Forensic Entomology

Insect Life Cycles & