The role of the subelytral spiracles in respiration in the flightless dung beetle Circellium bacchus
1 Ecophysiological Studies Research Programme, School of Animal, Plant and
Environmental Sciences, University of the Witwatersrand, Johannesburg, Wits
2050, South Africa
2 School of Physiology, Faculty of Health Sciences, University of the
Witwatersrand, Parktown 2193, South Africa
* Author for correspondence (e-mail: marcus{at}gecko.biol.wits.ac.za)
Accepted 21 January 2003
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Summary |
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Key words: discontinuous gas exchange cycle, Scarabaeidae, Circellium bacchus, beetle, spiracle, subelytral cavity
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Introduction |
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The water-saving role of the subelytral cavity is an attractive hypothesis
(Cloudsley-Thompson, 1964) with
circumstantial evidence to support it, such as the air inside the cavity
having a high water content (Zachariassen,
1991
; Cloudsley-Thompson,
2001
). However, the exact mechanism by which it works is still
unclear. Ahearn (1970
)
suggested that there is unidirectional, retrograde airflow in which the
thoracic spiracles are used for inspiration and the subelytral spiracles are
used for expiration. After air has been expelled from the spiracles, the
CO2 that has accumulated in the subelytral space is eliminated to
the atmosphere by lifting the elytra, creating an opening above the anus
through which CO2 could be expelled on an intermittent basis
(Nicolson et al., 1984
), thus
resulting in the discontinuous gas exchange cycle (DGC) measured from desert
beetles (Lighton, 1991
).
However, although the flightless dung beetle Circellium bacchus
(Coleoptera, Scarabaeidae) has a marked DGC
(Duncan and Byrne, 2000
), we
found CO2 emission to take place predominantly through one anterior
mesothoracic spiracle, and the airflow through the beetle to be predominantly
anterograde (or possibly tidal) when at rest
(Duncan and Byrne, 2002
).
The DGC is an intermittent discontinuity in external gas exchange that
typically consists of three periods
(Miller, 1981;
Kestler, 1985
;
Lighton, 1994
). First, there
is a closed period, where the spiracles are shut, which inhibits gas exchange
and respiratory water loss. Oxygen levels in the tracheae drop while
CO2 is largely buffered in the tissues and haemolymph. This is
followed by the flutter period during which slight opening of the spiracles on
an intermittent basis allows some normoxic O2 uptake through the
spiracles by diffusion and convection, but little CO2 or water
vapour is lost. The final CO2 burst period is triggered when the
accumulation of CO2 from respiring tissues causes some or all of
the spiracles to open widely or triggers a bout of active pumping movements.
The rapid unloading of CO2 minimises the time that the spiracles
are open and therefore reduces water loss
(Kestler, 1985
).
In our previous study (Duncan and
Byrne, 2002), the DGC was only found at one anterior, mesothoracic
spiracle, outside of the subelytral cavity, and showed only the closed and
burst periods. The flutter period was largely reduced or absent. We detected
no large, intermittent CO2 bursts from outside the elytral case,
which would correspond to the lifting of the elytra to expel accumulated
CO2. Water loss also occurred mainly via the mesothoracic
spiracle when it opened, and 90% of the CO2 expelled was eliminated
through this spiracle (Duncan,
2002
). Because the small amount of CO2 emitted was
measured from outside the subelytral cavity, we could not determine what role,
if any, the subelytral spiracles had in respiration.
There are seven pairs of spiracles that open into the subelytral cavity in
C. bacchus (a single metathoracic pair and six abdominal pairs;
Fig. 1). The shape and position
of the spiracles within the Scarabaeoidea are so variable that they have
little utility as a taxonomic character
(Richter, 1969); nevertheless,
the Scarabaeidae have them positioned under the elytra. Because these
spiracles represent a significant surface area for gaseous exchange, we would
expect them to be involved in respiration. The forward airflow (anterograde)
from the subelytral cavity to the anterior spiracles found in C.
bacchus implies that the subelytral cavity could act as a humidity
chamber but in a different manner to that previously proposed.
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The large size and quiet nature of C. bacchus allowed simultaneous
measurement of respiratory gas exchange from the anterior and posterior body
half of the beetle at rest. By drilling holes in the elytra, we were also able
to sample gas from the subelytral cavity at different stages of the DGC.
Because CO2 release rate is proportional to the degree of opening
of the spiracles (Kestler,
1985), we were able to examine the role of the subelytral
spiracles in respiration by infrared gas analysis and to conclude that they
are synchronised to the DGC found at the anterior spiracles. These findings
show that the subelytral spiracles play an active part in respiration at rest
and suggest that the subelytral cavity has a role in respiration in accordance
with its hypothesised function as a water-saving adaptation.
