Talk:Porcellio scaber
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[edit] Roly-Poly
In the South, we also refer to these as roly-polies because they roll up into a perfect little ball when uncovered.
[edit] Text dump of high-school level assignment about P. scaber
Released under the GPL for editing in the article. Ppe42 / L Howison, Wellington, New Zealand.
Taxonomy and general facts should be useful, some original research not so much.
Animal Study of Porcellio scaber
�Taxonomy
Kingdom Animalia Phylum Arthropoda Sub-Phylum Crustacea Class Malacostraca Order Isopoda Sub-Order Oniscoidea Family Porcellionidae Genus Porcellio Species Scaber
Alternative Taxonomy:
Kingdom Animalia Phylum Arthropoda Class Crustacea Order Isopoda Sub-Order Oniscoidea Series Ligienne Family Porcellionidae Genus Porcellio Species Scaber
Hierarchical Characteristics
Kingdom Animalia Animals are similar to plants in that they consist of a colony of specialised cells. They differ from plants because the cells do not have chloroplasts for photosynthesis or cellulose walls. Animals are heterotrophs (feed off other organisms). Animals are motile, with a few exceptions such as the sponges.
Phylum Arthropoda This is the largest phylum of Animalia. Arthropod bodies are divided into distinct head and body regions. Arthropods have jointed, paired legs and a chitin cuticle. This exoskeleton must be shed periodically, or moulted, for the arthropod to grow.
Sub-Phylum Crustacea Crustaceans have gills, which restrict them to aquatic habitats. A few crustaceans have adapted to non-aquatic habitats, but these must be very humid habitats. Crustacean body structure allows some species to grow larger than species in other arthropod classes (Arachnida and Insecta). They have at least two sets of antenna, although in some species the second pair may be atrophied and hard to observe.
Class Malacostraca
This is a group of crustaceans, delineated by having an 8-segmented thorax and 6 or 7 segments of the abdomen. Female Malacostra have reproductive organs on the 6th thoraic segment, while males have reproductive organs on the 8th thoraic segment. There is generally a pair of legs on each segment, giving 8 legs attached to the thorax and 6 legs attached to the abdomen.
Order Isopoda
Most members of the order Isopoda are aquatic forms that do not swim as other orders of Crustacea, but have a bottom-living or parasitic existence. They have seven pairs of thoraic limbs, which are used for walking rather than swimming. These limbs tend to be similar in structure. This leads to the Isopoda name – isos, Greek for ‘equal’, and podes, meaning feet. The first abdominal segment is fused with the head, and the next seven thorax segments each have a pair of legs, called pereopods. The abdomen has five pairs of pleopods, which the aquatic species use for swimming, and these are sometimes also used for gas exchange. Many Isopoda have dorsally flattened bodies.
Sub-Order Oniscoidea These are terrestrial isopods, living in damp conditions. It is thought they evolved from aquatic forms, which gradually became adapted to periods of terrestrial living on the seashore.
Family Porcellionidae These are woodlice that have an exoskeleton that extends over their legs (an overhang). Their uropods are spear shaped and flattened, and extend beyond the telson (fused tail segments). Despite their appearance, these woodlice cannot roll into a ball.
Genus Porcellio The heads of Porcellio have tri-lobed heads: two definite outer lobes and a central, almost triangular lobe.
Species scaber This species is generally slate grey all over (apart from variations due to moulting and age) with the head being the same colour as the body, while some other species have quite interesting colour schemes. The head and back (dorsal surface) of Porcellio scaber is covered in small wart-like nodules. Also, Porcellio scaber lacks a median stripe on its back.
�Introduction: General Woodlice Information
The common woodlouse, Porcellio scaber, is a small terrestrial crustacean that mostly inhabits terrestrial soil, helping decompose plants and animal matter.
Although woodlice resemble insects, being a small animal with segmented legs and a rigid exoskeleton, it is not an insect. One difference is in the seven pairs of legs (insects have six). Also, woodlice breathe through modified gills (thanks to their aquatic crustacean ancestors) whereas insects use trachea to transport oxygen directly to the tissues.
Woodlice move about on their legs (pereiopods) which are spread far apart under the body, giving them great stability. The legs are also quite long to give the animal speed while running. They can move directionally (taxis) for some purposes, and wander randomly (kinesis) for others.
Woodlice reproduce sexually, with the male fertilising the females eggs. The young grow in a brood pouch, growing in an egg-like structure for about a month, then hatching out and remaining in the brood pouch for a few days. After they are born the woodlice are capable of looking after themselves, but are not fully mature until they moult twice and gain the full set of segments and legs. Thereafter they must moult periodically to shed the rigid exoskeleton so they can grow larger.
Woodlice have various sensing organs: the compound eyes, movement and chemical sensors, and humidity sensors, possibly on the antenna. As woodlice are somewhat obscure, they have not yet been extensively studied and some sensor-like structures have not yet been identified.
Woodlice eat decaying plant matter, fungal spores, live plants, and their own faeces. Their liking for very young plants has led to them being a nuisance in some nurseries, but this is the only occasional when woodlice are pests, and they are easily controlled by pesticides as they do not develop resistance easily.
Undigested food is excreted in the form of faeces, and nitrogenous waste as ammonia gas, which the animals have a high resistance to. Although water is saved from urine, it is used by the permeable cuticle (required for ammonia excretion).
�Life Cycle
The woodlice life cycle is relatively simple because the animal does not require any parental care. After conception it develops from an embryo into a young woodlouse, which can live on its own. Two moults later it is mature and ready to start the life cycle again.
