Phototaxis & kinesis responses in maggots

John Cheverton

Issue 5, March 1996

Background information

Blowfly (Calliphora),larvae are easy to obtain from fishing tackle shops and are reliable subjects for studying behavioural responses. The adult fly lays eggs on animal carcasses, which the maggots burrow into as they feed. Later they emerge from the carcass, and crawl away to pupate, usually in the soil or a safe crevice. The method described here aims to discover whether maggots show phototaxis, photokinesis, or both. It is designed to allow statistical analysis of both responses using the X² test. It could also be done as an exercise in experimental design, to meet these criteria.

Practical requirements

• Darkened room, with individual shaded desk lamps (or other source of directorial light). Avoid having lights on different tables facing in opposite directions'
• Shallow plastic tray, large enough to hold a sheet of A4 paper, with Perspex cover.
• Sheet of A4 paper, marked out with concentric circles of 1.5cm and 15cm radius, divided into two 120° sectors towards and away from the light, and two 60° sectors at right angles to the light [as shown on next page].
• 20 fresh maggots per group, in a Petri dish. Spare dish. If previously refrigerated, allow the maggots at least 30 minutes to reach room temperature before the investigation.
• Stopwatch, and water-soluble marker pen. Ruler. Table of the X² distribution.

Method

1. Arrange the desk-lamp about 10cm from one end of the tray, shining at a shallow angle along the tray. Position the marked A4 paper in the tray.
2. Place one active maggot exactly at the centre of the small circle, and place the cover over the tray. mark the position of the centre on the cover.
3. Start the stopwatch as soon as the head of the maggot crosses the inner circle. Mark this position with a small 'a'. Every 10s, mark the maggot's position again, using the same symbol. NB: if the maggots are very lively 5s may be a better interval.
4. Record, in a suitable table, across which sector (+ve, -ve, or neutral) the maggot left the outer circle. Continue marking its position at regular intervals until it reaches the edge of the marked paper.
5. Put the used maggot in the spare dish, and choose a fresh one.
6. Repeat steps 2 - 5 until you have recordings from 10 maggots: use. a new symbol ('b', 'd., 'd/, etc) for each one. Take care to place the cover in the same position for each trial. Hint: If the symbols get crowded, stop after 5 maggots and do step 7 before continuing.
7. Place the Perspex cover on a large sheet of paper, and mark the relative position of the lamp. For each maggot, now make a table of the distance from the lamp to each position, and the speed of movement (in mm/s) from that position to the next. Clean the cover.

Analysis

8. Collect the class results for the number of observed departures in each direction. Given that each of the three directions cover the same angle, calculate expected numbers on the null hypothesis of random movement. Use the X² test to see whether your results differ significantly from the expected. Can you reject the null hypothesis?
9. Plot a multiple line graph of distance v speed, using a different symbol for each maggot, and connecting successive points with straight lines. If there is no obvious pattern, count the numbers of times the speed increases and decreases with distance. What would you expect?
10. Compare your data with the expected results using the X² test.
11. Interpret your results with respect to the type(s) of response shown, and the adaptive value to the maggots. How might the responses) change with the age of the maggots? Find out what type of sense organs are involved in the maggots' response.

Diagram of experimental apparatus