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Materials and methods |
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Volume of the subelytral cavity
To gain access to the subelytral cavity, one hole was drilled into each
elytron, near the anterior lateral margin, of five beetles using a 3 mm dental
drill. A short (5 mm) length of 3 mm-diameter tube (flexible nylon; Portex
Ltd, Hythe, UK) was sealed into each hole using a combination of bee's wax and
superglue. Between experiments, the tubes were closed off with dental wax
plugs, which allowed the beetles to survive for several months in the
laboratory. Therefore, we assume that the beetles were not incapacitated by
this procedure.
To estimate the volume of the subelytral cavity, the wax plugs were removed, and distilled water was gently pumped into one elytral tube using a 10 ml syringe until it appeared at the second tube. After approximately 5 s, the beetle was tipped on its side and air was pumped through the upper tube while the water was collected as it came out of the lower tube. This water was weighed to ±0.1 mg (Precisa 160A balance) and used to estimate the volume of the subelytral cavity.
Continuous measurement of CO2 emissions from the
subelytral spiracles
A flow-through respirometry system was used to measure CO2
emission in inactive beetles at room temperature (23±1°C).
Simultaneous sampling of gas emissions from the anterior (mesothoracic) and
posterior (subelytral) spiracles was performed in a set-up with two separate
compartments, similar to that described by Duncan and Byrne
(2002), where the head and
thorax are separated from the abdomen by a sheet of latex
(Fig. 2). Each compartment had
its own manometer, flow controller (Supelco flow meter) and gas analyser. Room
air, scrubbed of CO2 by a soda lime column, then humidified in a
water column, was drawn through each compartment of the respirometer either at
50 ml min-1 or at a rate that generated equal pressures in all
parts of the system. Initially, respiratory gases were sampled from the air
spaces in each chamber. To measure the CO2 output directly from the
spiracles in the subelytral cavity, the saturated air was drawn in through one
of the tubes inserted into an eltron and out through the other. Thus, the
CO2 emission from the subelytral spiracles was sampled in humid
air, as we assumed that passing dry air through the subelytral cavity during
the long periods required to make measurements would unduly stress the beetles
and alter their respiratory patterns. The air withdrawn from both sides of the
respirometer was dried using a magnesium perchlorate scrubber before
measurement in a gas analyser. Either a Licor CO2 analyser
(LI-6251) or Licor CO2/H2O analyser (LI-6262), both with
a resolution of 0.1 p.p.m., was used to measure the separate CO2
emissions. The length of the tubes connecting each compartment to its
respective analyser was kept to a minimum and was identical for each tube.
Readings of the volume of CO2 emitted were taken every 5 s and
recorded using computerised data acquisition software (Datacan V, Sable
Systems, Las Vegas, USA).
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Measurements were made on individual beetles that were first weighed to
±0.1 mg (Precisa 160A balance). The beetles were measured for a minimum
of 6 h in the dark, with the same conditions being used for all of the
beetles. Separate, simultaneous measurements of the CO2 emissions
from the mesothoracic and subelytral spiracles allowed us to compare our
results with previous work and to verify that the beetles remained inactive
during sampling (Duncan and Byrne,
2002). Sealing the elytral holes with dental wax enabled us to use
the same individuals for comparison of CO2 emissions from the
subelytral cavity with those from the anterior body half and those from
outside the elytral case.
Baseline drift of the gas analysers was corrected during analysis from measurements taken at the beginning and end of each trial with the respirometer chamber empty. All measurements were corrected to standard temperature and pressure (STP). The CO2 recordings were converted to rate of CO2 production in ml h-1. The discontinuous gas exchange cycle (DGC) characteristics were calculated as follows: the DGC frequency (= burst frequency) was calculated by determining the number of peaks of CO2 per second, and the DGC duration was taken as one complete cycle. The mean rate of CO2 emission was measured as the mean value over several complete DGC cycles, and integration of the area under the curve against time (in hours) was performed to obtain the emission volume.
Intermittent measurement of CO2 emissions from the
subelytral spiracles
To determine the gas composition within the subelytral cavity during the
different periods of the respiratory cycle, we alternated sampling between the
air inside the subelytral cavity and the air outside the elytral case, while
continuously measuring CO2 emission from the anterior spiracles.
