This exercise utilizes nightcrawler decomposition to guide students through the methods of science, including forming hypotheses, conducting an experiment, statistically analyzing data, and writing a lab report in the style of a scientific journal article. Additionally, students gain experience with dissection, anatomical terminology, and the biological process of animal decomposition. Students interested in forensic science will enjoy exploring the topic of decomposition. The materials for this experiment are inexpensive and easy to obtain, and several extension activities make this a versatile activity that can be used at many different educational levels.

Introduction

Inquiry-based learning has become the standard in science education (Prince & Felder, 2007). It encourages students to think critically and to advance their scientific literacy (Hall & McCurdy, 1990; Ahren-Rindell, 1999). As part of the transition to an inquiry-based learning model, secondary and postsecondary schools are moving away from traditional lab activities with predetermined outcomes that involve little critical thought by students. Not only are inquiry-based lab activities more effective for teaching the critical-thinking skills necessary to understand science, they also improve student interest in the course and subject matter (Ahren-Rindell, 1999). Student interest has been linked to higher retention rates in the sciences (Kinkead, 2003).

The importance of inquiry-based learning has been thoroughly demonstrated, but the degree to which it has been implemented is questionable (Singer et al., 2005; Gengarelly & Abrams, 2009). One issue with implementing inquiry-based learning at the college level is that students are not prepared to think at the level required for this type of learning. This can make students frustrated and give them the erroneous thought that they are not smart enough to be scientists. The experiment described here illustrates a potential intermediate between a traditional step-by-step laboratory activity and a purely inquiry-based activity. It provides clear instructions for students like a traditional laboratory activity, but takes them a step further by asking them to analyze and explain results like a scientist. This would also be useful at the high school level to help prepare students for the higher-level thinking they will experience in college courses.

This lab activity is a 4-week experiment that introduces students to use of the methods of science and to scientific writing. It was developed for an undergraduate, introductory organismal biology lab course. The course introduces students to the diversity of life on Earth and meets once per week for 3 hours. Approximately half of the students in this course are forensic science majors; therefore, learning about animal decomposition is relevant to their future careers. This experiment would also be appropriate for high school biology courses and meets the expectation of the Next Generation Science Standards that students be engaged in the process of science (NGSS Lead States, 2013). The forensic science application would be of interest to most students in grades 9–12, given the popularity of forensic-based television programs.

Nightcrawlers (Lumbricus terrestris) are used as the study animal in this experiment because they are larger than other earthworms and fresh specimens are easy to obtain and euthanize. Nightcrawlers exhibit decomposition similar to that of humans and other animals, in that they undergo putrefaction, which is the breakdown of tissues that leads to the production of gases as byproducts (Vass et al., 2002; Carter et al., 2007). Research has shown that microorganisms from the digestive and respiratory systems cause putrefaction to occur (Vass et al., 2002; Carter et al., 2007). In this experiment, gut transplants are performed in order to increase or decrease the number of microorganisms available for decomposition.

Methods

Materials

Each group of two to four students will require the following:

  • A laboratory notebook

  • Four large, euthanized nightcrawlers (purchased from a local bait shop)

  • Four 1-quart Ziploc plastic storage bags

  • Two scalpels with new blades (Carolina Biological Supply [CBS] no. 626642)

  • Two pairs of 4.5 inch, stainless steel dissecting scissors (CBS no. 621810)

  • Twenty to thirty dissecting pins (CBS no. 629130)

  • Two small, padded dissecting trays (CBS no. 629005)

  • One permanent marker

  • Latex or nitrile gloves

One thermometer and one 5.5-quart or larger plastic storage bin with a lid are required for the entire class.

Instructor Preparations

Nightcrawlers can be stored in a refrigerator for up to 2 weeks. Euthanize the worms by freezing immediately prior to dissection, because decomposition begins upon death. Freezing is the optimal method of euthanasia because it leaves the worm tissues intact without introducing additional moisture to the specimens, which could affect their decomposition rate (Carter et al., 2007). To euthanize the worms, put up to three worms in a plastic or glass container and then place the container in a standard freezer for 2 hours prior to the start of the laboratory. If worms are kept in the freezer longer than 2 hours, the tissues begin to break down and dissection becomes difficult. Placing more than three worms in a container increases the length of time it takes for them to freeze. Worms should be removed from the freezer 10–15 minutes before lab to allow them to thaw before dissection.

