Orthokinesis In Slaters Relative To Humidity

There are many different species slaters and for this experiment I chose to test the Porcellio scaber or more commonly known as the Rough Common Woodlouse. This humidity test links to the slaters ecological niche through examining its preferred habitat which is known to be dark, damp and sheltered areas. The Slaters that I collected for testing, I found under old flat pieces of wood that were on damp soil and leaf litter near the Waikato River. A structural adaption that slaters lack is a waxy cuticle layer that is used in most insects to minimise desiccation therefore they are more likely to dry out. As Slaters lack this adaptation of surviving unfavourable conditions, they use other adaptations to remove themselves from these conditions. For example slaters have an orthokinetic response to humidity and temperature. Orthokinesis is a non directional response of the change in the rate of movement due to a stimulus. This means that the slaters will increase their rate of movement when the humidity or temperature is unfavourable so they move out of that area quicker therefore minimising desiccation. Other adaptations are a negative phototaxis which means that they move out of areas that have higher light intensities so that they can minimise desiccation. Slaters are also found clumping together to avoid water loss. The adaptations of slaters are shown in there ecological niche of dark, cool and damp environments. Though knowing their ecological niche I am going to test how different humidity’s affect the rate of orthokinesis in slaters so I can determine their preferred humidity.

Aim:

The aim for this investigation is to determine whether orthokinesis (speed of movement) in Slaters is affected by increasing or decreasing humidity percentages in their test environment.

Hypothesis:

I think that the lower the humidity the faster the Slaters will travel as it will want to quickly return to an environment that has a higher humidity percentage. While when a Slater is in a high humidity environment the Slater will slow down or completely stop as it would have found a favourable environment. Therefore I think the Slater will move fastest when humidity is 12.5% (LiCl) and will stop moving when humidity is 93.5% (KNO₃).

Null Hypothesis:

Different humidity percentages will have not affect on the rate of movement in Slaters.

Controlled Variables:

Experiment is conducted in the same room and the same place on that room. (Middle desk of the project room)

Lights are off in all experiments so change of light intensity cannot cause a difference in the rate of movement.

Blinds are shut so no change in light intensity from natural light.

Air conditioner kept at 20°C so change in temperature cannot be the cause for change in rate of movement.

Wait 5 minutes for humidity percentage to change to the required humidity that will be created from each chemical so that Slaters have time to become accustomed to their new environment and respond how they naturally would.

Use a random selection of Slaters so a random part of the population is being tested and the experiment is relevant to the population.

Repeat test of each chemical / Humidity percentage 5 times in each trail and do 3 trials to give a fair test and make sure that the selection of data is large enough to accurately portray the population.

Place clear, heavy, plate piece of glass or other heavy clear material on the Petri dish. This holds the experiment in place and stops the humidity from being altered due to incoming or outgoing air into the surrounding environment.

Method:

Collect all the equipment.

75 Slaters; stored in an ice-cream container with damp soil, bark and raw potato for food. (Collect Slaters 2 days before experiment to allow Slaters to settle into their new environment).

The chemicals LiCl, MgCl, Mg(NO₃)₂, NaCl and KNO₃ which will be used to alter the humidity in the experiment to determine whether it is a stimulus that will affect orthokinesis.

5 Petri dishes. (Plus spare to balance glass sheet)

5 pieces of gauze or other breathable material.

Stop watch.

Ball of string.

White board marker.

Sheet of glass or clear plastic.

25 mL measuring cylinder.

Set the air conditioner at 20°C; close the doors, windows, shut the blinds and turn off the lights (this will insure the environment will be the same in all the experiments except for the humidity and that no other stimulus will affect the validity of the results). By setting the temperature at 20°C it will cause the chemicals to produce the already proved humidity.

LiCl- 12.5% humidity

MgCl – 33% humidity

Mg(NO₃)â‚‚ – 52.9% humidity

NaCl – 76% humidity

KNO₃ – 93.5% humidity

And wait 5 minutes to allow the temperature in the room to reach or drop to 20°C.

Starting with LiCl, add 15 mL of LiCl into a Petri dish (by measuring with a 25 mL measuring cylinder) which will create a humidity of 12.5% in the test environment. Place a piece of gauze (or other breathable material) over the Petri dish but be careful that the gauze doesn’t touch the chemical (LiCl) as the Slaters cannot touch it as it would result in injury to the Slaters and it would affect their orthokinesis and the results would be invalid.

