Hypothesis
I think that when the Calliphora larvae are subjected to the light, they will move away from the source to a region of the paper circle where the intensity of the light is not as strong, generally moving into the negative region, with a few in the neutral zones.
The reason for this is that the larvae needs to avoid the possibility of becoming prey to a predator, just before it is due to go into the pupal state. When the larvae are found in brighter areas, it becomes more visible to other animals, therefore putting itself in harms ways. If the larvae remain in darker areas, it increases it chance of survival (as larvae), also increasing the possibility of it becoming a bluebottle fly once it has come out of the pupal state.
Furthermore, I predict that the speed of the larvae will be dependant on where they are situated. If they are nearer to the light source, they will move away from it faster, slowing down as the intensity of the light decreases. The reason for this is due to the larvae needing to get out of the light as quickly as they can, reducing the amount of time that they exposed to potentially threatening situations.
Null Hypothesis
Due to information that I have seen about the way in which woodlice behave in a choice chamber, my null hypothesis would be that there would be an equal number of maggots in each section. That is to say the maggots will show no preference in which sector they finally finish.
Experimental Design
Apparatus:
- 15-25 Bluebottle blowfly larvae
- 60 Watt lamp
- Petri dish
- Pencil
- Stopwatch
- Ruler
- Compass
- Protractor
- Scissors
- A2 Sheets of paper
Note: Only use Calliphora larvae that are ready to pupate within a few days because this is the period in which they are most sensitive to light. Also, keep them in the fridge as this decreases their activity, so remain longer as larvae.
Making the marked sheets of paper: Take a sheet of A2 size paper. Using the compass, draw a circle 20cm in radius (40cm in diameter). At the centre of the circle, draw another circle of radius 2cm (diameter 4cm). Then with the protractor, divide the circle into four sectors: two of 120° and two of 60° as shown below:
Label the circle as above. Cut out, and repeat to produce as many as required.
Preliminary Work:
Before investigating the effect of light on Calliphora larvae, I had to ensure that the size of the circle that I would be using would be sufficient to allow the light source to decrease in intensity as it spread along the surface. At fist I began with circle of diameter 15cm. On a circle of this size, I found the light intensity had not decreased enough from one end to the other. I then moved up in size, using a circle of diameter 30cm. Unlike the 15cm circle, this did allow the light source to decrease in its strength. However the difference between centre and the ends was not large enough. Finally I tried a circle of diameter 40cm. This size circle allowed the light to lessen in strength sufficiently and was also easily reproducible by using a compass and not any other drawing methods. Whilst investigating the size of the circle, the height at which to have the light source above the table was also explored. Simply by just moving the lamp up and down, I found that a suitable height for the front end to be above the table was 1.5cm.
The next factor that I had to consider was the time intervals between marking the progress of the larvae.
Method:
To ensure that the following method can easily be followed, make sure all apparatus that needs to be used is within in easy reach as the experiments are carried out in the dark.
Firstly set up the apparatus whilst the lights in the room are still on.
Make sure that the circle of paper placed beneath the lamp is put in the correct way; the positive sector nearest the lamp with the negative furthest away.
After the apparatus is set up, remove the larvae from the fridge and allow them to sit for 10 minutes so they reach room temperature.
Once everything is ready, turn off all the light in the room, close all the blinds; as a result the room is lit only by the lamp being used in the experiment.
When ready, spin the pencil to provide a random direction in which to place the maggot. Place the maggot in the inner circle. As soon as the tail of the maggot leaves the circle, start the stopwatch. Make a mark on the inner circle to show the spot from where the maggot left. Here on, until the maggot reaches the outer circle, mark the movement every five seconds. Do not make marks by the maggots head as this will affect the direction in which they travel, instead mark near the end of the tail. When the maggot has left the circle, place it in petri dish so that it is not used again.
Repeat this process at least fifteen more times, using a new sheet of paper each time so that the new maggot cannot follow any trails of previous ones.