The subelytral cavity was sampled for short intervals on an intermittent
basis. Air was withdrawn from the posterior chamber of the respirometer from
either the space around the posterior end of the beetle or from within the
subelytral cavity. At the same time, air was continuously sampled from the
anterior chamber around the mesothoracic spiracles, which provided a benchmark
for the stage of the normal DGC at which we sampled air from the subelytral
cavity. By measuring the CO2 emission from the anterior spiracles
at the same time as intermittent sampling from within the subelytral cavity,
we were able to determine the gas composition within the subelytral cavity at
different periods of the normal DGC. The set-up was similar to that described
above, except that the humidifier was removed from the intake side and the air
was dried and scrubbed of CO2 before entering the respirometer. In
the posterior chamber, the air stream was periodically switched from passing
over the elytral case to passing through the subelytral cavity using a
solenoid valve switching mechanism (Sable Systems). The subelytral cavity was
sampled for 8-min periods, with the intervals between measurements ranging
from 20 min to 1 h. A Licor CO2/H2O analyser (LI-6262)
and O2 sensor (Qubit Systems, Ontario, Canada; resolution of 0.1%)
were used for the posterior chamber. Other parameters were treated as above.
We were unable to synchronise the intermittent samples periods with the DGC of
individual beetles as they only settle into their normal DGC after some time
in the respirometer, when they become inactive. Therefore, we relied on
fortuitous overlaps of the burst and closed period of the DGC from the
anterior spiracles with the sampling periods of the subelytral cavity
contents.
Statistics
Data are represented as means ± S.D. Sample size (N) is
indicated as either representing individual beetles or, in the case of gas
exchange characteristics, 3-10 discontinuous gas exchange cycles per beetle.
Unless otherwise noted, statistical comparisons were made either with the
Student's t-test or with analysis of variance (ANOVA). Significant
ANOVAs were followed with the NewmanKeuls multiple range test.
Regression analysis was done by the least-squares method, and the regression
lines were compared using analysis of covariance (ANCOVA).
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Results |
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Continuous measurement of CO2 emissions from the
subelytral spiracles
Carbon dioxide emissions from the anterior and posterior chambers of the
respirometer showed what we consider to be a typical pattern for C.
bacchus at rest. There is a marked DGC, with the majority of
CO2 being expelled at the anterior mesothoracic spiracles, and the
absence of a flutter period (Fig.
4A). However, continual sampling from the subelytral cavity
revealed a complete reversal of this pattern
(Fig. 4B). The majority of
CO2 emitted was now detected from the subelytral spiracles, which
clearly exhibited a flutter period before the burst period. Measurements from
six beetles showed a similar pattern and are summarised in
Table 1. By combining the
amounts of CO2 emitted from the subelytral space and the
mesothoracic spiracles, the total recorded from two halves of a beetle is
similar to that recorded from one beetle measured as a single entity; mean
CO2 partial pressure
(CO2) of
295.7±195.2 µl h-1 from six beetles of mean mass
6.94±2.37 g (this study) compared with mean
CO2 of
407±204 µl h-1 from seven beetles of mean mass
7.285±2.93 g (Duncan and Byrne,
2000
), respectively; t0.05,11=1.0,
P>0.05.
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During the continuous removal of air from the subelytral cavity, almost no CO2 left via the mesothoracic spiracle (Table 1). The rate at which CO2 emission occurred into the subelytral cavity was, on average, 22 times greater than that emitted through the mesothoracic spiracle at that time. The mean rate of CO2 emission and mass-specific rate of CO2 emission from the subelytral spiracles was significantly greater than that from the mesothoracic spiracle (t=3.33, P<0.001; t=5.14, P<0.01, respectively).
The characteristics of the DGC obtained from the subelytral spiracles are
given in Table 2. There is a
significant positive relationship between the length of the periods (closed,
burst and flutter) and the volume of CO2 emitted during those
periods. The slopes of these curves are identical but the intercepts are not
(ANCOVA; flutter period, F0.05,4,40=1.48; burst period,
F0.05,4,40=1.67, closed period,
F0.05,3,24=0.55). This indicates that the beetles adjust
their CO2 output by increasing the lengths of the respective
respiratory periods rather than increasing their frequency. Comparing the
values obtained for the DGC characteristics for the subelytral spiracles with
those reported for the mesothoracic spiracle
(Duncan and Byrne, 2002),
there are no significant differences in the frequency and duration of the
cycle, volume of CO2 emitted during the burst period and duration
of the burst period (P>0.05 in all cases).