Mating
When a male woodlouse comes across a receptive female, he stops, waves his antenna and then places them on the female. If she does not turn away he climbs onto her back and licks her head with his mouthpieces while tapping her back with his front legs. This goes on for several minutes. Then the male shifts diagonally across the female’s back, bends his tail under her and transfers sperm from his stylets (part of his genital pores on the pleopods) into her genital pore. After a few minutes this is complete and he can turn and repeat the operation on the other side.
Conception
The female has formed a brood pouch during the moulting prior to mating. The brood pouch is a fluid-filled space made between the females belly and a layer of overlapping plates called oostegites. Inside the female’s genital pores the eggs are fertilised by the sperm, and then pass through oviducts into the brood pouch.
Embryology
The eggs are initially a mass of yolk inside two membranes. The actual cells divide and grow from 1 to 2, to 4, to 8, to 16 cells over a few days, while migrating to the outside of the yolk mass. From here the mass of cells begins to differentiate and show signs of segmentation. By the 17th day, buds of limbs are present and organs have begun to form. The embryo has by now gradually increased in size until it bursts one of the membranes. The rest of the yolk passes into the digestive system and the head begins to form. By the 21st day, the body can move slowly. The woodlice embryo straightens out inside the membrane so it can grow larger. By the 26th day, the woodlice embryo has darkened, indicating that the cuticle has formed. The yolk has run out and the eyes have become pigmented. Movement increases until the membrane ruptures. The young now live inside the brood pouch for several days, and fluid in the pouch gradually disappears.
Birth
When the young are ready they simply crawl out of the brood pouch.
Maturation
At first the young woodlice have only 6 thorax segments, but after their first moult they have seven. After the second moult this segment also has legs, and the woodlice is mature. Subsequent moultings do not change its physiology.
�Special Features of Porcellio scaber
A woodlouse’s modified gills lose water very quickly, as does the rest of the body surface. Also, their faeces are quite watery. If the creature dries out it cannot function properly and will die. For this reason, they have evolved several behaviours that ensure they can acquire more water and inhabit only sufficiently moist environments that can prevent them from drying out (desiccation). Firstly they consume water through food and drinking, and by anal drinking, which is actually capillary action. This involves pressing the uropods against a moist substrate; water moves by capillary action through channels to wet the underside of the woodlouses body. Also, water may be absorbed through the cuticle if the animal is immersed in water, as in rain. Also, they prefer dark moist crevices, which both shelter them from the sun and protect the woodlice from predators. Shelter from the sun and its associated warmth is important; at a similar humidity, thirty degrees has a much greater drying action than ten degrees. Conversely, woodlice can also be too wet: saturated with water. This could occur by prolonged exposure to a very moist environment, or in rainy conditions. Woodlice in these conditions will seek a dry environment. They can lose water by reverse capillary action (anal drinking); water can pass out of the body if the substrate is dry enough. Also, the woodlice may be able to add water to the gut contents to increase water in the faeces.
The permeability of the cuticle, which restricts the animal to damp environments, may be necessary because of the excretion of ammonia gas through the cuticle. The advantage is probably due to the energy saved by not converting the ammonia into urea or uric acid.
Woodlice must moult because their hard exoskeleton, which provides protection and waterproofing to some extent, is not flexible enough to grow with them. Therefore the woodlice must periodically shed this exoskeleton. A few days before moulting, the woodlice becomes inactive and stops feeding. When the rear half of the body is ready, the skin splits and the animal pulls itself out using the front legs. A few days later the same process occurs with the front half. Often the cast skin is eaten. The woodlice can now grow before its new exoskeleton hardens. In captivity, they may be subject to cannibalism by other woodlice while moulting, but his probably does not occur in the wild.
Woodlice eat decaying plant matter, fungal spores, live plants, and their own faeces. Their liking for very young plants has led to them being a nuisance in some nurseries, but this is the only occasional when woodlice are pests, and they are easily controlled by pesticides as they do not develop resistance easily.
Woodlice have a very large requirement for copper which is due to their blood pigment hemocyanin which contains copper just as haemoglobin contains iron. Curiously, woodlice seem to require even more than is necessary for that purpose. It seems that they eat their own faeces because, although their food contains plenty of copper, it is not digestible until changed into a digestible form by bacteria in the faeces.
�Investigation Introduction and Planning
The Phototaxis and Orthokinesis investigations were previously specified. The two unspecified investigations were designed with two important observations in mind: that woodlice inhabit crevices between objects and are found both close to ground level and above ground level (between concrete blocks surrounding the compost heap).
Therefore, when the geokinesis investigation was designed, it was not known whether the woodlice would prefer the ‘down’ or lower chamber, or the ‘up’, higher chamber. The hypothesis was formulated as “The woodlice will prefer the lower chamber” because if woodlice persistently climbed upwards, they would be found up trees and on top on objects – places where they have not been observed.
The thigmokinesis investigation was more difficult, however. Research indicated that woodlice had a thigmokinesis response, which drove them to inhabit cracks and crevices which came into contact with more of their body. But a choice chamber with a crevice or crack, and another area without one, was difficult to construct because observation of the woodlice in at least one of the chambers was essential to the gathering of results. In other words, a simple piece of cardboard to lower the roof and form a crevice was not suitable because if both chambers were dark (only one variable can be tested during each experiment) it would be impossible to observe the woodlice and keep results. Therefore, three different types of crevices were tried throughout the investigation, and they proved to be roughly equivalent in effectiveness.
�Phototaxis Investigation
Aim and Introduction: To investigate the phototaxis response of Porcellio scaber in a light-dark choice chamber. Continuous data in the form of observations of the number of slaters on each side over time will be gathered by observing the slaters over a fifteen-minute period in the choice chamber. Ten slaters will be used as this is an easily handled and counted number and a large enough sample size for statistical analysis. The use of a choice chamber precludes the use of a control.
