Insects at Low Pressure :
Applications to Artificial Ecosystems
CHARLES COCKELL1*
DAVID CATLING 1
HILARY WAITES 2
1M/S 239-20, NASA Ames Research Center, Moffett Field, CA 94035-1000
2Department of Biological Sciences, Stanford University, Stanford, CA 94305-5020
*Address for correspondence. e-mail : ccockell@mail.arc.nasa.gov, tel : (650) 604 3615,
fax : (650) 604 1088
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Abstract
Insects have a number of potential roles in closed-loop life support systems. In this
study we examined the tolerance of a range of insect orders and life stages to drops in
atmospheric pressure using a terrestrial atmosphere. We found that all insects studied
could tolerate pressures down to 100 mb. No effects on insect respiration were noted
down to 500 mb. Pressure toleration was not dependent on body volume. Our studies
demonstrate that insects are compatible with plants in low pressure closed-loop and
artificial ecosystems. The results also have implications for arthropod colonization and
global distribution on Earth.
Keywords : insects - ecosystems - greenhouse - Mars - subambient - respiration chamber
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Content sentence
Reductions of pressure down to 100 mb can be tolerated by most insects. These
pressures are compatible with the use of insects as a component in low pressure closedloop ecosystems containing plants.
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INTRODUCTION
Insects can perform some functions in closed-loop life support systems at least as well as
automated systems and in many cases better, such as pollination and soil turn over in
non-hydroponic units. They can be used to clean up detritus since some insects are nonspecific in their choice of food. They may provide redundancy for food availability in
case of failure of crop plants. Many species are relatively robust in terms of their
tolerance of environmental conditions and they are not dependent on light. To date little
work has been undertaken on the inclusion of insects as a component of life-support
systems.
Although closed-loop artificial ecosystems can be operated at terrestrial
atmospheric pressures (≈1000 mb), low pressure environments in extraterrestrial
ecosystems are beneficial for a number of reasons. Firstly, low pressures in outer space
or the Martian surface reduce complexity of engineering design since thickness of
chamber and structural support can be reduced (5). This in turn reduces the mass of
material that must either be fabricated in an extraterrestrial environment or removed
from the terrestrial gravity well (2,8,23). Secondly, reductions in internal pressure
reduce the rate of gas and water leakage and thus the quantity of new gas and material
that must be introduced into the system. An ideal low pressure system is one in which
humans can operate with a simple mouth respirator and small gas tank for tending
plants and other components when needed (2), but where pressures are not so low as to
require complex EVA operations. Such a pressure would be around 400 to 500 mb with
oxygen and carbon dioxide partial pressures close to the terrestrial atmosphere. This
pressure might be lower for enriched oxygen and carbon dioxide atmospheres.
Studies on low pressure ecosystems have been focused mainly on plants which
are known to tolerate substantially reduced atmospheric pressures and reduced oxygen
tenions as ambient atmopheric pressures (18). Daunicht and Brinkjans (8) showed that
tomato plant growth and photosynthesis was not significantly reduced at 400 and 700
mb, consistent with early work by Rule and Staby using a wide variety of pressures (16).
They did not observe significant reductions of photosynthesis in tomato plants down to
166 mb. Andre grew rye grass down to 73 mb (1). Other experiments have generally
confirmed that plants can be grown at total pressures between 100 mb and 1000 mb
without detrimental effects, although increased loss of water from transpiration is
generally seen at lower pressures (2,8). Germination of seeds is also possible down to 50
mb (5), although in simulated Martian atmospheres of 95%CO2 and 0.07% O2, at a total
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pressure of 10 mb, seed germination required supplemental oxygen addition (20) which
concurs with previous data on seed germination at subambient pressures and in
extraterrestrial atmospheres (13,17,19,22).
Low pressure effects on insects are less well-known. As well as bizarre
experiments on ducks and kittens, Boyle undertook preliminary studies on caterpillars
and butterflies in a vacuum chamber (6), but his studies were not quantitative. Early
detailed studies on low pressure effects were reported by Woodworth (26,27) who
found that bumble bees could tolerate pressures down to 500 mb without significant
effects on respiration, although respiration was reduced at pressures below 500 mb.
Most insects rely on simple gas diffusion for taking in oxygen and expelling
carbon dioxide. This process is sufficient to deliver the required oxygen levels through
the insect tracheal systems and to prevent carbon dioxide build-up. Both processes
display an ideal interrelationship and thus are equally critical to insect function (14,24).
