held against the surface of their jug with an
elastic band around the jugs and passing over the thermocouple
wires. Jugs were of two “trunk” sizes, some contained
dry sand and some water-saturated sand, some were hung near the
ground and others more than a meter above the ground. Results
were generally as expected. Figure 6 shows the temperatures recorded
from the basic physical model, a large dry-sand jug, on a cold
winter day, with air temperature for comparison, and also sunlight
intensity as measured with a photographic exposure (visible light)
meter.
Note that the temperature/time courses of clay spheres
(model pupae) on different sides of a tree are not the same: the
S (south) clay sphere (black dots) warmed more and climbed from
freezing to nearly 28° C; the N sphere (open triangles) closely
followed air temperature; and that a brief shading at 460 min.
affected the S and W spheres but the E and N spheres scarcely
if at all. Many comparisons among such spheres and jugs are possible;
Figure 7 shows temperature/time plots for N and S clay pupae on
wet and dry jugs, with the moderating effect and thermal gain
from “tree water.” However, one photographed pupa was
discovered to be conspicuously arched out away from the tree,
suggesting that it should not be presumed that pupae fastened
to trees have no control over their body temperature; perhaps
they press against a warm tree to warm up, or arch out away to
cool down by increasing air insulation and circulation between
them and their too-warm tree.
The behavior of these juvenile fireflies raises many
questions that students can approach. Do larvae actually manipulate
with some precision their thermal gains from azimuth and height?-how
about thermal conditions in pockets between the ridges of a muscle
tree (Carolina beech)? Would a larva select a pupation site 15°
from a “precise target position” or “ideal directional
site,” if other pupae or a sheltering vine were positioned
there? Could a P. borealis juvenile be expected to integrate all
or some of the variables noted or discussed, to control the moment
when it, as an adult enters the competitive reproductive environment?
Would a male-to-be larva that was late getting to a tree accelerate
its development? Of course it would be absurd to ask whether a
larva could control its gender by adjusting its developmental
temperature.
Fundamental to comparing observations and sets of
observations, and of interest to the mathematically-minded, note
the problem of calculating statistical descriptions such as mean
positions and amount of spread in circular data, that is, of angular
positions around a tree-consider this: the average position of
a pupa 5° west of north and another 5° east of north,
is half of 355° + 005° and thus 180°, which is
true south! Nor is it simple and straightforward to compare the
means and deviations (spread) of samples to determine the likelihood
that they are “identical” (drawn from the same population).
Were my samples properly made?-my data show that more larvae climbed
smooth-barked trees (Fig. 5), but were there more smooth trees
in the woods; but, perhaps it is not relative abundance that should
be considered, but rather the identity of nearest neighbors to
trees actually climbed, because individual larvae may not move
far in the days or weeks before pupation. If you are interested
in physics or photo-journalism, can you suggest a better method
of measuring insolation (solar radiation), or a way to see infrared
patterns on and among the trunks of the trees that might be available
to tree-seeking larvae?
Figure 8 illustrates data that bear on several questions:
do azimuths of solitary P. borealis pupae show the same directionality?
(Fig. 8A); did hanging larvae that subsequently moved, have the
same near-southern azimuth? (Fig. 8B)-this question of course
relates to the (proximate) mechanism of orientation; do solitary
larvae and pupae that occur in protected sites deviate appreciably
from an approximate southern azimuth? (Fig. 8C).
On several occasions I found adult P. borealis males
attending pupae (Fig. 2). This raises questions related to mate
finding and competition: are males able to recognize female pupae?;
would guarding a sexually unidentified pupa have a better long
run payoff than searching with a signal light at night, and would
this probability and payoff change through the mating season?;
might males accelerate their eclosion to appear earlier in the
season to be ahead of and be waiting for unfertilized (high value)
females? This last speculation presently finds no support in the
azimuth and height data, assuming that accelerating males would
show different pupation azimuths and heights than females (Fig.
8E and F). Perhaps P. borealis fireflies in north central Florida
accelerate their seasonal appearance to avoid predaceous Photuris
species, which pupate in the soil and thus are stuck in a cold
cellar.
The pupation behavior of the smaller species P. limbicollis
stands in such contrast to that of P. borealis that it reinforces
the suspicion that there really is something significant occurring
in P. borealis, providing both encouragement to proceed and another
firefly subject for a comparative study. In my sample, P. limbicollis
pupated toward the north (Fig. 8D) and much lower on smaller trees
(Fig. 8G and H)-being low down on the north side of small trees
would result in a cooler-than-air temperature regime.