Variables: Tested: Light strength. Constant: Substrate type, humidity, slope (0°), temperature, chemical environment.
Hypothesis: The slaters will exhibit a negative phototaxis by spending more time on the dark side of the choice chamber.
Null-hypothesis: The slaters will show no phototactic response and have no preference for either side of the choice chamber.
Equipment: 1 Petri-dish choice chamber, 10 slaters, 1 clock or stopwatch, a circular black cardboard cover large enough to cover one side of the choice chamber, a strongly lit area large enough to contain the choice chamber (such as a sunlit bench), lengths of sellotape, a paper and pen (for recording results).
Method: Prepare the choice chamber by affixing it firmly to a flat surface. Tape the black cardboard over one side of the chamber so that it becomes dark. Place the choice chamber in a strongly lit area, for example sunlight from a window. Place the ten slaters into the lighted side of the choice chamber. Wait for five minutes as the slaters explore the choice chamber, then count and record how many are on the light side. Count and record the number of slaters on the light side every minute for another ten minutes. Remove the slaters from the choice chamber and place them back into their container. Repeat the experiment described by steps 1 to 5, three times. Analyse the data collected by graphing and statistical analysis.
Results, Experiment 1:
Phototaxis Results 1
Slaters: 10
Time Number Number (minutes) in light in dark 0-4 (exploring) 5 5 5 6 7 3 7 4 6 8 6 4 9 5 5 10 4 6 11 6 4 12 5 5 13 2 8 14 4 6 15 2 8 Total 50 60 Percent 45.5 54.5
This experiment went relatively well, except that the black paper on the dark side of the choice chamber was not stuck down firmly and some cracks permitted light into the dark chamber. The graph shows that over time, the slaters explored the choice chamber and eventually began to stay in the dark side.
The ‘Total’ and ‘Percent’ figures show the total number of slaters observed on each side during the whole experiment, as both numbers and percentages of the whole. This figure was used for statistical analysis.
Statistical Analysis, Experiment 1:
The null hypothesis states that the light and dark will not affect the slaters, that is, they will wander at random. The expected proportion of slaters would then be 50% on either side. This means that the expected Standard Error (SE) for ten slaters is:
SE = = 0.158
The difference between the null-hypothesis prediction (50% on each side) and the observed percentages (45.5% and 54.5%) is 0.045 (0.545 – 0.5)
To find ‘t’ the formula is: Difference from Expected Ratio Standard Error
‘t’ is therefore 0.045 = 0.285 0.158
The ‘t’ tables show that this has a probability of occurring randomly of more than 20%, so the overall result of this experiment is that it is not statistically significant. To be significant the probability should be below 5% and preferably below 1%
Discussion, Experiment 1:
Although the graph shows that at the end of fifteen minutes, the slaters showed a preference for the dark side of the choice chamber, the statistical analysis shows the overall results to be insignificant. This is a problem with the overall analysis: it does not show trends over time like the graph. The probable reason that the slaters did not show a very definite preference for the dark side is that the black cardboard covering the dark side was not securely taped down. The cracks permitted light into the dark chamber. Also, the light side was not in bright sunlight, but simply room lights. This meant that the difference in light strength between the two chambers was not very strong, and so the slaters did not exhibit a very strong phototactic response.
�Results, Experiment 2:
Phototaxis Results 2 Slaters: 9 (see text)
Time Number Number (minutes) in light in dark 0-4 (exploring) 5 2 7 6 1 8 7 3 6 8 1 8 9 0 9 10 1 8 11 2 7 12 1 8 13 0 9 14 1 8 15 1 8 Total 13 86 Percent 13.1 86.9
This experiment went very well, as the results obviously support the hypothesis. The graph shows that as soon as results began to be recorded, the slaters preferred the dark side of the choice chamber. While the slaters were being transferred to the chamber, one was lost. This was not discovered until after the experiment was finished.
Statistical Analysis, Experiment 2: Expected Standard Error (SE) for nine slaters = SE = = 0.167
The difference between the null-hypothesis prediction (50% on each side) and the observed percentages (13.1% and 86.9%) is 0.369 (0.869 – 0.5)
‘t’ is therefore 0.369 = 2.21 0.167
‘t’ tables show that this has a probability of just over 5% of occurring randomly. This is an acceptable value for significance. However this significance is a little doubtful as, to be really sure, the probability should be under 5%.
Discussion, Experiment 2: The graph shows that the slaters exhibited a preference for the dark side all through the experiment, and the statistical analysis shows the overall results to be significant. The doubt introduced by the statistical analysis is again probably due to the black cardboard, which was very difficult to tape down acceptably. Once again, the light strength gradient was not very pronounced, so the slaters did not exhibit a very strong phototactic response.
�Results, Experiment 3:
Phototaxis Results 3 Slaters: 8 (see text)
Time Number Number (minutes) in light in dark 0-4 (exploring) 5 1 7 6 3 5 7 2 6 8 1 7 9 1 7 10 0 8 11 0 8 12 0 8 13 1 7 14 0 8 15 0 8 Total 9 79 Percent 10.2 89.8
This last experiment was a success, as the graph shows that from the start of the experiment, the slaters preferred the dark side of the choice chamber. At the end of the experiment all the slaters were on the dark side. However, only eight slaters were counted during the experiment because while the slaters were being transferred to the chamber, one was lost. This was not discovered until partway through the experiment. Also, one of the slaters seemed to be incapacitated from the start: it turned on its back in the light side and did not seem to be able to move. It was also removed from the results.