Student Preparation

Instruct students to read the article “Beyond the grave – understanding human decomposition” (Vass, 2001) as homework before the experiment. This article provides relevance for the experiment by introducing students to the topic of animal decomposition (specifically human decomposition), including the process of putrefaction caused by microorganisms.

In-Class Preparation

Begin the initial lab period by explaining that students will use the methods of science to explore the effects of manipulating microorganism abundance on the decomposition of nightcrawlers. After this brief introduction, have students spend 10 minutes discussing the following questions in small groups, using information from the article they read.

  • (1)

    What role do gut microbes play in the process of decomposition?

  • (2)

    Develop a hypothesis regarding the relationship between the abundance of gut microbes and the rate of decomposition of nightcrawlers.

  • (3)

    What other factors could affect the rate of decomposition?

  • (4)

    How can you change the abundance of gut microbes present in a nightcrawler?

Spend 10 minutes allowing each group to share their answers with the entire class, and lead the class to determine that adding or removing the nightcrawler's gut allows the number of microorganisms to be manipulated.

Most students hypothesize that an increase in gut microbes will increase the rate of decomposition; therefore, adding an extra gut will cause the nightcrawler to decompose more quickly. However, students are usually undecided about whether an undissected nightcrawler or a nightcrawler with a removed gut will decompose more quickly, because the undissected nightcrawler will have more gut microbes but the nightcrawler with no gut will have less tissue to decompose. This indecision causes students to become curious to see the results of the experiment. After the class discussion, explain the importance of controls and then ask students to identify the controls in this experiment. Finally, instruct the groups to modify their hypotheses to explain what will happen to the nightcrawler with two guts, the one with no gut, the undissected nightcrawler, and the nightcrawler with an incision but an intact gut.

Prior to dissection, briefly explain the dissection instructions, safety protocols, and nightcrawler anatomy (Figures 1 and 2). You can find instructions for earthworm dissection and a description of its anatomy on the CBS website (Carolina Biological Supply Company, 2014).

Figure 1.

The anatomy of the anterior end of an earthworm. The arrow indicates the location where gut removal begins. Structures anterior to the arrow should be left intact. Copyright Carolina Biological Supply Company, used by permission.

Figure 1.

The anatomy of the anterior end of an earthworm. The arrow indicates the location where gut removal begins. Structures anterior to the arrow should be left intact. Copyright Carolina Biological Supply Company, used by permission.

Figure 2.

Dissection of worms. (A) The placement of pins before dissection. The ventral side of the worm should be facing up. (B) The placement of pins after dissection. (C) The gut removed from the “no-gut” worm (top) and next to the “two-guts” worm (bottom).

Figure 2.

Dissection of worms. (A) The placement of pins before dissection. The ventral side of the worm should be facing up. (B) The placement of pins after dissection. (C) The gut removed from the “no-gut” worm (top) and next to the “two-guts” worm (bottom).

Dissection Procedure

Each group of students should follow the procedure below.

  • (1)

    Briefly rinse each nightcrawler with distilled water in order to remove debris from its exterior and then allow it to hang dry for ~30 seconds.

  • (2)

    Apply different treatments to each of the nightcrawlers (Table 1).

  • (3)

    Assess the degree of decomposition of each nightcrawler using the “quantification of decomposition” scale shown in Table 2. Also note the presence of fungi on the worms and the presence and color of any liquid in the bags. Record observations in a lab notebook. Be sure to include the date.

  • (4)

    Place all the plastic storage bags containing the prepared worms in the class's plastic storage bin.

  • (5)

    At the beginning of lab for the next 3 weeks, record the degree of decomposition (Table 2), presence of fungi, and presence and color of liquid in the bag.

  • (6)

    At the end of week 4 of the experiment, record final observations and report all quantitative decomposition data using a Microsoft Excel spreadsheet (Figure 3). Class data will be combined and statistically analyzed using Excel.