The adult season of P. limbicollis is about three
weeks later than that of P. borealis, and limbicollis adults appear
with a versatile firefly predator belonging to the Photuris versicolor
complex. The (sexual) flash pattern of P. limbicollis males is
virtually identical with one flash pattern emitted by the males
of this Photuris, an instance of the pattern-matching phenomenon
seen in males of many Photuris species. What would P. limbicollis
gain by synchronizing with a pattern-mimicking predator, or is
limbicollis manipulating its adult season to avoid a critical
seasonal overlap with its congener P. borealis? If this is the
case, is the avoided overlap that with mate-seeking adults or
with first instar larvae that must find soft-bodied and perhaps
only minute gastropod prey in the same forest litter?
These fireflies clearly present sufficient questions
with respect to proximate mechanisms and ultimate consequences,
to provide fireflyers many years of intriguing “off-season”
field work. Find quiet and mysterious trails.
Acknowledgments
I thank Robin Goodson, John Sivinski, and Steve Wing
for reading and insightful comments on the manuscript, several
classes of firefly students for fresh views of fireflies and research,
and Flora MacColl for additional editorial assistance. Journal
Series No. R-05567.
References Cited
Anderson, Charles H., and John D. Murray, (Eds.).
1971. The Professors: Work and life styles among academicians.
Schenkman Pub. Co., Inc., Cambridge Massachusetts.
Booth, Wayne C. 1988. The vocation of a teacher:
rhetorical occasions 1967-1988, 353 pp. University of Chicago
Press, Chicago.
Buschman, Lawrent L. 1984. Biology of the firefly
Pyractomena lucifera (Coleoptera: Lampyridae). Florida Entomol.
67: 529-542.
Green, John Wagoner. 1957. Revision of the Nearctic
Species of Pyractomena (Coleoptera: Lampyridae). Wasmann Journal
Biology 15: 237-284.
Heinrich, Bernd. (Ed). 1981. Insect thermoregulation.
Wiley, New York.
Lloyd, J. E. 1997. Firefly Mating Ecology, Selection,
and Evolution, Chapter 10 [In] The Evolution of Mating Systems
in Insects and Arachnids. (Ed) J. Choe and B. Crespi. Cambridge
Univ. Press. United Kingdom.
Oleksa, James. 1996. Fireflies: beauty and beyond.
Fireflyer Companion 1 (2): 23-24.
Williams, Francis X. 1917. Notes on the life-history
of some North American Lampyridae. Journal of the New York Entomological
Society 25: 11-33.
Zar, Jerrold H. 1984. Biostatistical Analysis. 718
pp., [especially Chapters 24 and 25], Prentice-Hall, Inc., Englewood
Cliffs, New Jersey.
Fig. 1. Locations of specimen-label records for P.
borealis and P. limbicollis from several North American collections.
Woodland Pyractomena species in addition to these two probably
also pupate up on the trunks of trees or shrubs.
Fig. 2. Male P. borealis with a P. borealis pupa,
sex unknown.
Fig. 3. Directional orientation on tree trunks of
P. borealis pupae during three winters, at the Possum Creek-Hog
Town Creek flood plain site.
Fig. 4. The height of P. borealis pupae on tree trunks.
Fig. 7. The comparison of temperatures of model pupae
at north and south positions on a dry-sand jug and a wet-sand
jug; a physical model examining the influence of tree water content
on pupal temperature.
Fig. 8. Graphs illustrating data that are pertinent
to some basic questions about P. borealis pupation biology, and
the remarkably contrasting behavior of P. limbicollis. (A) Azimuth
positions of solitary P. borealis pupae that presumably were not
influenced by others; (B) Azimuths of P. borealis larvae that
did not remain in position, showing that they abandoned what would
seem to be a good angle-though they may have moved to fine-tune
their positioning(?); (C) positions of P. borealis larvae and
pupae situated in sheltered locations showing that the shelters
did not have highly deviant azimuths; (D) The north-easterly azimuth
orientation of P. limbicollis pupae; (E, F) Azimuth and height
positions of male and female P. borealis. (G) Heights of pupal
positions of both species; (H) Trunk diameters (DBH, diameter
breast height) of pupation trees of both species.
Fig. 5. The height of P. borealis pupae on trees
with different bark roughness.
Fig. 6. The basic physical model of a tree with pupae.
The tree was a photographic chemical jug filled with dry sand,
painted flat black up to the sand level; the model fireflies were
1 cm clay spheres, painted black, each with a thermocouple inside
Department of Entomology and Nematology
University of Florida, Gainesville