Exploring the World of Insects and Arthropods

• Human Regulation and Reproduction
• Further Education (Key Stage 5)

Exploring the World of Insects and Arthropods

Learning Objective

At the end of the lesson, you will be able to:

• Identify different stages of the life cycle of insects and arthropods.
• Explain the impact of temperature on insect development.
• Compare the decomposition stages to insect development.

What do you already know about Forensic entomologists?

Recap!

A decomposing body follows a regular pattern and predicable changing habitats. The stages include: 

- Fresh

- Bloat/Putrefaction

- Active/Advanced Decay

- Dry/Skeletisation

Forensic Entomologists

Apply their knowledge of entomology to provide information for criminal investigations. Their job may include: 

• Identification of insects at various stages of their lifecycle.
• Collection and preservation of insects as evidence.
• Determining PMI estimate.
• Testifying in court to explain insect-related evidence.

What does PMI stand for? And what does it mean?

Forensic Entomologists

Insects help forensic scientists estimate how long the victim has been dead. Other uses include:  

• Movement of the body
• Manner and cause of death
• Association of suspects with the death scene
• Detection of toxins, drugs or even DNA

Introduction to Insects and Arthropods

Insects and arthropods are diverse species that play crucial roles in ecosystems. They encompass a wide range of species, including beetles, butterflies, ants, and spiders.

Did you know? Maggots can be used to test for the presence of poisons or drugs. Some drugs can even speed up or slow down an insects development. 

Life Cycle Stages

The life cycle of insects and arthropods includes egg, larvae (maggot), 1st instar, 2nd instar, 3rd instar, prepupal stage, pupa (puerperium), and adult (imago). Each stage has distinct characteristics and behaviours. 

A standard life cycle means that it can be 

used as evidence in court. 

How long does it typically take for a blow fly to complete its life cycle?

1. 2-4 weeks
2. 1-2 weeks
3. 3-6 months
4. 1 year

What do blow fly larvae feed on?

1. Decaying organic matter
2. Seeds
3. Nectar
4. Fresh fruits

What is the first stage of blow fly life cycle?

1. Pupa
2. Adult
3. Larva
4. Egg

Life Cycle Stages

The main insects used in investigations are in two orders: 

1. Flies (Diptera) - Specifically blowflies  
2. Beetles (Coleoptera) - Specifically dermestid beetles

Species succession may also provide clues for investigation. Some may feed on a fresh body, whereas others may prefer to feed on one that has been dead for a length of time.  

Insects of Death

Flies can arrive within minutes. They lay eggs that hatch to maggots. Maggots feed on soft, mushy body parts. More insects arrive to feed on the body and each other. 

             Blowfly                                         Flesh fly                                      House fly

Insects of Death

Some beetles feed on the body, some on maggots, and some on other beetles. 

Blow Fly Metamorphosis

Blow Fly Metamorphosis: Eggs

1st - Adult flies lay eggs on the carcass, especially at wound areas or around the openings in the body such as the nose, eyes, ears, anus, etc. within 48 hours of death.  

2nd - Eggs hatch into maggots (larvae) in 12-24 hours

Blow Fly Metamorphosis: Larvae

3rd - Larvae continue to grow and molt as they pass through their various instar stages. 

• 1st Instar - Approx. ?mm after ? hours
  
  
• 2nd Instar - Approx. ?mm long after ? days
  
  
• 3rd Instar - Approx. ?mm long after ? days

Blow Fly Metamorphosis: Larvae

3rd - Larvae continue to grow and molt as they pass through their various instar stages. 

• 1st Instar - Approx. 2mm after 24 hours
  
  
• 2nd Instar - Approx. 11mm long after 2.5 days
  
  
• 3rd Instar - Approx. 15mm long after 5 days

Blow Fly Metamorphosis: Pupa

4th - The larvae develop into pupa after burrowing in surrounding soil or a safe dark space. 

5th - Adult flies emerge from pupa cases after 6-8 days

Relating flies to the stage of decomposition

Now put this into a cycle or spider diagram to embed your learning. 

Take a photo to upload to the next slide.  

Stage of Decay	Timeframe	What happens in the body?	What happens with the bugs?
Fresh	0-4 days	Temperature falls, autolysis occurs	Flies begin to arrive
Bloat	4-10 days	Body swells due to gasses, temperature increases	Flies and maggots present
Active/Advanced	5-11 days	Body flattens, bacteria/maggots break through the skin	Larvae begin to pupate
Dry/Skeletonisation	10-25 days	Reduced to hair and bones	Flies reduced and replaced by other arthropods

Upload your images of your cycles/spider diagrams

00:22

What is the body temperature change also known as?

00:58

What is generally the first type of insect to hone in on a human body?

01:42

What are the different larval stages called?

03:41

What does mass spectrometry allow scientists to produce?

Behavior and Location

The stages of the life cycle significantly impact the behavior and location of insects and arthropods. For example, larvae may have specific feeding habits and habitats, while adults engage in reproduction and foraging.

Effects on Insect Activity

The most important environmental factors in decay:

• Temperature
• Access by insects
• Depth of burial
  

Flies do not live in the extreme cold

As a rule, the higher the temperature, the faster the decomposition rate.

Temperature Effects

Temperature has a significant impact on the developmental stages in the life cycle of insects and arthropods. It can influence the rate of growth, behavior, and survival of individuals.

Processing a Crime Scene for Insect Evidence

1. Death-Scene observations
2. Collection of meteorological (Weather) data
3. Two collections: Collect live insects, and collect and preserve other insects from the body at the crime scene
4. After body removal: Collect insects from the surrounding soil.  

Write down 3 things you learned in this lesson.

Have students enter three things they learned in this lesson. With this they can indicate their own learning efficiency of this lesson.

Write down 2 things you want to know more about.

Here, students enter two things they would like to know more about. This not only increases involvement, but also gives them more ownership.

Ask 1 question about something you haven't quite understood yet.

The students indicate here (in question form) with which part of the material they still have difficulty. For the teacher, this not only provides insight into the extent to which the students understand/master the material, but also a good starting point for the next lesson.