The Response of Blowfly Larvae to Light.

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BIOLOGY INVESTIGATION:

THE RESPONSE OF BLOWFLY LARVAE TO LIGHT

Background Information

This section contains general information about blowflies relevant to this investigation, leading to the reasoning of a prediction in the next section.

Blowflies

Blowflies are large-eyed flies, e.g. bluebottles Calliphora spp. and greenbottle Lucilia sericata. They lay their eggs in decaying meat and other foodstuffs, or in the case of greenbottle and L. cuprina (shown in Figure 5 on Page 13) in living sheep! The lifecycle of the fly that I will be using for my experiments (bluebottles) is shown in Figure 1. The numbers on the inside indicate the approximate days elapsing. Bluebottle is a name given to two similar species of true flies, Calliphora erythrocephala and C. vomitora. They have a four-stage life (metamorphosis) cycle, consisting of an egg stage, hatching into a larval stage, which metamorphoses into the pupa, from which the adult (imago) eventually hatches.

Each female bluebottle fly lays about 600 eggs, which hatch in about a day. Under favourable conditions the larvae pupate after in-between four days and a week, and emerge as adults a fortnight later, after twelve to seventy-two hours as pupae. This is summarised in Figure 1.

As the larva hatches out of the egg, it's sole purpose is to feed so that it has enough energy for the metamorphosis during the non-feeding pupal stage. At the larval stage, a developing bluebottle has no protection from radiation, and it's white skin would make it stand out strongly to predators. Later towards the pupal stage it develops protective pigment, and changes to a less conspicuous brown colour.

As the eggs are laid in a foodstuff, it is only natural that it would be beneficial to the larva to stay there in the nutrient-rich, insulated, moist, and predator-free environment until it has enough energy and it pupates, and develops into a fly, when it requires to be in the open. It has, needless to say (nature is almost always perfect), evolved a mechanism to satisfy these changing needs.

Responses to light

Innate behaviour, or instinct, is generally taken to be pattern of behaviour elicited by specific stimuli and fulfilling vital needs of an organism. It is demonstrated in its purest form by many lower animals, including insects.

The two types of behaviour in response to light which insects are capable of are:

* * Phototaxis- movement of the whole organism towards light (positive phototactic response), or away from light (negative phototactic response)

* * Photokinesis- the increase of rate of movement and/or change in direction of movement of the whole organism

Blowfly larvae have photoreceptors on each side of their heads, as shown in Figure 2. To gauge from which direction light comes, they move their heads from one side to the other. If they were to merely increase their rate of movement and/or change in direction of movement with light intensity (a kinesis), should they be on the surface, or reach the surface, they would instantly make themselves vulnerable to predators. This kind of movement would be too easy for predators to spot, although it stops when the organism is buried deep enough, to be of a good advantage to them. A change in allele frequencies resulting from their vulnerability in this situation would rather favour the evolution of direct movement away from the light (taxis). The ideal would be if the movement was away from the light source, and slowed down as the maggot reached the centre of its abode, where the light intensity was at its least, so that it stayed there. This is a negative phototactic response

Prediction

Based on the reasoning above, here is what I think will happen.

I therefore predict that the larvae will demonstrate a negative phototactic response. This will mean that they will:

* * Move away from a light source

* * Move away faster, the greater the intensity of the light

* * Move away at a smaller angle to the direction of the light, the greater the intensity of the light

* * Respond more strongly to more damaging (higher frequency and therefore energy) wavelengths, i.e. those closer to the Ultra-Violet end of the spectrum of visible light.

Plan

To investigate the validity of the above theory, I have devised the following plan. (The numbers in superscript reference their justification later in this document.)

OUTLINE: I intend to take five blowfly larvae and subject them to high and low light intensities and different frequencies, and record the direction and magnitude of their response.

Apparatus:

* * Black-lined box

* ? Approximately 30 by 20 cm

* ? With a shelve and mirrors at one narrow end, as shown

* ? Lined with matt black paper and with a smooth black bottom(3) (with 1cm2 grid)

* * Light source- neon strip lamp(4)

* * Light Meter probe on a Logit box(5)

* * Stop Clock/Timer(6)

* * Narrow paper scoop

* * Container to keep larvae in fridge(7)

* * 40 blowfly larvae

Chronological order of the experiment:

* * Set up apparatus as shown

* * Black out room, so that there is just enough light to see by, but not enough to affect experiment.(8)

* * Get five maggots out of the fridge, as needed for one experiment(9)

* * Allow them to warm to room temperature(10)

* * Set the variable resistor to the correct intensity, or fit filters, using light meter to check the value of the light intensity(11)

* * Switch lights in room off

* * Pick a larva up using the narrow paper scoop, with the head at the front end

* * Place the larva in the centre point of the base of the box, its right side facing the light source, shutting the lid and starting the timer at the same time as switching the lamp on

* * After 10, 20, and 30 seconds(12), note the position of the larva on the grid by looking through the peephole, recording the co-ordinates on the table shown below.

* * Do this with all 5 larvae(13)

* * Repeat this with red, green and blue filters, alternately, using the voltage pack to vary the output of the bulb, ensuring equal intensity readings on the light meter(14)

* * Repeat this with unfiltered light of 2, 3, 4 & 5 times the light intensity(15)

This is the table on which I will record my results for each colour/intensity:

Larva

Quality

Temperature

Co-ordinates 10

Co-ordinates 20

Co-ordinates 30

White

2

White

3

White

4

White

5

White

Red

2

Red

3

Red

4

Red

5

Red

Blue

2

Blue

3

Blue

4

Blue

5

Blue

Larva

Intensity

Temperature

Co-ordinates 10

Co-ordinates 20

Co-ordinates 30

2

3

4

5

2

2

2

3

2

4

2

5

2

3

2

3

3

3

4

3

5

3

Etc.

I will carry out the following statistical analysis on my results (on the spread sheet, together with the results):
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* * Use trigonometry to calculate the angle at which the larvae moved initially, intermediately, and finally(16), the direction being in the form of the number of degrees by which the path of the larva differed from the direction of light

Tan q = X-component of movement

Y-component of movement

* * Compare my results to the outcome predicted by the null hypothesis using the chi-squared test to check their validity.(17)

To do this I must first use the results of the above trigonometrical calculation to split the responses up into groups: those ...

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