The Bug Dude presents The Skeptical Entomologist
The Effects of Alcohol on
Oxygen Consumption of Acheta domesticus
ABSTRACT
Chemical
processes are the basis by which all life on earth functions. The set of all
chemical reactions which take place inside of an organism is known as
metabolism. Metabolism can be influenced by several factors, including diet,
activity, the organism’s immediate environment, and exposure to chemicals. To
test the effects of alcohol consumption, this experiment was designed to look
at the effects of alcohol on the nymphs of Acheta
domesticus, better known as the European house cricket. One group was used
as a “sober” control, while two were exposed to alcohol. Crickets from the
three different groups were placed in microrespirometers, which were then
placed inside tepid water baths; a solution of potassium hydroxide (KOH) was
used to determine the rate of respiration from the crickets. Experimental
results suggest that exposure to alcohol does affect metabolic and respiratory rate
of A. domesticus.
INTRODUCTION
Chemical reactions
are essential for life to exist. Metabolism is the collective term for the
chemical changes which occur inside an organism’s cells. It can be broken down
into two distinct processes: catabolic (deconstruction) reactions and anabolic
(construction) reactions. Catabolic reactions, in which complex sugars,
proteins, and lipids are broken down, are often associated and completed with
the use of enzymes. The overall metabolism of an organism is largely dependent
on the nutrients available; on the chemicals which are found in its immediate
vicinity; and on whether the organism produces and maintains a constant
temperature.
Many different
organisms could be used to scientifically investigate the complex relationships
which affect an organism’s metabolism. One of the easiest organisms is the
European house cricket, Acheta domesticus,
which is hardy and easy to breed in captivity. Due to its hardiness and ease of
care, A. domesticus is largely
captive bred for uses in science, as a feeder animal in the pet trade, and,
increasingly, for use in the culinary arts. As European house crickets are
ectothermic invertebrates, their metabolism is highly influenced by their
immediate environment. Crickets raised at lower ambient temperatures have been
observed to require longer intervals in which to reach adulthood when compared
to crickets raised even a few degrees higher; A. domesticus also used more energy in its metabolism (Booth, Kiddel,
2007). Especially relevant to this experiment is the method by which insects
take in atmospheric oxygen; while many other animals have a respiratory system
which includes lungs, insects such as A.
domesticus lack such complex structure. Oxygen is taken in through holes on
the sides of their bodies known as spiracles; these openings lead to an inner
system of trachea and tracheoles, through which gas exchange occurs. Also of
relevance to this experiment is the circulatory system of insects: such animals
have an open circulatory system, in blood sinuses play the role of the
cappilaries found in closed circulatory systems. Also of interest, the “blood”
of arthropods is not actually blood, but rather an analogous substance known as
“hemolymph”, in which hemocyanin acts as a proxy for hemoglobin.
In this
experiment, the focus was the effect of alcohol on the metabolism and
respiration on A. domesticus. Alcohol
is a complex organic molecule (such as C6H10O5),
which produces various effects when consumed; effects range from impaired mental
abilities to increased blood pressure, and induces higher degrees of
detrimental until eventually causing death if sufficiently high quantities are
consumed. The effects of alcohol consumption on cricket nymphs was measured
with a device known as a microrespirometer, a simple device, easily constructed
for an impromptu experiment, which measures the volume of gas inside. (Lee Jr., 1995)
A microrespirometer is
constructed of weights, a syringe, a micropipette, and is relatively simple to
make. While this device can be used to
determine the rate of metabolism, the resultant figures may vary significantly,
and add an amount of error to the experiment. (Van Voorhies., 2008) The experimental null
hypothesis was that there would be no alterations on the respiration and
metabolic rates of A. domesticus. The
alternative hypothesis was that A.
domesticus, when exposed to alcohol, would exhibit altered rates of
respiration as well as metabolism.
MATERIALS AND METHODS
To study the
effects of alcohol on the respiration and metabolism of Acheta domesticus, three separate microrespirometers were
constructed out of a syringe, a micropipette tube, and three metal washers (to
ensure stability while immersed in the water bath). Prior to construction, each
syringe was checked to ensure proper seals. Using a hot glue gun, nine metal
washers were glued together in groups of three. Once the glue had been allowed
to set, each group of three was then glued onto the bottom of a syringe
plunger. Then, three micropipette tubes were glued into the tips of the three
syringes, once again via a hot glue gun. Once the seals had sufficiently dried,
the parts were assembled. The three microrespirometers were ready to be used,
and each one was weighed.
Six Acheta domesticus nymphs were then
obtained, of which three were immediately distributed into one of the three
microrespirometers; the new weight (cricket plus apparatus) was recorded. The remaining
three individuals were placed in a plastic container in which cotton swabs
bathed in alcohol had been placed, thus ensuring the animals would be exposed
to the alcohol without drowning. The microrespirometers were then placed in
baths of tepid water, resting on the three washers. Each cricket was allotted
five minutes to acclimate to the air/water temperature differential. Upon the
conclusion of the acclimation period, a drop of potassium hydroxide (KOH)
solution was added to the tip of the micropipette tube, after which the three A. domesticus underwent observation for
20 minutes. The respiration rate was determined through the movement of the KOH
solution down the micropipette tube; as the animal consumed more atmospheric
oxygen, the KOH solution was pulled down the tube a proportional amount. The
height of the KOH solution was recorded every five minutes (0; 5; 10; 15; 20
minutes) throughout the trial, and the behavior of the A. domesticus was noted as well.
