In this activity, students will learn that insects can be used as a tool in a criminal investigation to predict the time of death of a person or an animal by a Forensic Entomologist. Students will analyze and graph multiple data sets related to the life cycle of a blow fly in order to better understand how the blow fly can be used as evidence in an investigation.
This activity will help prepare your students to meet the performance expectations in the following standards:
- Scientific and Engineering Practices:
- Using Mathematics and Computational Thinking
- Analyzing and Interpreting Data
- Engaging in Argument from Evidence
By the end of this activity, students should be able to:
- Interpret and construct the timeframe for a life cycle from provided data.
- Predict how environmental factors such as temperature may affect the life cycle of an insect.
- Describe how the life cycle of a blow fly is used in forensic entomology.
- Analyze and graph data points for multiple data sets, while comparing the data sets.
- Write a claim based on data and support the claim with thoughtful reasoning.
This activity supports students’ understanding of:
- Data Analysis
- Forensic Chemistry
Teacher Preparation: 5-10 minutes
Lesson: 60 minutes
- Student Handout
- 3 colored pencils/markers
- No specific safety precautions need to be observed for this activity.
- This activity was created as part of the 2022 Chemists Celebrate Earth Week (CCEW) public awareness campaign. CCEW is celebrated the week of April 17–23, 2022 with the theme, “The Buzz About Bugs: Insect Chemistry.” Through this theme, students, teachers and all those interested are encouraged to explore the many ways insects in nature can help us (promote biodiversity, pollinate fruits and vegetables, produce honey and silk, etc.) or bug us (give us itchy bites, spread disease, etc.), and the exciting chemistry at play in the lives of insects.
- For years, chemists have been promoting a better world through recyclable plastics, cleaner-burning fuels, phosphate-free detergents, environmental monitoring, and green chemistry initiatives. To promote the positive role that chemistry plays in the world, ACS established the Chemists Celebrate Earth Week (CCEW) public awareness campaign. During CCEW, ACS members and chemistry enthusiasts celebrate by coordinating events and communicating the importance of chemistry. Read more about CCEW.
- This activity was created to give students the opportunity to learn about the field of forensic entomology. Using insects, specifically blow flies to aide an investigation is an important aspect to a crime scene investigation. Most frequently this technique is used to help investigators determine time of death.
- Additional background information and classroom activities related to forensic entomology can be found through some of these helpful resources:
- It should be noted that the life cycle data sets used in the student handout were created based on information found in the Visible Proofs Exhibit: Entomology in Action for the blow fly at 70°F, from the National Library of Medicine (NLM), in combination with the calculation for determining accumulated degree hours (ADH).
- The student handout does not include an image of the life cycle of a blow fly (although they are asked to sketch the life cycle in question 2). But it might be helpful to show an image for students to fully comprehend how the insect changes appearance and develops over time. This image from NLM might be helpful.
- An Answer Key document for this activity has been included for teacher reference.
For the Student
Scientists known as Forensic Entomologists use insects as a tool in criminal investigations. One of the ways that insects can aide an investigation is by helping to predict the time of death of a person or an animal. While this might seem a little gross, it is actually extremely helpful, especially in an investigation where a timeline is an important part of determining if a crime has been committed.
One insect, called a blow fly is particularly helpful to a forensic entomologist due to its predictable life cycle. Blow flies are known to lay eggs on human and animal remains and based on the stage in the lifecycle of the insect, a forensic entomologist can make a prediction about how long a body has been deceased.
- There are 5 stages of the blow fly life cycle. However, the time in the cycle can change based on the surrounding temp described in the data table below:
|Life Cycle Stage||Duration of Time at 70°F|
|Eggs to larval instar 1||23 hours|
|larval instar 1 to larval instar 2||27 hours|
|larval instar 2 to larval instar 3||22 hours|
|larval instar 3 to pupa||130 hours|
|Pupa to adult blow fly||143 hours|
*Note: The time listed is for each individual stage of the life cycle. Time listed is not cumulative.
- Draw a diagram that describes the life cycle of the blow fly below:
- What is the length of time (in hours) of the entire life cycle of a blow fly?
- Insects require a certain amount of thermal energy to develop from one stage to the next throughout an insect life cycle, this is known as accumulated degree hour. With this in mind, make a prediction about the development of a blow fly from the pupa stage to the adult in a warm, tropical climate during July. Explain your reasoning citing data from the table above.
- Make a line graph using the data in question 1 that plots time for each stage of the blow fly lifecycle on the y-axis, and the corresponding stage of the life cycle on the x-axis. When plotting points, remember that the time needs to accumulate throughout the life cycle so that the graph shows the entirety of the life cycle. Reorganizing the data in the below table may be helpful—some hints have been provided.
|Start of Stage||Time (hours)|
|Larval Instar 3||72|
- How many days does it take for a blow fly to develop from an egg to larval instar 3 at 70°F? Show your work:
- How many days does it take for a blow fly to develop from larval instar 3 to an adult blow fly at 70°F? Show your work:
- The table below describes the 5 stages of the blow fly life cycle at 60°F and at 80°F. Using a different color pencil for each data set, add the information to your graph in question 2.
*Remember to analyze the data as you did in that question prior to plotting the data points. Use the space provided below for any calculations needed.
|Life Cycle Stage||Duration of Time at 60°F||Duration of Time at 80°F|
|Eggs to larval instar 1||27 hours||20 hours|
|larval instar 1 to larval instar 2||32 hours||24 hours|
|larval instar 2 to larval instar 3||26 hours||19 hours|
|larval instar 3 to pupa||152 hours||114 hours|
|Pupa to adult blow fly||167 hours||125 hours|
- If a deceased body was found in an environment where it was 70°F and it had evidence of blow fly pupa on it, what can be said about the body?
- If a deceased body was found in an environment where it was 60°F and it had evidence of blow fly pupa on it as well as adult blow flies, what can be said about the body?
- If a deceased body was found in an environment where it was 60°F and it had evidence of blow fly eggs on it, what can be said about the body?
- Based on the data provided, what claim can be made about the impact of temperature on the life cycle of a blow fly? Be sure to justify your response with reasoning, citing data.
- Are any limitations to the claim you made in part a? Explain.
- What are some real-world challenges that might occur for a forensic entomologist who is attempting to use blow fly evidence in a criminal investigation? List at least 2 potential challenges and explain why each is a challenge.