Table 1.
Instructions for applying treatments to the nightcrawlers.
WormTreatment
Undissected control worm 
  • Do not dissect. Place in a Ziploc bag labeled with group name and “Undissected Control.”

 
Dissected control worm 
  • Dissect and slightly loosen the gut from the body wall but do not remove it.

  • Place in a Ziploc bag labeled with group name and “Dissected Control.”

 
No-gut worm 
  • Dissect next to the two-guts worm, pinning open the body cavity as you go (Figure 2B).

  • Loosen the gut posterior to the gizzard (Figure 1), then remove it by using the scalpel to gently roll it onto the dissecting pad, next to the two-guts worm (Figure 2C).

  • Place the gutted worm in a plastic storage bag labeled with group name and “No Gut.”

 
Two-guts worm 
  • Dissect next to the no-gut worm, pinning open the body cavity as you go (Figure 2B).

  • Gently place the gut from the no-gut worm along the gut of the two-guts worm using a scalpel.

  • Place the worm in a plastic storage bag labeled with group name and “Two Guts.”

 
WormTreatment
Undissected control worm 
  • Do not dissect. Place in a Ziploc bag labeled with group name and “Undissected Control.”

 
Dissected control worm 
  • Dissect and slightly loosen the gut from the body wall but do not remove it.

  • Place in a Ziploc bag labeled with group name and “Dissected Control.”

 
No-gut worm 
  • Dissect next to the two-guts worm, pinning open the body cavity as you go (Figure 2B).

  • Loosen the gut posterior to the gizzard (Figure 1), then remove it by using the scalpel to gently roll it onto the dissecting pad, next to the two-guts worm (Figure 2C).

  • Place the gutted worm in a plastic storage bag labeled with group name and “No Gut.”

 
Two-guts worm 
  • Dissect next to the no-gut worm, pinning open the body cavity as you go (Figure 2B).

  • Gently place the gut from the no-gut worm along the gut of the two-guts worm using a scalpel.

  • Place the worm in a plastic storage bag labeled with group name and “Two Guts.”

 
Table 2.
Quantification of worm decomposition. Only whole numbers were used.
Degree of DecompositionDescription of Decomposition Level
All worm tissue is present; no liquid is present. 
Most worm tissue is still present; minimal liquid is present; liquid may appear brown or black. 
Abundant tissue is present; some black liquid is present. 
Half of the worm tissue is present; abundant black liquid is present. 
Less than half of the worm tissue is present; abundant black liquid is present. 
No worm tissue is present; the worm has completely liquefied and the liquid is black. 
Degree of DecompositionDescription of Decomposition Level
All worm tissue is present; no liquid is present. 
Most worm tissue is still present; minimal liquid is present; liquid may appear brown or black. 
Abundant tissue is present; some black liquid is present. 
Half of the worm tissue is present; abundant black liquid is present. 
Less than half of the worm tissue is present; abundant black liquid is present. 
No worm tissue is present; the worm has completely liquefied and the liquid is black. 
Figure 3.

Sample class spreadsheet combining data from all sections.

Figure 3.

Sample class spreadsheet combining data from all sections.

Additional Notes to the Instructor

During week 1 of the experiment, the introductory discussion takes ~30 minutes. Dissections and initial observations take ~1 hour. After the dissections and observations are completed, spend ~45 minutes introducing students to the topic of scientific writing and the expected format of their lab reports. Students spend ~15 minutes at the beginnings of the following three lab periods to record observations of worm decomposition. The remaining time for those weeks is spent doing unrelated laboratory activities.

The instructor should record the room temperature daily using a standard thermometer or the thermostat in the room. These data should be provided to the students at the end of the experiment to consider as an additional factor affecting decomposition.

The bags may leak, so it is good practice to store them in a leak-proof container. In order to control the smell of decomposition, we placed the lidded plastic storage box in a larger storage box and kept it in the back corner of the lab. This worked well to contain the smell the first time we conducted the lab during the winter trimester, but it was not enough when the lab was conducted during the spring trimester, when temperature and humidity were higher. If the smell is not contained, place the small storage box under a fume hood or in an area with excellent ventilation. The key is to keep the container in a fairly constant environment throughout the experiment to control for the effects of variation. However, this is one area in which this lab can be modified (see “Extension Activities” below).