Once the
control/sober trial had reached completion, the micropipette tubes were properly
disposed of, and replaced with fresh ones; the simple machines were then
weighed, as were the original microrespirometers. The sober crickets were then fed
to captive Pacific chorus frogs (Pseudacris regilla). Then, the three
experimental A. domesticus, which had
become “intoxicated” via breathing the alcohol fumes (which had evaporated off
the cotton swabs), were obtained and placed in the fresh microrespirometers.
The cricket/microrespirometer combination was weighed, and the weight of the
cricket determined. The refreshed microrespirometers were then placed, plunger
down, in the tepid water bath, and, once again, the nymphs were allotted five
minutes to acclimate to the ambient temperature of the water. Once acclimated,
one drop of KOH solution was added to each micropipette tube, and, as in the
control/sober trial, the nymphs were observed for a duration of twenty minutes.
Again, the height of the KOH solution was noted every five minutes.
Once the experimental
trial had been completed, the intoxicated A.
domesticus were humanely dealt with. The raw data was compiled and
calculated: the volume of the capillary tube was divided by the length of the capillary
tube (units: µl/mm); the
resultant value was then multiplied by the absolute value of the height differential
(final minus initial, in mm). The resultant value is then divided by the mass
of the cricket, and, finally, that value is divided by the time of the trial
run (in hours; 20 minutes = 1/3 hours). These calculations were carried out for
each microrespirometer in each trial. Once completed, they resulting values
were averaged, and the trials were compared to each other. The experiment was
over.
RESULTS
Once the data had
been calculated, the results were compared. The experiment yielded results
which would be highly unlikely if the null hypothesis were true. The
intoxicated A. domesticus nymphs
exhibited an immensely higher level of oxygen consumption than sober nymphs. Intoxicated
individuals were calculated as consuming oxygen at a rate of 29.19 µl/(g*h), whereas the control
trial averaged a rate of 12.60 µl/(g*h). Exposure to high levels of
concentrated alcohol seemingly caused an increase of over twice the normal
oxygen consumption; as oxygen consumption can be used to determine metabolic
rates of minute arthropods, it can be reasonably stated that the alcohol
affects metabolic process of A.
domesticus, at least while the animals are nymphs.
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DISCUSSION
The
experiment successfully rejected the null hypothesis. The results make it
readily apparent that exposure to alcohol at sufficiently high levels have an
apparent effect on oxygen consumption and, by tentative association, metabolic
rates of A. domesticus nymphs. While
the direct effect on nymph metabolic processes cannot be directly determined
from so simple and impromptu an experiment, it does provide enough information
to warrant further study. Exposure to alcohol certainly increases respiration
rate.
New
Mexico State University scientist have concluded
the measurements taken by such simple instruments to be insufficient in accounting
for all of the complexities which are present in the metabolic processes of an
animal, even as small as a cricket. As such, it is important to remember that
no direct claims regarding the metabolic processes of A. domesticus can be rejected (or fail to be rejected) from this
experiment; this experiment only serves to examine whether or not alcohol
affects the rate of oxygen consumption in such animals. As the results deviated
highly from what would be expected until a normal distribution, it would not be
unreasonable to assert that alcohol consumption does indeed produce observable
results on Acheta domesticus. The
experiment does leave behind several unanswered questions: did exposure to
alcohol induce such effects only because the animals were nymphs, or would
adult A. domesticus exhibit the same
behaviors? Was the rate of respiration due exclusively to the exposure of
alcohol, or was it a combination of “drunkenness” and exposure to the lower
temperature of the water bath? Were the results influenced by the stress and
mild trauma of the experimental processes, such as being caught, being dropped
into a syringe, and so on? Were the respiratory rates directly correlated to
the metabolic processes of the animals, or is it simply an indicator which may
be used as a rule of thumb, without directly attesting to the unseen chemical
processes proceeding within a living cricket? Such questions can only be determined
through more experimentation, using more sensitive technologies, using
different populations of Acheta domesticus.
Also
worth noting is the physical behavior of the intoxicated crickets; those
exposed to alcohol seemed more inclined to behave sluggishly than their sober
counterparts (conversely, one or two drunk individuals were severely
hyperactive when compared to sober animals). This could also be attributed to
the consumption of high concentrations of alcohol fumes; such high levels of a
poisonous gas could have very likely interfered with normal functioning of the
central nervous system. While it is possible to receive such an assortment of
individuals exhibiting these behaviors from a normal population distribution,
the probability of doing so is so small that it is more likely the observed
behaviors were alcohol induced, and not the result of genetic predisposition or
temperament.
REFERENCES
Booth, DT; Kiddell, K. "Temperature and the energetics of
development in the house cricket
(Acheta domesticus)." J
Insect Physiol.
2007 Sep;53(9):950-3. Epub 2007 Mar 30. (http://www.ncbi.nlm.nih.gov.ezp.lib.cwu.edu/pubmed/17481649)
Lee, Jr. Richard A.
"Using microrespirometers to measure O2 consumption by insects
& small invertebrates" The
American Biology Teacher. Vol. 57, No. 5 (May, 1995), pp.284-285
Van
Voorhies, Wayne A.; Melvin, Richard G.; O. Ballard, J. William; Williams,
Joseph B.
"Validation of
manometric microrespirometers for measuring oxygen consumption in small arthropods".
Journal of Insect Physiology Volume 54, Issue 7, July 2008, Pages 1132-1137, ISSN 0022-1910,
10.1016/j.jinsphys.2008.04.022.
(http://www.sciencedirect.com/science/article/pii/S0022191008000863)