Other systems exist to supplement or improve efficiency of gas exchange, such as active
ventilation in larger insects via abdomen movements. Ventilation may also be improved
by air sacs — enlarged tracheal areas. Gills and oxygen bubbles are used in aquatic
insects to localize oxygen stores and many insects have the ability to tolerate short
periods of oxygen debt. In studying pressure effects, other factors may also influence
response, such as the habitat of the insects being studied. For example, insects that live
in anaerobic conditions such as wood-boring beetles have respiratory systems adapted
to low oxygen and high carbon dioxide tensions (15).
In this study we examined the effects of low pressure on a wide range of insect
orders and stages to gather preliminary data on pressure effects for closed-loop life
support systems and artificial extraterrestrial ecosystems that may contain an insect
component.
MATERIALS AND METHODS
All experiments were conducted in an environment chamber at the NASA Ames
Research Center. The chamber is a 14" high, 10" diameter stainless steel cylinder with
computer-driven control and datalogging. Pressure was precisely controlled using
automatic solenoid flow controllers and a variable-speed turbomolecular pumping
system. Pressure was monitored using MKS Baratrons to high accuracy
(0.15%Reading) and resolution (0.001 mb in the 0-13 mb range, 0.1 mb at higher
pressures). A 6" diameter glass port on the top of the chamber allowed for insect
observation throughout the experiments.
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In this study we used a terrestrial atmosphere. The effects of different gas blends
will form the subject of a future publication. The chamber bears a resemblance to
chambers that have previously been used to study plants at subambient pressures (e.g.
2,8,21).
Insects were obtained from the wild or from Wards Biology (Rochester, New
York). They were acclimated in laboratory cages for three to seven days prior to the
experiment with food and conditions appropriate for each species. Insects were chosen
with a number of considerations in mind. First, a variety of orders that might be
represented in closed loop systems were studied. Second, insects were chosen with a
variety of body volumes to study whether body volume and thus efficiency of gaseous
diffusion into the tracheal system under reduced atmospheric pressures was a
significant factor for insect selection. Third, insects with a variety of respiration control
abilities (i.e. diffusion control, ventilation control in larger insects, and aquatic gill
systems) were analyzed. Fourth, different life stages were studied in some of the
species.
For each experiment five individuals were placed in a plastic petri dish that were
large enough to allow for insect movement. Holes in the top of the petri dishes allowed
for gas exchange in the chamber. In all experiments a small wad of water-soaked cotton
wool was introduced into the petri dish to allow for a saturating water vapor pressure
during the experiments. It remained wet throughout all the experiments conducted in
this study. For all experiments, gas temperatures were between 23 and 25˚C. For
Odonata spp. nymphs, the nymphs were placed in a beaker containing 50 mL of pond
water and the beaker covered in a permeable layer of cotton wool to allow gas
exchange.
Ambient atmospheric compositions (21% O2, 78%, N2, 0.03%CO2) was used for
experiments. Pressure was dropped from ambient (1,013 mb) in 100 mb decrements to
400 mb when pressures were dropped at 50 mb decrements. When total pressure was at
100 mb, decrements were reduced to 10 mb. At each new pressure, insects were given
several minutes of acclimation period. At the end of the experiments insects were
released.
RESULTS
As pressure was reduced in the chamber most insects showed signs of increased
movement, suggesting irritation due to pressure changes. In almost all insects studied
no significant change in behavior or locomotion was noted down to 500 mb consistent
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with earlier studies on bumble bee respiration (26,27). Below this pressure a sluggish
behavior was noted. Insects would become immobile for a period before acclimating to
the new conditions and would then remain immobile for extended periods of time.
Below 300 mb an improvement was noted in some species. Full mobility and a return to
control type behavior occurred between 300 mb and 200 mb. Further reductions in
pressure below 200 mb were seen to result in reductions of mobility again. At pressures
below 150 mb, larger insects were found to switch to a greater reliance on ventilation
respiration, as evidenced by large and pronounced movements of their abdomens. This
was often accompanied by visible distensions of their abdomens.
For all insects studied the pressure below which complete mobility ceased and
the insects showed evident signs of respiratory shutoff was found to be quite well
defined and reproducible. For example, for Pogonomyrmex spp (harvester ants) this
pressure was 30 mb corresponding to an oxygen partial pressure of 6 mb. We define this
pressure in this paper as the 'critical pressure' referring to the pressure in an artificial
ecosystem below which these insects would cease to function using a terrestrial
atmosphere. Table 1 shows the critical pressure values for the insects studied. Figure 1
illustrates this data as a body volume vs. critical pressure graph.