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Maintaining the same flow rate (50 ml min-1) at the anterior and posterior chambers during continuous sampling of the subelytral cavity flushed almost all of the CO2 out of the cavity (Fig. 4B), whereas equal pressures in both respirometer chambers and a lower flow rate through the subelytral cavity allowed a build up of CO2 in the cavity, and more CO2 left from the mesothoracic spiracle. Nevertheless, the pattern of CO2 emission was the same in both cases.
Intermittent measurement of gas composition in the subelytral
cavity
Intermittent sampling from the subelytral cavity did not disturb the normal
DGC seen in C. bacchus (Fig.
5) and allowed us to gather a more complete picture of events
during the respiratory cycle. At any time, the air inside the subelytral
cavity is high in CO2 and water vapour, while the concentration of
O2 is lower than atmospheric. The absolute concentration of
CO2 sampled changed according to whether or not air had recently
been expelled through the mesothoracic spiracle
(Fig. 5;
Table 3). The amount of
CO2 in the subelytral cavity was significantly lower when sampled
after CO2 had been expelled from the mesothoracic spiracle (i.e.
during the closed period of the mesothoracic spiracle;
Fig. 5A) than when measured
immediately before or during the burst period of the mesothoracic spiracle's
DGC (Fig. 5B) (F0.05,2,35=7.754, P=0.0016, N=5
beetles). In one specific individual in which several measurements were
obtained from all three stages of the mesothoracic DGC, the same result was
obtained (F0.05,2,11=5.26, P=0.025). Thus, the
concentration of CO2 within the subelytral cavity is lowered after
CO2 has been emitted through the mesothoracic spiracle.
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Removal of CO2 from the subelytral cavity immediately before or during the burst period of the mesothoracic DGC influenced the amount of CO2 emitted through the mesothoracic spiracle (Table 4; Fig. 5). If a large amount of CO2 was removed from the subelytral cavity, less was emitted through the mesothoracic spiracle. The total amount of CO2 lost from the beetle was calculated and, in some cases, it compares well with the total amount of CO2 emitted from the mesothoracic spiracle (Table 4). Therefore, the total amount of CO2 released is similar by whichever route CO2 is emitted or sampled from the beetles.
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The method for sampling water concentration used in this study did not enable the water content of the air in the subelytral cavity to be determined accurately. However, the data in Table 3 show that this air was kept at a high humidity, regardless of the stage of the DGC cycle at the mesothoracic spiracle. These data indicate that, should the elytra be opened, the proportional increase in water loss will be approximately 74% (Table 3).
The O2 concentration of the air within the subelytral cavity was
lower than the air around the elytral case, irrespective of the time of
sampling (Fig. 5). Because
small differences of O2 concentration could be due to baseline
fluctuations, we could not compare the differences in O2
concentration within the subelytral cavity at the different DGC periods of the
mesothoracic spiracle. Substantial drift in the baseline readings of the
O2 measurements occurred during the sampling period of several
hours, which is typical for these analysers; nevertheless, a qualitative
assessment of O2 consumption can still be made. The noticeable drop
in O2 concentration when air was sampled from within the subelytral
cavity was not due to dilution by CO2 because an
H2O/CO2 scrubber was placed before the O2
analyser in the experimental set-up
(Withers, 1977).
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Discussion |
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Spiracular coordination
Distinct functions and capabilities of thoracic and abdominal spiracular
groups, working in coordination, were found in the desert ant Cataglyphus
bicolor (Lighton et al.,
1993). The authors concluded that the thoracic spiracles act as a
high-capacity entrance to the tracheal system, whereas the abdominal spiracles
are more specialised and play a major role during the early flutter period of
the DGC, which we also propose for C. bacchus. The same pattern of
spiracular use is employed by both insects for the same purpose of water
retention, but we would expect that the effectiveness would be enhanced by the
presence of the subelytral cavity in C. bacchus, which will allow for
the maintenance of a high relative humidity over the posterior spiracles.
Sampling air from inside the subelytral cavity revealed a flutter period
that was not apparent from outside the subelytral cavity. The flutter period
in Cecropia pupae starts when the endotracheal O2
concentration drops below about 3% (Levy
and Schneiderman, 1966) and enables the bulk inflow of
O2 into the trachea along its concentration gradient while
minimising the loss of H2O. In arid-dwelling beetles, a large
proportion of the CO2 expelled exits during the flutter period.
