An Investigation into the Effects that Different Light Intensities have on the Speed of Woodlice:

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An Investigation into the Effects that Different Light Intensities have on the Speed of Woodlice:

Introduction:

Woodlice are one of the easiest terrestrial arthropods to obtain in large numbers throughout the year. Porcellio scaber, the most common species is not difficult to identify with the naked eye, and is large enough to be handled without employing special techniques. Their behaviour is relatively simple yet providing perfect examples of kinesis and taxis.  A species, which shows taxis, can orient the direction of their movement with respect to the source of an external stimulus whilst kinesis is a steady-state dependence of the movement velocity on the stimulus intensity.iii Thus it is clear that kinesis will be the important response in my investigation as I am not concerned with the stimulus direction.

Woodlice lack a waxy cuticle on their exoskeleton and have a high surface area to volume ratio. This means that they are prone to desiccation. As a result, much of their behaviour is concerned with reducing water loss. Woodlice will therefore tend to congregate in more humid, cooler and darker regions where their water loss will be lower.

Although light does not affect the physiological state of woodlice in the same way as humidity and temperature, it plays a very important part in woodlice behaviour. Woodlice have a negative photokinesis- they move faster as the light stimulus increases. This has great survival value because bright places tend to be drier and dark places tend to be damper. Thus the light response reinforces the humidity response in that it means they will spend more time in humid conditions where they will not desiccate. There are wide differences in the strength of the light stimulus in different species. The less rapid the water loss, the less intensely negatively photokinetic the species will be.i 

Hypothesis:

Graph showing the effect of light intensity on the speed of woodlice:

My HA hypothesis therefore is that the speed of woodlice will increase as the light intensity increases until the maximum speed of a woodlouse is reached. Thus there will be a positive correlation between light intensity and the speed of movement. My Ho hypothesis is that there will be no significant correlation found between light intensity and the speed of movement. Consequently, I will use the Spearman’s rank correlation coefficient statistical test or the Product Moment correlation coefficient, depending on the distribution of my data, in order to test this hypothesis.

The distance values that I will use for my experiment, which will alter the light intensity, will be 10cm, 20cm, 30cm, 40cm, 50cm, 60cm, 70cm, 80cm, 90cm and 100cm as this will enable me to obtain a full range of results within a conceivable amount of time, ensuring that I am able to get a complete analysis of the effect of light intensity on the speed of woodlice. I am aware that the highest light intensities that I employ may result in no further increase in the speed of woodlice as at a certain light intensity the maximum speed of woodlice will be reached but it will be interesting to see at which light intensity this is achieved, if at all. I will obtain these different light intensities by using the light from a projector and suspending the projector at different lengths. I have chosen to use a projector light because if I was to use a bulb, the heat given off may affect the reliability of my investigation by raising the temperature at lower distances. I will measure light intensity using a light probe, which will be placed, in a fixed position at a fixed angle (using black tape) throughout the investigation. I have decided that I am going to investigate 10 woodlice at the same light intensity in order to obtain an average speed at each light intensity, which will make my results more reliable.

Control Variables:

The other factors, excluding light (my input variable), that affect the activity of woodlice will need to be controlled throughout my experiment.  If I did not control them, they would affect the reliability of my investigation into the effect of light intensity.

Method:

Apparatus List:                                                

  • Large plastic container x2- one to act as a long-term culture for the woodlice and the other for the investigation.
  • Smaller plastic container with wet cotton wool- to keep the woodlice, which are under investigation, hydrated before they are transferred to the experimental container.  
  • Projector- to produce a range of light intensities by suspending at different distances.
  • Light probe- to measure the light intensity. 
  • Thermometer- to monitor the temperature to make sure it remains relatively constant.
  • Fixed camera connected to a video player and television screen- to display and record the paths of the woodlice.
  • Acetate and coloured pens- in order to draw the paths of each woodlouse.
  • Blue tack- needed to hold the acetate firmly in place.
  • Ruler- needed to measure the distance that each woodlice travels.
  • Paint Brush and plastic tea spoon- these are needed to help transfer the woodlice from their culture container to the experimental container.
  • Black Tape- to secure the light and humidity probes.
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First of all, a large number of woodlice will need to be collected which are both roughly the same size and of the same species. Place these in the long-term culture container. After collecting the woodlice, set up the apparatus as shown above.

Suspend the projector so that the first distance of 10cm is obtained. Then extract 10 woodlice from the long-term culture container and place ...

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The student has followed the conventions expected at this level and at some points they have exceeded them, the only improvement that I could suggest would be to include a null hypothesis, this is basically a hypothesis that says that your tested variable will have no effect on the test subject. The spelling grammar and scientific language are all to a very high standard.

The extensive reasoning behind the hypothesis is a feature not normally seen at this level of work, this really gives the reader an insight into why the student has come up with the hypothesis that they did. The fact that the control variables have been placed in a table it makes it very easy to read which will allow the examiner to find the marks easier. The fact that all of the equipment has an explication is a very high level skill that I have only just grasped this skill as somethings are obvious why you would need them! It is also a logical progression through the investigation as you can see how one section leads on to the next.

The student has answered the question really well with everything being present in the essay that is needed, the scientific explanation and layout is to a very high standard.