Insects held above these critical pressures could survive for long periods of time.
Both harvester ants and Gryllus domestica (house crickets) were maintained for 24
hours at 100 mb without major detrimental effects, although they were more sluggish
than laboratory controls. Insects maintained for extended periods (>24 hours) at 200 mb
were found to be largely unaffected by pressure.
Reductions below the critical pressures for most insects could only be tolerated
for short periods of time. Ants reduced to Martian surface pressures (6 mb) could
sustain this pressure for several seconds before a need to be revitalized by a return to
100 mb. They died after ten minutes at 6 mb. Sarcophaga bullatta (blow fly) larvae and
Odonata spp. (dragonfly) nymphs could sustain longer periods (approx. 10 minutes) at
6 to 10mb pressures.
In the case of dragonfly nymphs, temperature of the water dropped as
evaporation increased at lower pressures, which is a potential problem for low pressure
aquatic units. The nymphs we studied were still alive, although they were sluggish at 11
mb when the water began to boil at low (<10˚C ) temperature. The effects of boiling
water and drop in temperature made it difficult for us to ascertain the true critical
pressure of these insects at fixed temperatures.
Dehydration was also a problem at low pressures (<100 mb). Smaller insects
including ants, Drosophila melanogaster (fruit flies) and Trilobium confusum (flour
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beetles) were observed to congregate around the cotton wool and drank more
frequently at pressures below 100 mb.
DISCUSSION
Despite the large number of parameters that may affect gas exchange in different
species of insects, we found that the critical pressures of insects fell within the same
order of magnitude for our experimental conditions i.e. tens of millibar. A lower limit
for the oxygen partial pressure suitable for animal life is the level below which
respiration ceases in comtemporary organisms, known as the 'Pasteur point'. Many
existing organisms adapt from respiration to fermentation below this level. The Pasteur
point is about 1% of the present atmospheric level (PAL) of oxygen, equivalent to 2.1 mb
partial pressure. Indeed, Berkner and Marshall (4) have suggested that the beginning of
the Cambrian in the geological record and the appearance of animals was linked to the
level of oxygen reaching the Pasteur point. The Pasteur point for oxygen would be
equivalent to a total atmospheric pressure in our experiments of about 10 mb. Thus our
results are consistent with this hypothesized lower limit.
For all insects studied, pressures down to 500 mb had no effect on insect
function, which is consistent with the earlier data on bumble bees (26,27) and cadelle
beetles (12). Given that mean atmospheric pressure at the summit of Mt. Everest is 400
mb, our data also suggests that atmospheric pressure in all high altitude regions on the
earth should not be a limiting ecological factor for insects, although other factors such as
vegetation and temperature may well be.
As a peripheral observation to the focus of this paper, the data also provides a
further demonstration of the potential for insects to be globally distributed by winds at
high altitude. This also applies to arachnids which have similar respiration systems. The
critical pressures that we measured for many of these insects correspond to an altitude
of approximately 20 kms. After the eruption of Krakatau in 1883 in which the island
became devoid of life, spiders were the first invertebrates to repopulate the islands of
Rakata and Anak Krakatau (the remnants of Krakatau), presumably by wind currents
(25). 'Ballooning spiders' are known to be capable of flying in the wind like kites, using
threads attached to vegetation. When the wind becomes strong enough the threads
break propelling the spiders to high altitudes, sometimes several km's. High altitude
wind dispersal probably also occurs for insects picked up in wind currents and in
storms. Our work demonstrates that pressure effects would certainly not be a limiting
factor for high altitude arthropod migrations at heights up to 15 kms (100 mb) in the
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troposphere. Insects could theoretically survive pressures up to 25 km (about 25 mb) but
here the coldness, aridity and ozone concentration of the stratosphere would likely be
limiting. Reductions in temperature to below freezing will become a negative factor for
many insects at these altitudes, although reduction of respiration by lowered
temperatures may also improve survivability during transit at low pressures for some
insects.
Below 500mb the insects we studied showed a reduction in locomotor efficiency
which then improved to control levels of movement below 300 mb. This is consistent
with improved respiration noted by Woodworth in bumble bees at below 300 mb (27).
This improvement in respiration may be due to complete opening of the spiracles at low
pressures to allow for free gas exchange, as opposed to higher pressures, where spiracle
bursts and fluttering that are not completely adjusted to the new pressure conditions
may be inhibiting respiration.