Lighton (1991
) found that 24%
of the total CO2 release occurred during the flutter period for
nine species of Namib Desert tenebrionid beetles, with one species reaching
47%. Duncan et al. (2002
)
reported values of 48% and 29% for two species of Negev Desert tenebrionid
beetles. By contrast, C. bacchus released only 10% of the total
CO2 measured from the subelytral spiracles during the flutter
period. In flighted dung beetles, the flutter period could also be an
important means of eliminating CO2 whilst enabling inflow of
O2 and reducing water loss
(Lighton and Garrigan, 1995
).
Our unpublished results on flighted dung beetles (without sealed elytra)
indicate that the subelytral spiracles are the main site of gas exchange in
these beetles at rest (M. J. Byrne and F. D. Duncan, unpublished data). We
have measured a distinct flutter period that persists for 30% of the duration
of the DGC in C. bacchus, which compares well with the above
species.
Continuous sampling from the subelytral cavity did not disturb the normal
pattern of the DGC seen in C. bacchus, just as holes into the elytra
of the desert tenebrionid Onymacris plana caused no change in its
cyclic respiratory pattern (Bartholomew et
al., 1985). This suggests that the DGC is driven from
CO2 concentration within the trachea rather than from
CO2 or H2O levels outside the spiracles (Lighton, 1996).
The total amount of CO2 removed from the beetle during the gas
exchange cycles remained fairly constant
(Table 4) regardless of whether
it was removed from the subelytral cavity directly or exited naturally
via the mesothoracic spiracle. Continuous sampling did, however,
shift the bulk of gas exchange from the anterior to the posterior spiracles
when a high flow rate was maintained through the subelytral cavity.
Intermittent sampling or reduced flow rates through the subelytral cavity
allowed water vapour and CO2 to build up in the cavity; therefore,
we assume that these are maintained at elevated levels throughout the normal
DGC. Zachariassen (1991
)
showed that experimental manipulation of the relative humidity of the air
inside the subelytral cavity of a tenebrionid beetle had a strong effect on
the rate of water loss. The volume and pattern of CO2 accumulation
within the subelytral cavity of C. bacchus suggest that it is
expelled from the subelytral spiracles rather than diffused through the
cuticle, as in termites (Shelton and
Appel, 2000
).
Role of subelytral cavity in water conservation
The air within the subelytral cavity of C. bacchus has a high
water vapour content, which was kept relatively constant during the course of
the DGC. Ahearn and Hadley
(1969) showed that removal of
sections of the elytra increased water loss from desert tenebrionids. We
calculated that if the subelytral cavity of C. bacchus was open,
there would be an approximately 74% increase in the rate at which water is
lost from the posterior body. Thus, lifting the elytra to create an aperture
above the anus through which CO2 could be expelled, as previously
suggested for tenebrionids by Nicolson et al.
(1984
), would cause a
substantial increase in the rate of water loss. Depending on the timing of
this opening, water would be lost from the respiratory passages, as well as
from the subelytral cavity. By restricting CO2 exchange with the
atmosphere to only one mesothoracic spiracle, water loss is confined to a
small area of the total respiratory system
(Lehman, 2001
). In addition,
these respiratory passages are only open to the atmosphere for short periods
during the mesothoracic spiracle's burst period. Water loss rates increase
when the mesothoracic spiracle opens but this only contributes 4% to the total
water loss rates (Duncan,
2002
). Thus, the evolution of a tidal airflow and a subelytral
cavity could be important in reducing water loss in these arid-dwelling
beetles.
The subelytral cavity as a CO2 sink and O2
sponge
Differential control of the mesothoracic and subelytral spiracles in a
coordinated DGC has been shown in these results; with respect to other insects
such as dragonflies (Miller
1961), ants (Lighton et al.,
1993
) and moth pupae (Slama,
1999
), this is not remarkable. However, combined with the
discovery of anterograde airflow and the use of a single spiracle for
CO2 emission in C. bacchus, these findings allow us to
extend the hypothesis of the subelytral cavity as a water-saving device as
follows.
The anterior mesothoracic spiracles are shut during the closed period,
while the subelytral spiracles progress from being closed to rapid fluttering.
