Changes in earthworm responsiveness
An animal’s responsiveness to a particular stimulus can often show changes in the short term. In these investigations you can study changes in the earthworm’s responsiveness to the stimulus of touch.
The worms have to be left for significant periods of time between stimulations in this investigation. Students could complete a study exercise during these times. However they need to pay attention to their test subject (the worm) to ensure it does not experience dehydration or unnecessary stresses from being tested. If you have access to a suitable video camera, you could record the investigation in the first year and use the recording as evidence in future years.
Apparatus and Chemicals
For each group of students:
Petri dish, 1
Water in a dropper bottle
Seeker, blunt, 1
Stopclock or stopwatch, 1
Liner for Petri dish to simulate rough surface of inside of burrow (Note 3)
For the class – set up by technician/ teacher:
Earthworms in a suitable temporary vivarium (Notes 1 and 2)
Health & Safety and Technical notes
Take sensible hygiene precautions after handling worms or the soil in which they have been kept.
1 Collect the worms well in advance. Excessive handling will render the worms unresponsive; they must be allowed time to recover before the start of the experiment. Collect worms by digging in the soil in some part of your school grounds or a domestic garden. If you encounter sharp objects or animal faeces, stop digging and choose a new site. If you find only a few worms, these two methods might encourage more to surface.
Worm collection method 1 Rhythmic rocking
Gather a group of 8-20 students in a circle in the area where you want to collect worms. Ask them to rock gently back and forth on the soles of their shoes, from heel to toe and back again. There is no need to stamp or make vigorous movements, just rock steadily for a couple of minutes. Worms should surface, unless the weather has been very dry in which case the worms may be very deep in the soil.
Worm collection method 2 Mustard
The OPAL worm survey website suggests the following technique for collecting worms. Dig a pit 10 cm deep and 20 cm x 20 cm wide, and collect worms from the soil you extract. Then add the contents of a sachet of mustard (as supplied in restaurants) to 750 cm3 of water. Pour this into the pit to encourage deep-dwelling worms to surface. This is not toxic or harmful to the worms in the long term.
2 Keeping worms: set up a box or bucket with ventilation holes. Add add some moist soil (not compacted) with added leaf litter and/or potting compost. Keep in a cool place – outdoors is better than in the laboratory or prep room. Make sure it does not dry out. Set up smaller containers, such as beakers or plastic cups, containing pieces of moist sphagnum moss. These are to hold the worms in immediately before and during practical investigations.
3 Simulate the burrow using a liner: mix some coarse grit or sand with a similar volume of artists’ clay (borrow some from your art department), or use some lightly compressed garden soil. Make a ring of liner around the edge of the Petri dish, or in a ring within a crystallising dish. If there are short tunnels in the liner long enough for the worm to burrow into, but not to hide itself completely, you can stimulate the part of the worm’s body still exposed.
Teachers should be careful to introduce earthworms for this practical in a way that promotes a good ethical attitude towards them and not a simply instrumental one. Although they are simple organisms which may not 'suffer' in the same way as higher animals, they still deserve respect. Animals should be returned promptly to their natural environment or a suitable holding tank after being tested. This supports ethical approaches in field work where animals are returned to their habitat after observations have been made.
a Collect some worms for the investigation (Note 1).
b Moisten the inside of a Petri dish with several drops of water, place the worm in it, and leave for at least 15 minutes under dim illumination.
c Stimulate the worm by touching it for one second with a blunt seeker, and note the response.
d Investigate the effect of repeated stimuli by giving the worm stimuli at fixed intervals of between 10 and 20 seconds, keeping the duration and strength of the stimuli as similar as possible. It is important that the worm is stimulated in about the same place on its body each time and that the light intensity is at a low and unvarying level. Observe and record the worm’s response each time. Continue until the worm has ceased to respond to five stimuli in succession.
e Once the worm has become unresponsive to a particular stimulus, continue to stimulate in a new position on the body. Record all responses as before.
f Allow an unresponsive worm a period of 10-20 minutes without stimulation. Stimulate it again; if the response has recovered, continue as in step d. If recovery does not take place, allow a longer rest period.
g If possible, repeat the test procedures on subsequent days with the same individuals, returning them to a suitable holding tank in between tests.
The effect of dual stimulation on responsiveness
h The contact of the worm’s body with the walls of its burrow is a source of stimulation that operates when specific stimuli are applied. Repeat the procedure as in step d above, using a worm whose body wall is in contact with the rough surface. As much as possible of the body wall should be touching the surface. Stimulate as before, recording the responses until the worm ceases to respond.
i If there is time, vary the light intensity to see if this modifies the worm’s responsiveness.
A knowledge of the reflex arc involved in the earthworm’s escape response is desirable but not essential.
The response to a particular stimulus can wane as a consequence of the presentation of the stimulus. A waning of responsiveness usually has some degree of specificity to the stimulus used, and is therefore unlikely to be due to muscular fatigue. A recovery of responsiveness may indicate that more than one process underlies waning.
Earthworms respond to a single stimulus with a single, rapid withdrawal of the part stimulated. This is often called the ‘escape response’, reflecting the likely survival value of the reflex. With repeated stimulation, the response diminishes in magnitude and eventually disappears. A group of worms may show a wide variation in the rate of waning, particularly when the stimulus strength is difficult to control.
The response should reappear when another part of the worm is stimulated – provided that the point of stimulation is sufficiently far from the first point of stimulation. Neighbouring segments, however, may also be affected by the waning and may not produce a response when stimulated. The recovery of responsiveness immediately after stimulation in a new place shows that waning is not due to muscular fatigue. If waning is shown to affect neighbouring segments, then sensory adaptation can also be ruled out. Waning appears to be a property of the nervous system.
Although responsiveness recovers immediately with stimulation at a new point of the body, the response is likely to be of smaller magnitude and to wane more quickly. Responsiveness may recover rapidly, but may not be complete.
The response should be greater, the greater the degree of contact between the worm and the rough surface. Earthworms maintain a high degree of contact – positive thigmotaxis – with the walls of their burrows and this can be a powerful stimulus affecting responsiveness to other stimuli.
Bright light would be expected to increase responsiveness and reduce the rate of waning.
Rapid waning to a stimulus proving to be innocuous will save time and energy that would otherwise be spent making unnecessary response. Changes to the degree of contact and the intensity of light can be related to the most likely function of the response; this would be to draw a worm back into its burrow from the surface.
Many organisms show a period of rapid recovery followed by a period of slow, or even no, further recovery. This suggests that more than one process may be responsible for waning. The term ‘habituation’ is often used to refer to both short-term and long-term changes in responsiveness. However WH Thorpe restricts its use to more permanent effects. Students should understand that a term like this embraces behavioural phenomena with apparent similarities which may result from diverse underlying processes.
Health & Safety checked, September 2009
Download the student sheet Changes in earthworm responsiveness (62 KB) with questions and answers.
Association for the Study of Animal Behaviour website, including guidelines for the treatment of animals in behavioural research and teaching.
(Website accessed October 2011)
CLEAPSS guidance leaflet PS 03 'Keeping and using animals and plants: towards a school/science department policy'
Leaflet suggesting the questions you should address as a department or school to develop your school policy on providing a context for practicals like this one and others.