Statistical Analysis

At both high school and introductory college levels, it is useful to introduce students to statistical analyses beyond simple descriptive statistics. Use the lab period the week after the experiment ends to walk students through the process of using Excel to complete an analysis of variance (ANOVA) of the combined class data (Figure 3). This should be conducted in a computer lab where each pair of students has computer access, though working individually provides more experience for the students. Begin with a brief lecture on the importance of statistical analysis in biology and then provide a brief explanation of ANOVA and what it tests. An Excel spreadsheet with the combined class data should be provided to the students (Figure 3). Walk the students through the ANOVA, discuss the results as a class, and explain how to report the results in a lab report.

Results

Assessment

Students must individually complete a lab report in the style of a scientific journal article. The process of writing this lab report begins during the initial week of the experiment, when the instructor teaches students how to write a scientific paper and how to write using scientific tone. This instruction should include how to cite sources, and students should cite “Beyond the grave – understanding human decomposition” by Arpad Vass (2001), as well as an additional credible source that they find on their own. Each section of the paper (Introduction, Materials and Methods, Results, and Discussion) should be due on a different date. We allowed a 1-week turnaround time for grading the first attempts, and grading was based on whether or not they contained the correct elements of each section and on the overall writing quality (Table 3). Students should make necessary revisions to each section, combine them into one complete lab report, and turn in a final draft during the last lab period of the term. This final draft is graded using the rubric shown in Table 3.

Table 3.
Grading rubric for the final lab report. The first attempts for each section were graded using the same criteria.
TitleDescriptionPoints
Introduction 
  • Concise, informative, and relevant (2 points)

  • Context for the study is provided (6 points)

  • Hypotheses are stated and follow logically from the provided context (2 points)

 
 
Materials & Methods 
  • Procedure is explained with enough detail that it could be repeated (6 points)

  • Methods of data analysis included (2 points)

  • Written in past tense (2 points)

 
 
Results 
  • Results are presented clearly (3 points)

  • Results are written in paragraph form (1 point)

  • Figures and/or tables showing trends are included (3 points)

  • Figures and/or tables are labeled properly (2 points)

  • Figures and/or tables are referred to in the text of the Results section (1 point)

 
 
Discussion 
  • Hypotheses are supported or rejected (2 points)

  • Conclusions follow logically from the results (6 points)

  • A connection is made to the context presented in the introduction (2 points)

 
 
Writing Mechanics 
  • Proper spelling, punctuation, and grammar are used (3 points)

  • Sentences are constructed with care and include a subject and a verb (2 points)

  • Paragraphs are constructed with care and demonstrate good flow (2 points)

  • Species names are in the proper format (1 point)

 
 
Citations 
  • Citations follow APA format (2 points)

  • Citations are included in text and in a Works Cited section at the end of the document (3 points)

 
 
 Total 45 
TitleDescriptionPoints
Introduction 
  • Concise, informative, and relevant (2 points)

  • Context for the study is provided (6 points)

  • Hypotheses are stated and follow logically from the provided context (2 points)

 
 
Materials & Methods 
  • Procedure is explained with enough detail that it could be repeated (6 points)

  • Methods of data analysis included (2 points)

  • Written in past tense (2 points)

 
 
Results 
  • Results are presented clearly (3 points)

  • Results are written in paragraph form (1 point)

  • Figures and/or tables showing trends are included (3 points)

  • Figures and/or tables are labeled properly (2 points)

  • Figures and/or tables are referred to in the text of the Results section (1 point)

 
 
Discussion 
  • Hypotheses are supported or rejected (2 points)

  • Conclusions follow logically from the results (6 points)

  • A connection is made to the context presented in the introduction (2 points)

 
 
Writing Mechanics 
  • Proper spelling, punctuation, and grammar are used (3 points)

  • Sentences are constructed with care and include a subject and a verb (2 points)

  • Paragraphs are constructed with care and demonstrate good flow (2 points)

  • Species names are in the proper format (1 point)

 
 
Citations 
  • Citations follow APA format (2 points)

  • Citations are included in text and in a Works Cited section at the end of the document (3 points)

 
 
 Total 45 

It is important to discuss the results as a class before students begin to analyze them. Although the statistical analysis is one method of analyzing the data, it should be explained to students that they also collected qualitative data (presence of fungi, presence and color of liquid) and that all types of data should be discussed. This discussion allows students to see that there can be multiple explanations for scientific findings and encourages them to critically analyze results.