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Place a randomly selected Slater from the ice-cream container on the gauze, place the lid on the Petri dish and place the glass or plastic sheet on the top of the Petri dish balancing it on both sides by putting other unused Petri dishes under the glass as well. This will keep pressure on the Petri dish so humidity cannot escape through gaps between the lid and dish. If it were to escape it would make results invalid as humidity wouldn’t be accurate.

Wait 5 minutes ( timing on the stop watch) to allow the LiCl (the chemical) to reach the already identified humidity percentage by causing a reaction that makes Hâ‚‚O either move into or out of the chemical which alters the water content in the air of the environment. This waiting period also allows the Slater to have time to settle into the new environment and take in the humidity. This stops fear and a still changing humidity from influencing the rate of movement and causing the results to be invalid.

After waiting 5 minutes, time another minute on the stopwatch and during this time follow the path of the Slater with a whiteboard pen drawing on the glass. Stop tracing when the minute is up.

Take off the glass sheet without smudging the whiteboard pen tracings and remove the Slater from the Petri dish and place it into another ice cream container with soil, bark and food so that the Slater doesn’t get mixed up with the Slaters still to be tested. Place the lid back on the Petri dish as quickly as possible.

Lay a length of string along the whiteboard marker line from the start to the end. Get as exact as possible and mark on the string where the whiteboard marker starts and ends.

Take the marked string and lay it flat on the table and measure between the two marked points to find the distance that the Slater travelled. Record data in a data table. Sub the distance into the formula v=d/t (speed equals distance that the Slater travelled divided by the time taken to travel it). This formula will give you the average speed of the Slater during this test and therefore the orthokinesis of the Slater. Use 60 seconds as your measurement of the time taken because the measurement of distance was taken over 1 minute.

Remove the whiteboard marker from the glass sheet with a clean cloth. Repeat steps 2 – 9 four more times using LiCl, until you have 5 travelling distances of Slaters under the humidity created by LiCl. This data will create unbiased results once the 5 pieces of data is averaged.

Repeat steps 2 – 11 using the different chemicals (MgCl, Mg(NO₃)â‚‚, NaCl, KNO₃) using a separate Petri dish for each chemical, this will give data of different speeds of Slaters under the different humidity’s. This will give you data that will allow you to compare and contrast speeds relative to the humidity. Record all data in a data table.

Steps 1 – 11 are classified as 1 trail. Do at least 3 different trail so that a large proportion of the population is accounted for in the data.

Data and Graphs:

12.5% 33% 52.9% 76% 93.5%

1 2.5%

Conclusion:

The data that I have collected supports my hypothesis that the lower the humidity the faster that the slaters travel, while, when the humidity is high, the slaters will have a slower rate of movement. The scatter graph shows that at 76% and 93.5% humidity slaters travelled significantly slower than at 33% and 52.9% humidity. However at the extreme humidity’s of 12.5% and 93.5% humidity I found that my hypothesis was incorrect as the results didn’t support my assumption that 93.5% humidity would cause the slaters to stop moving while 12.5% humidity would cause the fastest rate of orthokinesis. The graph shows that 12.5% humidity had a slower rate of movement than 33% humidity and that 93.5% humidity had a faster rate of movement than 76% humidity. The “best fit” curve show that the rate of movement increases as the humidity moved either side of the preferred humidity. This experiment also proves that humidity has an effect on orthokinesis in slaters and therefore the null hypothesis is incorrect.

Discussion:

I designed and completed this experiment to discover whether or not different humidity’s will affect the rate of orthokinesis in Slaters. Through my data I found that humidity does affect orthokinesis in Slaters. When the humidity was at 76% the Slaters moved at their slowest therefore showing that Slaters are at their preferred humidity; they are moving at a slower rate as they don’t need to move away quickly as they are already in favourable conditions.

At both 33% and 52.9% humidity the Slaters moved significantly faster than they did at the preferred humidity of 76%. This presents the idea of unfavourable conditions as they are travelling faster and therefore wasting energy in order to spend less time in these conditions, and by moving away they are less likely to suffer from desiccation. Slaters are extremely sensitive to desiccation and don’t possess many structural adaptations to protect themselves from desiccation. Unlike most insects, Slaters lack a waxy cuticle layer, this layer helps to prevent drying out as it minimises evaporation of water from the exoskeleton. Therefore they rely on their orthokinetic response to remove themselves from areas that cause desiccation by speeding up their rate of activity. This reaction makes it more likely that they will move out of the unfavourable conditions quickly so they spend less time in an area that can cause desiccation and death.