Statistical Analysis, Experiment 3: Expected Standard Error (SE) for eight slaters = SE = = 0.177
The difference between the null-hypothesis prediction (50% on each side) and the observed percentages (10.2% and 89.8%) is 0.398 (0.898 – 0.5)
‘t’ is therefore 0.398 = 2.25 0.177
Referring to ‘t’ tables shows that the observed result has a probability of just over 5% of occurring by chance. This is acceptable to show that the results are significant. However the significance is a little uncertain as, to be really sure, the probability should be under 5%.
Discussion, Experiment 3: The graph shows that the slaters exhibited a preference for the dark side all through the experiment, and the statistical analysis shows the overall results to be significant. There was no problem with the light strength gradient in this experiment as the black cardboard was taped down securely and the choice chamber was placed in strong sunlight. Although the overall statical analysis was not conclusively significant, the graph showed that at several points in the experiment all the slaters on the dark side. This shows their strong negative phototaxis.
�Combined Statistical Analysis: Overall, 10 + 9 + 8 slaters were used. The expected Standard Error (SE) for 27 slaters is: SE = = 0.096
The combined percentages are: Light side: 24.2% Dark side: 76.8%
The difference between the null-hypothesis prediction (50% on each side) and the observed percentages is 0.268 (0.768 – 0.5)
‘t’ is therefore 0.268 = 2.79 0.096
The ‘t’ tables show that this has a probability of occurring randomly of much less than 1%, so the overall result of the entire investigation is that the phototactic effect is definitely statistically significant.
General Discussion:
The statistical analysis of the individual experiments has a small uncertainty because of the small numbers of slaters used. Collective analysis for all the experiments shows that the experiments are definitely significant.
These experiments all showed that slaters are negatively phototactic. This means that they move away from bright light and prefer dark environments. The response is affected by the strength of the photo-gradient – if there is a large difference between the dark and light areas or not. When the difference is pronounced, the phototaxis response is stronger.
The investigation demonstrates an important behavioural response and how it can be helpful to the slaters for exploiting their niche. The negatively phototactic behaviour drives them into dark shelters during the day, which helps prevent desiccation (drying out) and also hides the slaters from predators. At night, the difference in light strength inside and outside the shelter is small enough that the phototactic response is overcome and the slaters can emerge from their shelter to feed.
Problems: The problem of the black cardboard, which admitted some light around the edges, was solved during the third experiment by using a lot of tape to ensure it was stuck down firmly. Another problem with this experiment could have been the tapping used to discourage the slaters from stopping. This is unlikely to have affected results since the tapping was used only occasionally and it tended to alert all the slaters present. One more problem could have been the way the slaters were placed in the choice chamber. Because the black cardboard covered the entrance hole of the dark side, the slaters had to be placed into the light side. This is unlikely to have affected results, however, because the slaters immediately explored all parts of the choice chamber. Airflow in and out of the hole on the light side only is also unlikely to have been a factor in the slaters preference for sides since there was very little airflow.
In future experiments, it is recommended that all ten of the slaters should be placed into the choice chamber by putting five on each side, and then covering both holes with sellotape before placing the black cardboard cover over the dark side. Then the chamber should be placed in a bright light such as sunlight. Also, tapping the slaters should be avoided as it is not really necessary. Finally, to be very accurate, the experiments should be repeated ten times, rather than the three times as in this investigation.
Conclusion:
In conclusion, the combined statistical analysis shows that the phototactic effect is definitely present.
The results show that the slaters prefer the dark side to the light side and this supports the hypothesis and disproves the null-hypothesis.
The response is helpful to the slaters because they seek shelter during the day, hiding from the hot sun and predators. At night, when there is very little light, the phototaxis response does not occur, so the slaters emerge to feed.
�Orthokinesis Investigation
Aim and Introduction: To investigate the orthokinesis response of Porcellio scaber in a humid-dry choice chamber. Continuous data in the form of observations of the number of slaters on each side over time will be gathered by observing the slaters over a fifteen-minute period in the choice chamber. Ten slaters will be used as this is an easily counted and handled number, and a sufficiently large sample size for statistical analysis. The choice chamber means that a control is unnecessary.
Variables: Tested: Humidity. Constant: Substrate type, light strength, slope (0°), temperature, chemical environment.
Hypothesis: That the slaters will exhibit an orthokinetic behaviour by spending more time on the humid side of the choice chamber.
Null-hypothesis: That the woodlice will show no orthokinetic behaviour and have no preference for either side of the choice chamber.
Equipment: 1 Petri-dish choice chamber, two extra Petri dishes, metal grille, approximately 5 grams of granulated calcium chloride, lukewarm running water, a small piece of paper towel, 10 slaters, 1 clock or stopwatch, lengths of sellotape, a paper and pen (for recording results).
Method: Place a thin layer (2-3 mm) of calcium chloride in the bottom of one of the extra Petri dishes. The calcium chloride will absorb water vapour and make the surrounding air less humid. Soak the piece of paper towel in room-temperature water and place into the other Petri dish. Assemble the choice chamber by placing the water and calcium chloride dishes side-by-side and placing the metal grille on top. Then tape the choice-chamber Petri dishes on top, aligned with the Petri dishes below the grille. Ensure there are no cracks underneath the apparatus where the slaters can escape. Seal the entrance holes with sellotape and wait ten minutes for a humidity gradient to build up. Remove the entrance-sealing tape and place the ten slaters quickly into the humid side of the choice chamber. Re-seal the entrance holes and wait for five minutes as the slaters explore the choice chamber, then count and record how many are on the humid side and how many are on the dry side. Count and record the number of slaters on the humid and dry sides every minute for another ten minutes. Remove the slaters from the choice chamber and place them back into their container. Repeat the experiment described by steps 1 to 7, three times. Analyse the data collected by graphing and statistical analysis.