Health and Safety
Within this investigation, a few safety issues arouse. The main issue was the handling of sharp instruments such as the compass and scissors. Care had to be taken when drawing and cutting out the circle so as to avoid cutting myself. Another issue in this investigation is the ethical implications of using living organisms. The Calliphora larvae were used so that no harm came to them from being subjected to the light source. The lamp being used was not too powerful and so this did not have any long-term effects on them as they were seen to be reacting in the way suggested even when not being tested. Also the larvae did not encounter any chemical stimuli that could cause imbalances in their bodies.
Results
See the following tables and graphs.
Analysis
My hypothesis stating that the larvae will move away from the light source has been shown to be correct. Fifteen out of the seventeen maggots tested moved into the negative region of the circle. The two that did not enter the negative sector settled in the neutral zone.
As you can see from the table and graph showing the angle at which the maggots finally leave the circle, the majority are found within the angles 160° and 200°. The larvae found between these angles can be seen on the circles to be the furthest away from the light source. This is further proved by the standard deviation of the angles. The value of 32.52 places the majority of the angles within this range.
My hypothesis stating that as the larvae move away from the light stimuli, their speed will decrease, has been shown to be partially correct. As you can see from the table and graph showing the speed of the maggot (until they reach the edge of the circle), the majority do show an overall decrease in their speed, but a few do speed up the further they have travelled. One of the better examples of this is maggot 15. This maggot ended up in the neutral sector of the circle, this possibly being a possible cause in its speeding up.
My null hypothesis stated that the maggots would show no preference in which sector they finish. To prove that there is a relationship between the site of a light stimulus and the direction in which they travel, I will use a Chi Squared statistical analysis.
Where:
O=Observed Value
E=Expected Value
Degrees of freedom (d.f.)= 2
Using a Chi2 statistics table, look 5% probability at d.f. =1
The critical value given is 5.99.
As the Chi2 value of 23.4 is significantly greater than the critical value of 5.99, the null hypothesis can be rejected because there is a relationship between the location of the light stimulus and the direction in which the maggot travels.
Evaluation
To conclude, when Calliphora larvae are in close proximity with a light source, they have a tendency to move away and seek darker surroundings. The reason that the maggots moved away from the light source is due to a pair of symmetrically placed light receptors on their heads. The placement of these receptors produces the swinging of the head from side to side as the larva is moving. When it swings to the left, the left-hand receptor is exposed to the light behind it; this evokes a bend to the right, away from the light. But on swinging to the right, the right-hand receptor is illuminated, which causes it to swing to the left. These alternately evoked swings make the larva move in a roughly straight line. This directed movement involving alternate testing of the environment by bending its body is called a klinotaxis. This therefore describes the reason as to why the majority of the maggots travelled in a straight line. The maggots move into the darkness in order to pupate, that is, to grow into the mature form of the bluebottle blowfly. The maggots’ movement into darker areas shows that they are negatively phototaxic.
When producing the tables of distance and speed of the maggots, I encountered a problem with my measuring technique that I had not considered. On the trail marked for maggot 15, the distance of the first two marks from the light source was only 0.1cm. This number itself is far too small a distance for the maggot to travel whilst the receptors on its right-hand side are being illuminated. The actual distance covered by the maggot in those five seconds was more than 4.5cm. An error arose here because the maggot was moving towards the light source, so it displacement was far less than the distance it had travelled. This error led to an inaccuracy in the speed that the maggot was moving.
Other minor problems were also encountered. The first of these is that it was impossible to remove all background light from the laboratory. With all the blinds and doors were closed; there was some light from the extractor fan entering the room. I think that the effect of this small light source was negligible. This is because the light was entering from high on the walls and from the opposite end of the laboratory.
Another problem that was came across was the heating effect that the lamp was having on the paper circle and as a result on the maggot. This factor may have played a crucial role in the experiment as the maggots may not have been just moving away from a light source, they may also have been moving away from a heat source.
Further Investigations:
To further investigate this experiment, see the effect of changing the intensity of white light on how the maggot moves, whether it is faster or slower, or the average angle they travel is considerably different.
Another further investigation would be to find out what effect different colours of light have on the orientation of the maggot.
Bibliography
I would like to thank the following for providing information for this project:
- Microsoft® Encarta 98 Encyclopaedia
- World Book 1997 Encyclopaedia (standard edition)
- Encyclopaedia Britannica 2002
- Advanced Biology 1 2002 (student resource and activity manual)