This allows oxygen to enter the tracheal system from the subelytral cavity
down its diffusion gradient. However, for O2 to continue diffusing
into the trachea via the subelytral spiracles during the 20 min
flutter period, it will have to be replenished from the atmosphere, leading to
a paradoxical role for the subelytral cavity where it will have to allow
entrance of O2 from the atmosphere while retaining water vapour and
CO2. This mirrors the proposed action of the tracheae and spiracles
during the DGC, where CO2 will theoretically unload from the
respiratory system 15 times slower than O2 will load, despite
similar diffusion coefficients (Snyder et
al., 1995; but see Lighton and
Garrigan, 1995
). Gorb
(1998
) has shown the
subelytral cavity of an arid-adapted tenebrionid to be tightly sealed by
microtrichia, and similar structures have been found in C. bacchus
(M. J. Byrne and F. D. Duncan, unpublished data). Our results from this and a
previous study (Duncan and Byrne,
2002
) demonstrated differential movement of O2 into the
subelytral cavity and consumption from the subelytral cavity without a
corresponding emission of CO2.
Retention of CO2 as a consequence of water retention may then
actually be advantageous, whereby elevated levels of CO2 in the
subelytral cavity serve a dual function. Carbon dioxide can be sequestered in
the subelytral cavity, increasing the overall CO2 capacity of the
body and, in that way, lengthening the DGC period, thereby reducing
respiratory water loss rates (Kestler,
1985; Lighton et al.,
1993
) and contributing to a diffusion gradient that will draw
atmospheric O2 into the subelytral cavity. Water vapour in the
subelytral cavity will also contribute to water retention in the flutter
period by reducing its concentration gradient between the tracheae and ambient
air (Kestler, 1985
).
This state will persist until, finally, O2 diffusion and
replenishment from the atmosphere will lead to a build up of N2 and
CO2 in the subelytral cavity and in the tracheae
(Schneiderman, 1960) and the
hypoxic trigger point is reached at the mesothoracic spiracle, which opens.
These are the largest spiracles and appear to have the largest diffusive
capacity and, therefore, are best suited for CO2 exchange en
masse (Lighton et al.,
1993
). Consequently, large amounts of CO2 are expelled
from the anterior spiracle in its burst period. Whether or not this is
achieved by active ventilation is unknown to us, and the theoretical arguments
in favour (Kestler, 1985
) or
against (Lighton et al., 1993
)
are still unresolved.
The burst period of the anterior mesothoracic spiracle is probably preceded
by the burst period of the subelytral spiracles but with considerable overlap
(Figs 4,
5). However, very little
CO2 was detected outside the subelytral cavity to indicate a burst
period from the posterior spiracles, providing further evidence of the
subelytral cavity being CO2 tight
(Fig. 4A). Therefore,
CO2 from the subelytral cavity must be moved forward through the
body, as we have shown O2 to do
(Duncan and Byrne 2002), and
be expelled at the mesothoracic spiracle during its burst period. We do not
know if active ventilation is responsible for this movement of gas. However,
observations of dorsal abdominal pumping through holes drilled in the elytra,
and the lack of correlation between body size and the volume of the subelytral
cavity (Fig. 3), suggesting
that it can be voluntarily altered by the beetle, both indicate that
CO2-laden air could be pumped forwards out of the subelytral
cavity. Small CO2 volleys, overlaying the burst period from both
the anterior and posterior spiracles, can be seen in
Fig. 4A,B and are indicative of
active ventilation. The anatomical route of CO2 from the subelytral
cavity to the mesothoracic spiracle still needs to be resolved, but the system
of large air sacs attached to the trachea coupled with abdominal pumping could
be involved in moving CO2 through the body through longitudinal
tracheae in C. bacchus
(Wasserthal, 1996
).
C. bacchus is a monotypic, canthonine dung beetle, which is rare
and restricted to fragmented patches of dense, arid bush in South Africa
(Scholtz, 2000). It is assumed
that competition from large, flighted dung beetles has confined it to these
areas, where its apterous state and large size are assumed to be advantageous
in minimising water loss (Chown et al.,
1995
). Klok (1994
)
found individuals of C. bacchus to be the most resistant to
desiccation of the 12 species of dung beetles he tested. While these findings
only correlate with our assumptions about the role of the subelytral cavity in
reducing water loss, we can assume that, if our hypothesis is correct, this
mechanism will be widespread among related species. Thirty percent of the
Canthonini are flightless, and most are found in tropical or subtropical
stable habitats (Scholtz,
2000
). Secondary loss of flight is widespread in the Scarabaeidae
and is polyphyletic within the Scarabaeini
(Harrison, 1999
). Flightless
members of the subfamily, from the genus Pachysoma, occur in the West
Coast deserts of southern Africa, and look superficially similar to C.
bacchus. Convergent evolution of a large, rounded body shape is assumed
to improve water saving in these species
(Chown et al., 1998
), and
discovery of a shared respiratory mechanism would provide strong support for
its role in water retention.
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Acknowledgments |
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References |
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