�Results, Experiment 1:
Orthokinesis Results 1
Slaters: 10
Time Number Number (minutes) in wet in dry 0-4 (exploring) 5 2 8 6 3 7 7 3 7 8 6 4 9 5 5 10 4 6 11 2 8 12 5 5 13 3 7 14 4 6 15 3 7 Total 40 70 Percent 36.4 63.6
This first experiment was not successful. The slaters spent a lot more time on the dry side then the wet side throughout the experiment. However, there is a good explanation for this. The slaters for this round of the experiment were being kept in a very humid container, so they may have been over-saturated with moisture and therefore preferred the dry side.
Statistical Analysis, Experiment 1:
The null hypothesis states that the humid and dry sides will not affect the slaters, that is, they will wander at random. The expected proportion of slaters would then be 50% on either side. This means that the expected Standard Error (SE) for ten slaters is: SE = = 0.158
The difference between the null-hypothesis prediction (50% on each side) and the observed percentages (36.4% and 63.6%) is 0.136 (0.636 – 0.5)
To find ‘t’ the formula is: Difference from Expected Ratio Standard Error
‘t’ is therefore 0.136 = 0.861 0.158
The ‘t’ tables show that this has a probability of occurring randomly of more than 20%, so the overall result of this experiment is that it is not statistically significant. To be significant the probability should be below 5% and preferably below 1%.
Discussion, Experiment 1
Even the apparent preference of the woodlice for the dry side is not statistically significant. However, their prior over-saturation is a likely explanation.
�Results, Experiment 2:
Orthokinesis Results 2
Slaters: 10
Time Number Number (minutes) in wet in dry 0-4 (exploring) 5 2 8 6 4 6 7 3 7 8 1 9 9 3 7 10 8 2 11 5 5 12 6 4 13 6 4 14 8 2 15 3 7 Total 49 61 Percent 44.5 55.5
This experiment was also a failure. The results seem to support the null-hypothesis because the slaters spent an almost equal amount of time on each side.
Statistical Analysis, Experiment 2: Expected Standard Error (SE) for ten slaters = SE = = 0.158
The difference between the null-hypothesis prediction (50% on each side) and the observed percentages (44.5% and 55.5%) is 0.055 (0.555 – 0.5)
‘t’ is therefore 0.055 = 0.348 0.158
‘t’ tables show that this has a probability of more than 40% of occurring randomly, so the results are not statistically significant.
Discussion, Experiment 2
These results support the null-hypothesis. Once again, however, the lack of orthokinetic response can perhaps be explained by the extremely humid conditions of the slaters accommodation prior to the experiment.
�Results, Experiment 3:
Orthokinesis Results 3
Slaters: 10
Time Number Number (minutes) in wet in dry 0-4 (exploring) 5 5 5 6 3 7 7 1 9 8 3 7 9 3 7 10 4 6 11 6 4 12 7 3 13 5 5 14 8 2 15 7 3 Total 52 58 Percent 47.3 52.7
Again, the results are disappointing and support the null-hypothesis. However the slaters were still in their extremely damp container prior to the experiment, so they may not have required a humid environment as the hypothesis suggested.
Statistical Analysis, Experiment 3: Expected Standard Error (SE) for ten slaters = SE = = 0.158
The difference between the null-hypothesis prediction (50% on each side) and the observed percentages (47.3% and 52.7%) is 0.027 (0.527 – 0.5)
‘t’ is therefore 0.027 = 0.171 0.158
Referring to ‘t’ tables shows that the observed result has a probability of over 40% of occurring by chance. This shows that the results are not statistically significant.
Discussion, Experiment 3
Once again, the orthokinesis experiment has yielded an inconclusive result. However, the over-saturation explanation is still applicable.
Combined Statistical Analysis:
Overall, 30 slaters were used, so the expected Standard Error (SE) is: SE = = 0.091
The combined percentages were: Humid side: 42.7% Dry side: 57.3%
The difference between the null-hypothesis prediction (50% on each side) and the observed percentages is 0.073 (0.573 – 0.5)
‘t’ is therefore 0.073 = 0.802 0.091
Referring to ‘t’ tables shows that these results could have occurred by chance with more than a 40% probability. The combined results are therefore still statistically insignificant.
General Discussion:
Although this experiment did not show any orthokinetic effect, and the hypothesis was not supported by any of the results, the response probably does exist. It was not shown by the Porcellio scaber specimens used in this experiment because they were kept in a very moist environment just before the experiment, so did not need to enter the humid side of the choice chamber to prevent desiccation (drying out). Therefore, in future experiments of this type, it is recommended that the slaters used be kept in a low or medium humidity environment before experimentation, or be collected from the wild immediately before the experiment.
Problems: The woodlice could also have shown no orthokinesis response because: The calcium chloride might not have been fresh, that is, it might have already absorbed some water. There was not enough time for a humidity gradient to build up inside the choice chamber. The woodlice tended to clump together on the dry side and not move to the humid side. Another effect was involved, for example perhaps the slaters liked the smell of the calcium chloride.
These are not likely explanations for the observations made, however.
To avoid these possibilities, it is recommended that future experiments investigating orthokinesis use more airtight equipment, with humidity monitors to check each chamber, and ensure that a humidity gradient is present. Also, different methods of increasing and decreasing humidity might be more desirable.
Conclusion:
In conclusion, the results obtained during this investigation did not support the hypothesis, and did endorse the null-hypothesis.