At 12.5% humidity Slaters travelled slower than when they were at 33% humidity even though it is a more unfavourable condition. The 12.5% humidity at 20°C has more drying power that 33% humidity at 20°C so therefore desiccation will being to occur earlier at 12.5% humidity when compared with 33% humidity. The earlier desiccation means that the Slaters cannot physically travel at increased speeds as the desiccation causes problems with the Slaters respiration. The Slaters transport oxygen using pseudo trachea which are small hollow air tubes which carry the oxygen to the Haemolymph. Moisture is needed in the pseudo trachea to dissolve the oxygen and allow it to diffuse into the Haemolymph. Without the moisture the oxygen cannot dissolve and therefore cannot enter the Slaters body for it to use and without oxygen the Slater will die. The Slaters orthokinetic response at 12.5% humidity is to increase its rate of activity in order to get out of those conditions. This is what causes the increase of speed compared to the speed of Slaters at the preferred humidity. However because of the early desiccation, the Slaters activity rate is limited because of restricted respiration therefore the speed of the Slaters at 12.5% is less than the speed of the Slaters at 33%.

While at 93.5% humidity although the rate of orthokinesis was similar with that or 76% humidity Slaters still moved slightly faster at 93.5% humidity. This is because 93.5% humidity the conditions are not completely favourable as the humidity is so high that it causes the Slaters to become overloaded with water. This effect can also be seen when Slaters leave their shelters after heavy rain as they need to transpire the water that they have taken on. When a Slater takes on too much water they cannot respire properly as the distance that the oxygen needs to diffuse becomes too long therefore the Slater doesn’t receive the necessary oxygen that it needs to function this will result in the drowning and death of the Slater. This is why when at 93.5% the rate of movement of Slaters increase from the speed of Slaters at the preferred humidity of 76% even though it is only a slight increase in the rate of movement as the 93.5% humidity is only slightly more unfavourable than 76% humidity.

Evaluation:

The results of my tests, which have been place in the scatter graph above show that there is a significant relationship between humidity and the rate of orthokinesis in slaters. As the r² value is 0.9703 it can be seen that 97% of my data’s variation can be contributed to the change in humidity rather than any other variable. This means that my conclusion is valid.

During my experiment I had to control many variables so that my results were valid and to prove that the change in the rate of orthokinesis was due to the change in humidity and not due to another variable. For example through my research I found that Slaters are nocturnal animals and that light intensity can also affect their rate of orthokinesis; Because of this, when I did my testing I closed all the blinds and turned off all the lights. By creating this environment I replicated the time period (night) in which Slaters are most active so that I could see significant differences in their rate of movement. By turning off all the lights and closing the blinds I also eliminated another variable that is known to change the rate of movement in Slaters. This meant that my results were valid as I insured that my data wasn’t a result of changes in light intensity but was due to the change in humidity.

I set the air conditioner at 20°C during all my experiments as change in temperature is also a variable that can affect the rate of orthokinesis. By having the temperature the same though all my experiment I eliminated it as a changing variable and once again insured that changes in the speed of Slaters was due to change of humidity and not another variable. Another reason that I set the air conditioner to 20°C is because that the chemicals I used required that temperature in order to react and produce the predetermined humidity. Drying power of humidity’s also can change due different temperatures. For example the drying power of 33% humidity at 20°C can be different to the drying power of 33% humidity at 30°C which could affect the rate of desiccation in Slaters and therefore affect the data on the rate of movement. So by keeping the same temperature in all experiments I made sure that the data was valid and not a result of different drying powers due to different temperatures.

By allowing timing before each testing it meant that the Slaters weren’t out of their comfort zone and by the time it came to testing the results were based on the Slaters natural response to the change in humidity and not by the fact that they were under stressful conditions. Also by taking a large sample size and repeating the test numerous times through different trials I made sure that the data I collected was an accurate representation of the population’s reactions to changes in humidity and how it affects their rate of movement. The large random sample size means that the results were not based on one type of Slaters change in rate of movement; for example the change in rate of movement in old Slaters. Therefore by having a random selection of Slaters I was incorporating all types of Slaters so the data I collect was an accurate response of the populations change in the rate of movement relative to change in humidity. The repeat trials also meant that my results could be conclusive and when I came across an outlier that would have disrupted my results I would have be able to successful recognise any significant outliers and retest them to use in my average. Because I controlled these variables, my data and conclusion must be valid as the only stimulus left that could have affected orthokinesis is humidity.