Most insects could tolerate pressures down to 100 mb, which is equivalent to a
partial oxygen pressure of 21 mb and a carbon dioxide partial pressure of 0.03 mb using
an ambient terrestrial atmosphere. Insects left for extended periods of time at 100 mb
were found to suffer some locomotor deficiency and at these pressures dehydration
through loss of water through the spiracles may become a major factor. Similar loss of
water through increased transpiration has been observed in plants at less than 400mb
(8). Although we have defined the critical pressure as the pressure at which the insects
no longer functioned, most insects showed signs of locomotor inefficiency at pressures
slightly higher than the critical pressure. In general, insects at or above 150 mb seemed
to function without major limitations.
In some larger insects such as cockroaches, the critical pressure was found to be
comparatively low despite the fact that they have relatively large body volumes to
supply with oxygen. This may be explained by the fact that active ventilation in larger
insects probably overcomes the theoretical diffusion problem they encounter at low
pressures. We found no well-defined relationship between body size and critical
pressure but larger insects with active ventilation systems in general fared better than
smaller insects relying on diffusion. The observation of large abdomen movements and
greatly distended abdomens in larger insects at pressures below 150 mb supports this. It
also suggests that although large insects may survive below 150 mb, they are under
considerable respiratory stress. For large insects in a reduced pressure closed-loop
ecosystem, an optimum is not only above the critical pressure, but also above the point
at which greatly increased ventilation respiration is required (approx. 150 mb).
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Insects taken to pressures lower than their critical pressures died within ten
minutes, but could be revived if taken back to higher pressures in less than one minute,
suggesting that at very low pressures many insects can tolerate a short period oxygen
debt. Some insects could survive these lower pressures for longer (between five and ten
minutes) . These included the blow fly larvae, which naturally inhabit anaerobic
environments in dead meat and have plastron respiration structures which allow for a
larger area of gas exchange across a water/air interface and the dragonfly nymphs,
which have gill systems like other aquatic insects (11). This is consistant with the
generally lower critical pressures of these types of insects. The observations also show
that depressurization to below 30mb in a closed loop system in space or on the Martian
surface would kill most adult and larval stage insects, although locust eggs have been
reported to tolerate a complete vacuum for short periods of time (3).
Aside from respiration problems, dehydration is a complicating factor at low
pressures when spiracle control of water retention is lost. Below 150 mb all small insects
we studied (fruit flies, flour beetles, harvester ants) congregated around the wet cotton
wool in the petri dish and drinking rates increased. We did not observe a similar
problem in larger insects and this may be partly due to the larger surface area to volume
ratio in smaller insects in which dehydration may be a greater problem. These results
concur with earlier results obtained on Tenebriodes mauritanicus at pressures of 40 mb
at which loss of water was noted (9,12). Experiments conducted without wet cotton
wool in the dishes in which the ambient humidity was lower did not result in
significantly changes to critical pressure over short time periods. However, it is known
that ambient humidity does affect longer-term water loss from insects (9,14) and thus in
a low pressure ecosystem water supplies to the insects must be sufficient.
In closed-loop systems, CO2 partial pressures may be increased to stimulate crop
growth (2), for example a 200 mb atmosphere using terrestrial gas compositions, but
with a 1.8 mb CO2 partial pressure enrichment has previously been suggested. Some
insects may tolerate high CO2 concentrations such as the tetse fly which at ambient
pressures may tolerate greater than 10% carbon dioxide at ambient 21% oxygen levels (a
1:2 ratio) (7). For insects with more anaerobic lifestyles (e.g. flesh beetles) higher than
ambient CO2 concentrations in closed-loop systems will not cause a significant
reduction in function, particularly where the total pressures are significantly above the
critical pressures. Provided total pressures are not close to critical pressures, most
insects would survive in a partially enriched CO2 environment.
In order to compensate for such increases in CO2 and reductions in pressure,
atmospheres may be enriched with O2. Monro et al., (12) found that respiration of the
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grain-infesting cadelle beetle, Tenebroides mauritanicus was just over 1.5 times greater
in a 46 mb pure oxygen atmosphere than a 46 mb air atmosphere, demonstrating
mitigation of low pressure (and thus also carbon dioxide) effects by oxygen enrichment.
However, it is important to note that respiration of insects in enriched low pressure
oxygen atmospheres is still lower than the respiration in air at ambient atmospheric
pressures with the equivalent oxygen partial pressures. This result has now been
confirmed for a number of pest insects (9,10,12). Clearly respiration is reduced in
response to drop in pressure per se, as well as reductions in oxygen partial pressure. The
most likely explanation is that drops in pressure (below about 500 mb) result in reduced
respiration in order to curtail water loss. Oxygen enrichment may therefore be used to
improve respiration, but probably not to completely counter the effects of low pressure.