This was probably because the slaters were kept in a very humid environment before the experiment, so they were not in any immediate danger of desiccation. They therefore did not need to stay in the humid side of the choice chamber, and may in fact have preferred the dry side if they were over-saturated with water.
Because woodlice cannot control water loss through the cuticle, they cannot stand dry conditions such as warm sunny days. Although this is a vulnerability, several behaviours, including orthokinesis, ensure that woodlice inhabit humid habitats where they will not dry out. The uncontrolled water loss (and gain, in some cases) dictates their habitat but brings other benefits, possibly energy savings during nitrogen excretion. �Geokinesis Investigation
Aim and Introduction: To investigate whether Porcellio scaber has a geokinesis behaviour, that is, prefers to spend time at the bottom of a slope, or the top. Continuous data in the form of observations of the number of slaters on each side over time will be gathered by observing the slaters over a fifteen-minute period in the choice chamber. Ten slaters will be used as this is an easily handled and counted number and a large enough sample size for statistical analysis. The choice chamber means that a control is unnecessary.
Variables: Tested: Slope (17°). Constant: Substrate type, humidity, light strength, temperature, chemical environment.
Hypothesis: That the slaters will prefer to stay at the bottom of a slope.
Null-hypothesis: That the woodlice will show no geokinesis response, by having no preference for the upper or lower part of a slope.
Equipment: 1 Petri-dish choice chamber, 10 slaters, 1 clock or stopwatch, a large thick piece of cardboard (20 x 45 cm), sellotape dispenser to use as a prop, protractor, lengths of sellotape, a paper and pen (for recording results).
Method: Firmly tape the choice chamber onto the cardboard sheet, ensuring there are no cracks around the edges where the slaters could escape. Place the prop under one end of the cardboard sheet and check that the angle of the slope is fairly small – around 17°. Place the cardboard sheet on the level again and place the ten slaters into one of the chambers. Wait a few seconds for the slaters to disperse and then prop the sheet up on an angle again, so that one of the choice chambers is at the top of the slope and one is at the bottom. Wait five minutes to give the slaters a chance to explore the chamber. Count and record how many woodlice are on the ‘up’ side of the choice chamber and how many are on the ‘down’ side. Count and record the number of slaters on the higher and lower sides, every minute for another ten minutes. Remove the slaters from the choice chamber and place them back into their container. Repeat the experiment described by steps 1 to 6 three times. Analyse the data collected by graphing and statistical analysis.
�Results:
Geokinesis Results 1
Slaters: 10
Time Number Number (minutes) in higher in lower 0-4 (exploring) 5 2 8 6 3 7 7 1 9 8 0 10 9 1 9 10 0 10 11 1 9 12 2 8 13 6 4 14 7 3 15 6 4 Total 29 81 Percent 26.4 73.6
The results of this experiment show that the slaters first spent time at the bottom of the slope, then, towards the end of the experiment, several went to the upper part of the slope. Overall, they appear to prefer the choice chamber at the lower part of the slope.
Geokinesis Results 2
Slaters: 10
Time Number Number (minutes) in higher in lower 0-4 (exploring) 5 3 7 6 3 7 7 2 8 8 3 7 9 5 5 10 3 7 11 4 6 12 3 7 13 5 5 14 5 5 15 4 6 Total 40 70 Percent 36.4 63.6
This experiment also shows that the slaters prefer the lower side of the choice chamber.
� Geokinesis Results 3
Slaters: 10
Time Number Number (minutes) in higher in lower 0-4 (exploring) 5 5 5 6 2 8 7 2 8 8 1 9 9 3 7 10 2 8 11 1 9 12 5 5 13 5 5 14 4 6 15 3 7 Total 33 77 Percent 30.0 70.0
This last experiment is similar to the first: the slaters spent more time on the lower part of the slope.
Collective Statistical Analysis:
Individually, each experiment is not statistically significant. For example, this is the analysis for Experiment 1: Expected Standard Error (SE) for ten slaters = SE = = 0.158
The difference between the null-hypothesis prediction (50% on each side) and the observed percentages (26.4% and 73.6%) is 0.236 (0.736 – 0.5)
‘t’ is therefore 0.236 = 1.494 0.158
‘t’ tables show that this has a probability of about 15% of occurring at random, so the results are not statistically significant.
When the results are combined and calculated for the thirty slaters used overall, the geokinesis effect is significant: Expected Standard Error (SE) for thirty slaters = SE = = 0.091 The combined (average) percentages were: Upper chamber: 30.9% Lower side: 69.1%
The difference between the null-hypothesis prediction (50% on each side) and the observed results is 0.191 (0.691 – 0.5)
‘t’ is therefore 0.191 = 2.099 0.091
The ‘t’ tables show that these results have a less than 5% probability of occurring at random, so they are definitely statistically significant.
General Discussion:
The graphs of experiments 1, 2 and 3 show a clear trend for the slaters to prefer the lower slope. The individual results involved too few slaters for their results to be statistically significant. However, the combined analysis shows that the geokinesis effect is real and not a statistical fluke.
So the combined results of the investigation support the hypothesis: That woodlice have a weak geokinesis which causes them to spend more time at the bottom of a slope than at the top. Possibly they were attempting to descend the slope rather than ascend it but that is not really what was tested in this investigation.
Geokinesis behaviour is probably valuable to woodlice survival because: A lot of shelter is found close to the ground, and less shelter is found up slopes. It is more energy efficient to descend slopes rather than climb them.
Problems: Very little experimental error was possible in this simple experiment. The equipment was carefully set up so that other variables were the same for each choice chamber: light strength, temperature, humidity, substrate (the cardboard) were all checked to ensure they did not become a variable.
