Orientacions per al professorat

Aqueta pràctica es pot portar a terme a 2n ESO dins del tema de la funció de relació en animals. Es pot realitzar amb altres invertebrats com ara cucs de terra, tot i que les larves de Calliphora es poden trobar en botigues especialitzades de pesca. Es tracta d'esbrinar quina serà la resposta d'una larva de mosca a la llum. Així es pot parlar dels tactismes. Es podria fer una comparació amb vegetals si posem unes setmanes abans una planta al costat de la finestra del laboratori i parlar alhora dels tropismes.
Una idea pel tractament de la diversitat a l'aula seria fer aquest experiment amb una única font de llum i proposar com a ampliació l'ús de dues làmpares situades de forma oposada.

Guió per a l'alumnat

1. Introduction

How do you think light affects animals? Do all the animals like the light or not? Today we will study the response of an insect larvanamed Calliphora towards the light. You can repeat this experiment at home with any other animal you wish to test as long as you can track its movement easily.

2. Problem

We need to determine if this animal moves towards the light, against it or it is not affected by this factor.

3. Hipothesis

Choose between this three possibilities:
A The larvae won't show any special response to light.
B The larvae will move towards the light.
C The larvae will move against the light

4. Experimental design

4.1. Materials

  • Glass square 20x20cm
  • Black card
  • One or two lamps (camp lamps should be ok but they should be identical)
  • Permanent pen
  • Several pieces of paper to mark the path
  • Ruler and protractor
  • Calliphora larvae

4.2. Procedure

  1. Take a piece of black card, about 30 cm square, and fix it to the bench with tape, making it as flat as possible. Use two clamp stands and clamps to support a sheet of glass horizontally about 5 cm above the card.
  2. Place one lamp above the edge of the card, halfway along one side. Optional: Place a second lamp on an adjacent side of the card to make a beam at 90° to the first. Ensure that the bulbs are of the same strength. Test with a light meter if necessary. Draw a faint mark on the card to indicate the positions of the beams of the lamps.
  3. Black out the laboratory and set up barriers to cut out the light from other sets of apparatus.
  4. Ensure the experimental lamp is the only lamp that will influence the larvae in your investigation.
  5. Make faint marks on the card to show the positions of the beams from each lamp.
  6. Collect ten Calliphora larvae in a Petri dish.
  7. Release a single larva near the centre of the card. Follow its movements by marking the glass with a suitable felt pen or wax pencil every few seconds.
  8. Repeat each investigation several times with the same individual, starting with it pointing in a different direction each time.
  9. Collect the data from each investigation by tracing the maps onto paper (or photographing the glass if you can).
  10. Clean the marks from the glass before repeating with other individuals.
  11. Investigate the response of larvae to a single lamp on one edge of the card.
  12. You could also investigate the response of the larvae to the second lamp:
  13. when it is switched on as the larva crosses the path of its beam and the first lamp is switched off
  14. when it is switched on as the larva crosses the path of its beam and the first lamp stays on
  15. Return the larvae to their holding box.
  16. With a ruler and protractor, measure the angles of the animals’ paths in relation to the beams.


Describe the larvae’s response to light. If you had time describe the response to one source of light, changing light source and with two sources of light.
Have you been able to determine if your starting hipothesis was right?


Before you try to write any conclusion, try to ask these questions:
  1. Why have you been asked to use 10 larvae?
  2. Do all the larvae respond the same way? How could you explain any differences?
  3. How did you decide when you had collected enough data about the behaviour of any particular individual in any particular situation?
  4. Is the response to light directional or random?
  5. If the response is directional, is it towards the light (photopositive) or away from the light (photonegative)?
  6. What other factors could be affecting the larvae’s response? How did you minimise or eliminate their effects?
  7. How could these responses affect the larvae’s survival in their normal life?