Extension Activities

This experiment can be adapted to various academic levels and to various degrees of inquiry and content.

  1. Students could learn nightcrawler anatomy while dissecting the worms at the beginning of the experiment. Worm anatomy is often used at the high school and college levels as an introductory dissection activity and an introduction to invertebrate anatomy.

  2. Students could characterize microbes they obtain by streaking bacterial plates and isolating bacterial colonies using various techniques. These techniques can be found in any microbiology textbook.

  3. Students could test the effects of humidity, temperature, pH, etc., on the rate of worm decomposition. Research on human remains indicates the importance of various environmental factors in the rate of decomposition (Vass et al., 2002).

  4. Students could present their findings to the class. This would be most effective if each group were testing a different environmental variable, as indicated above.

  5. Students could complete peer assessments of the drafts of their research papers in addition to, or in place of, instructor feedback.

Conclusions

This experiment provides an engaging way to introduce the methods of science, statistical analyses, and scientific writing to students. One student was so excited by the experiment that she volunteered to conduct the experiment a second time on her own for the purposes of this article. The introduction to statistical analyses is especially valuable because it allows students to conduct analyses on data they collect. They were excited to see that the ANOVA showed significant differences between the treatment groups. While this is not a purely inquiry-based laboratory experiment, it could serve as a critical step between traditional, predetermined laboratory activities and more independent, inquiry-based learning.

I thank Lindsay Boyer for her detailed work on this project, Zarah Pratz for her help with the photographs, and Drs. J. Michael Campbell and Jason A. Bennett for their continuous flow of ideas, feedback, and support.

References

References
Ahern-Rindell, A.J. (
1999
).
Applying the strategies of inquiry based learning and cooperative group learning to promote critical thinking
.
Journal of College Science Teaching
,
28
,
203
207
.
Carolina Biological Supply Company
(
2014
).
Earthworm dissection guide
. .
Carter, D.O., Yellowlees, D. & Tibbett, M. (
2007
).
Cadaver decomposition in terrestrial ecosystems
.
Naturwissenschaften
,
94
,
12
24
. .
Gengarelly, L.M. & Abrams, E.D. (
2009
).
Closing the gap: inquiry in research and the secondary science classroom
.
Journal of Science Education and Technology
,
18
,
74
84
.
Hall, D.A. & McCurdy, D.W. (
1990
).
A comparison of a Biological Sciences Curriculum Study (BSCS) laboratory and a traditional laboratory on student achievement at two private liberal arts colleges
.
Journal of Research in Science Teaching
,
27
,
625
636
.
Kinkead, J. (
2003
).
Learning through inquiry: an overview of undergraduate research
.
New Directions for Teaching and Learning
,
93
,
5
17
.
NGSS Lead States
(
2013
).
Next Generation Science Standards: For States, By States
.
Washington, DC
:
National Academies Press
.
Prince, M. & Felder, R. (
2007
).
The many faces of inductive teaching and learning
.
Journal of College Science Teaching
,
36
(
5
),
14
20
.
Singer, S.R., Hilton, M.L. & Schweingruber, H.A. (Eds.) (
2005
).
America's Lab Report: Investigations in High School Science
.
Washington, DC
:
National Academies Press
.
Vass, A.A. (
2001
).
Beyond the grave – understanding human decomposition
.
Microbiology Today
,
28
,
190
192
.
Vass, A.A., Barshick, S.-A., Sega, G., Caton, J., Skeen, J.T., Love, J.C. & Synstelien, J.A. (
2002
).
Decomposition chemistry of human remains: a new methodology for determining the postmortem interval
.
Journal of Forensic Sciences
,
47
,
542
553
.