One alternative explanation for the results is possible: The woodlice might have preferred the lower chamber because its lower edge, where the slaters tended to congregate, formed a sort of crevice with the sloped floor that they might have preferred to the upper chamber, which did not have the same kind of crevice. However, this explanation is unlikely because the degree of crevice formed at an angle of 17° is not very different from that formed at any other part of the choice chamber. Recommendations for future investigations into geokinesis would be to use a slope inside a larger container. This would eliminate the possible ‘crevice effect’ discussed above. If a choice chamber were used again, however, placing five slaters on each side instead of all ten on one side would be desirable. To be very accurate, the experiments should be repeated ten times. (Rather than the three times as in this investigation.)
Conclusion:
In conclusion, the results of this experiment support the hypothesis: Porcellio scaber possesses a weak geokinesis, which causes them to prefer the lower part of a slope to the upper part. This would be helpful to the woodlice for finding shelter and saving energy.
�Thigmokinesis Investigation
Aim and Introduction: To investigate the presence of a thigmokinesis response in Porcellio scaber. Continuous data in the form of observations of the number of slaters on each side of the choice chamber over time will be gathered by observing the slaters over a fifteen-minute period. Ten slaters will be used during each experiment, as this is an easily handled and counted number and a large enough sample size for statistical analysis. The use of a choice chamber means that a control is unnecessary.
Variables: Tested: Physical environment. Constant: Substrate type, humidity, light strength, slope (0°), temperature, chemical environment.
Hypothesis: The woodlice will show a thigmokinesis response, by spending more time in an area which has cracks and crevices that touch more of their surface area, than in a similar flat area which touches a smaller part of their surface area.
Null-hypothesis: The slaters will show no thigmokinetic response and have no preference for either side of the choice chamber.
Equipment: 1 Petri-dish choice chamber, three thick cardboard sheets (20 x 45 cm), craft knife for shaping cardboard, 10 slaters, 1 clock or stopwatch, lengths of sellotape, a paper and pen (for recording results).
Method: Prepare the choice chamber with one of the methods of making crevices, making sure that it is taping down firmly so no slaters can escape. Place the ten slaters into one side of the choice chamber and wait five minutes while they disperse through the choice chamber. Count and record how many woodlice are on the ‘flat’ side of the choice chamber and how many are on the side equipped with crevices. Count and record the number of slaters on the flat and crevice sides, every minute for another ten minutes. Remove the slaters from the choice chamber and place them back into their container. Repeat the experiment described by steps 1 to 5 three times, using the different methods for making crevices. Analyse the data collected by graphing and statistical analysis.
Crevices: During the investigation three types of crevices were used:
Experiment 1: A piece of corrugated cardboard was placed in one of the choice chambers to raise the floor so that the space between the floor and the roof was a tight crevice. A small ramp provided access between the choice chambers. However the cardboard did not fit sufficiently tightly and the slaters were able to crawl down the edges and into the corrugations in the cardboard. The woodlice definitely preferred it there: once they found the crevices, they did not emerge.
Experiment 2: Because of the problems involved with the floor-raising method, a thick sheet of cardboard was used, and shallow slots or gaps were cut into the floor of one of the choice chambers.
Experiment 3: It was observed that during the second experiment, the slaters tended to follow the walls and some did not seem to ‘find’ the crevices in the centre of the chamber. Therefore, the crevice system was extended to touch the edges of the chamber.
Results, Experiment 1:
Thigmokinesis Results 1 Slaters: 9 (see text)
Time Slaters Slaters (minutes) in flat by crevices 0-4 (exploring) 5 4 5 6 4 5 7 3 6 8 2 7 9 2 7 10 1 8 11 1 8 12 1 8 13 1 8 14 0 9 15 0 9 Total 19 80 Percent 19.2 80.8
This first experiment involved attempting to create a crevice by placing a piece of corrugated cardboard one of the choice chambers to raise the floor so that the space between the floor and the roof was a tight crevice. A small ramp provided access between the choice chambers. However the cardboard did not fit sufficiently tightly and the slaters were able to crawl down the edges and into the corrugations in the cardboard. The experiment is not seen as a failure, however, since the corrugated cardboard acted as a different system of crevices to that intended – the choice chamber still consisted of one plain chamber and a different one with crevices.
Only nine slaters were counted for the results of this experiment because from the start, one of the woodlice appeared incapacitated: it turned on its back and did not appear to be able to turn back over again. This woodlouse was removed from the results on the flat side (where it was).
Statistical Analysis, Experiment 1: Expected Standard Error (SE) for nine slaters = SE = = 0.167
The difference between the null-hypothesis prediction (50% on each side) and the observed percentages (19.2% and 80.8%) is 0.308 (0.808 – 0.5)
‘t’ is therefore 0.308 = 1.84 0.167
Referring to ‘t’ tables shows that the observed result has less than a 10% probability of occurring by chance. This means that the results are not statistically significant.
Discussion, Experiment 1
Although the slaters definitely preferred the crevices to the flat side, the statistical analysis does not show this. If the experiment had continued for a few more minutes, then the analysis would be different.
The main problem with the crevices in this experiment is that they involved an additional variable: it was quite dark inside the corrugated cardboard crevices, so this complicates what environmental factor the woodlice were seeking – darkness, a tight crevice, or both? The next two experiments remedied the problem by using different methods of making crevices, that left them open to light.