In general therefore, higher carbon dioxide levels and pressures above 200 mb make
plants and insects compatible in closed loop systems in terms of atmospheric
considerations. A compromise can be quite easily reached. Further direct studies on the
combination of low pressures and differing gas mixtures are merited.
As well as direct studies on the insects, we also observed that low pressures did
not adversely affect insect behavior and development. At pressures below 300 mb,
mating behavior in the milk weed bug was unaffected and ovulation in the garden
cricket was normal. American cockroaches also laid egg cases normally. Pupae of blow
flies hatched normally after being sustained at 200 mb for 24 hours. It should be noted
that depending on size of insect, low pressures may affect flight efficiency. This will be a
factor for closed-loop ecosystems using winged insects such as flies and bees. On the
surface of Mars of course the 0.375 g gravity regime may partially compensate for loss of
flight efficiency as a result of a low pressure atmosphere, although exact dynamics of
flight may be changed. Further effects on long term effects of pressure on insect
behavior and development and processes such as metamorphosis are merited.
Finally, the results we have obtained have relevance to the speculations on the
terraforming of Mars - the transformation of Mars into an earth-like world by planetary
scale atmospheric engineering. We suggest here that total atmospheric pressures would
need to be at least 200 mb to prevent extreme desiccation in an insect component in a
newly established Martian biota. Oxygen partial pressures should be at least ~10 mb or
higher at any given total Martian atmospheric pressure to sustain insect life with this
requirement rising at higher CO2 concentrations.
CONCLUSIONS
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This initial work shows that insects, like plants, can survive low pressure closedloop ecosystems. Reductions of pressure that can be tolerated by plants can be tolerated
by a wide variety of insects and insect life stages. Although many insects possess
different respiratory capabilities, most insects can tolerate pressures down to 100 mb
and will operate between 200 mb and 1 bar without major loss of function. Between 500
mb and 1000 mb the insects we studied showed no pressure effects at all. Insect body
size is not related to pressure tolerance in a simple way because the mode of respiration
and natural environments of the insects are important in determining the critical
pressure at which they cease to function.
Closed loop ecosystems may have differing gas compositions depending on their
construction and location. For this reason, data on insects at a given pressure in
combination with the specific gas mixture, particularly with respect to combined effects
of CO2 and O2 partial pressures, may need to be reexamined for specific designs. The
chamber used in our experiments has ports for precise feed of gases of predetermined
composition into the chamber using a gas blending apparatus (Environics Inc.) and
these studies will form the subject of a later publication. Given the similarity in critical
pressures in many of the insects and their life stages found here, insects can easily be reexamined for new gas mixtures. Just a few adults can be used as model systems with
wide applicability to other insect orders, species and life stages. As useful model
systems with wide availability and a great deal of ancillary literature, the American
cockroach (Periplaneta americana) and fruit fly (Drosophila melanogaster) which span
the range from small insects to large insects employing active ventilation are
recommended. Odonata nymphs could be used as representatives of aquatic insects.
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Acknowledgements
The authors wish to thank the NASA Ames Research Center DDF program through
which this work was supported.
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BIOGRAPHICAL SKETCHES
Charles Cockell received a D.Phil in Molecular Biology from the University of Oxford in
1994. He is currently a Research Scientist at the NASA Ames Research Center
specializing in UV radiation studies and studies of life in extreme environments.
Specifically, previous insect research has focused on undertaking moth collections from
locations such as Indonesia and Mongolia and studies on behavioral adaptations in the
praying mantis.
David Catling received a D.Phil. in Atmospheric Physics from the University of
Oxford in 1994. He presently holds a National Research Council Associateship at NASA
Ames Research Center. His research interests are in planetary climatology, microsensor
development for planetary exploration, and the reciprocal interaction of lifeforms with
planetary atmospheres. Currently, his particular research focus is on studies of the
Martian climate, theoretical and experimental.
Hilary Waites is an undergraduate at Stanford University, where she is a Biology
major. During her undergraduate career she has spent several terms in residence at
Stanford's Hopkins Marine Station, where she has conducted subtidal research
investigating the effects of otter predation on a snail population and a study of enzyme
adaptation in congeneric snails distributed along a temperature gradient. Ms. Waites
contributed to the project during a volunteer summer internship at NASA-Ames
Research Center.
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