Results, Experiment 2:
Thigmokinesis Results 2
Slaters: 10
Time Slaters Slaters (minutes) in flat by crevices 0-4 (exploring) 5 2 8 6 4 6 7 3 7 8 2 8 9 1 9 10 1 9 11 2 8 12 1 9 13 4 6 14 4 6 15 3 7 Total 27 83 Percent 24.5 75.5
This was a relatively simple experiment to carry out: the crevices consisted of some long slots, about the width and depth of a woodlouse, cut out of the cardboard floor of one side of the choice chamber. The graph shows that they preferred this side, but only some of the slaters actually got into the crevices; the others followed the walls and managed to ‘avoid’ the crevices. The reason shallow crevices were used was to allow a similar amount of light in to all parts of the choice chamber, which ensures that only the thigmokinesis response was being tested, rather than the phototaxis response.
Statistical Analysis, Experiment 2: Expected Standard Error (SE) for ten slaters = SE = = 0.158
The difference between the null-hypothesis prediction (50% on each side) and the observed percentages (24.5% and 75.5%) is 0.255 (0.755 – 0.5)
‘t’ is therefore 0.255 = 1.61 0.158
‘t’ tables show that the observed result has a probability of around 20% of occurring by chance. This means that the results are not statistically significant.
Discussion, Experiment 2:
Although the individual analysis still shows that the results could have happened by chance, the collective statistical analysis should show a different answer, because it will take all three experiments into consideration.
The only apparent problem with this experiment is that the slaters could conceivably have been attracted to the ‘crevice’ side by the smell of the newly cut cardboard. However, this is unlikely.
Results, Experiment 3:
Thigmokinesis Results 3
Slaters: 10
Time Slaters Slaters (minutes) in flat by crevices 0-4 (exploring) 5 6 4 6 4 6 7 4 6 8 3 7 9 1 9 10 3 7 11 0 10 12 1 9 13 1 9 14 2 8 15 1 9 Total 26 84 Percent 23.6 76.4
This experiment involved crevices similar to but a little more extensive than those used in Experiment 2. The problem observed there was that some of the slaters didn’t find the crevices because of their tendency to follow the outer wall. The crevices in this experiment, then, were extended to the walls to ensure that all the slaters encountered them. The difference was very small however –the totals of the two experiments differed by just 1.
Statistical Analysis, Experiment 3: Expected Standard Error (SE) for ten slaters = SE = = 0.158
The difference between the null-hypothesis prediction (50% on each side) and the observed percentages (23.6% and 76.4%) is 0.264 (0.764 – 0.5)
‘t’ is therefore 0.264 = 1.67 0.158
‘t’ tables show that the observed result has a probability of less than 20% of occurring by chance. This means that the results are not statistically significant.
Discussion, Experiment 3: The individual statistical analysis still shows that the results could have happened by chance, but the graph suggests that the slaters do have a definite preference for the side of the choice chamber with the crevices. As with Experiment 2, the problem with this experiment is that the slaters could conceivably have been attracted to the ‘crevice’ side by the smell of the newly cut cardboard. This is unlikely, however. Collective Statistical Analysis: Expected Standard Error (SE) for twenty-nine slaters = SE = = 0.093 The combined (average) percentages were: Flat side: 22.5% Creviced side: 77.5%
The difference between the null-hypothesis prediction (50% on each side) and the observed results is 0.275 (0.775 – 0.5)
‘t’ is therefore 0.275 = 2.957 0.093
Referring to the ‘t’ tables reveals that these results have a less than 1% probability of occurring at random - so they are definitely statistically significant.
General Discussion:
The overall statistical analysis shows that the results are mathematically significant. Although the experiments were a little different from each other, they all involved a plain chamber and a chamber with crevices, so the results were still acceptable. So it has been shown that the slaters prefer the creviced side. This supports the hypothesis and refutes the null-hypothesis, making the investigation a success. Slaters would possess a thigmokinesis response to attract them to small crevices, for example within leaf litter or under objects. These areas tend to be damp, dark and safe from predators, so thigmokinesis could help the slaters survive and exploit their niche.
Problems: The main problem with this investigation is the nature of the crevices. Perhaps a different method of measuring thigmokinesis is called for, for example having one chamber with a smooth plastic floor, and another with a rough, scratched floor. The trouble with this approach is the difficulty in ensuring that all other variables are unaffected by the inherent differences in the materials: could differences in heat conduction, colour, or chemical composition be responsible for the preferences of the woodlice instead? Conceivably the last two experiments could have been affected by the presence of the freshly cut cardboard. It did not appear to smell, however, and this is only a very small possibility. Future experiments investigating orthokinesis might try using different apparatus, for example a large chamber with one half being a crevice and the other side being open. A different method of observing the slaters would have to be used in this case, for example an infra-red camera inside the (darkened) chamber. Also, to be very accurate, the experiments should be repeated ten times, rather than the three times as in this investigation.
Conclusion:
In conclusion, Porcellio scaber possesses a thigmokinesis response, which causes specimens to prefer an area where a larger proportion of their body can touch the substrate, more than a similar flat area where less of the body is in contact with the surface.
Thigmokinesis would be useful to the slaters because it would act in combination with other behaviours to draw them to shelter: dark, damp cracks and crevices such as under branches or in leaf litter.
�Bibliography
Websites:
http://www.geocities.com/CapeCanaveral/Hangar/7649/wlice.htm Woodlice Online, Greg McKenzie. Last updated: 19/04/00
Books:
Allan, Richard, and Greenwood, Tracey. Year 13 Biology Student Resource and Activity Manual 2000. Biozone International, Ltd, 1999.
Sutton, S.L. Woodlice. Ginn & Company, 1972.
Resources:
Form Seven Biology Slater Study, New Zealand Correspondence School. —Preceding unsigned comment added by Ppe42 (talk • contribs) 11:02, 26 March 2008 (UTC)
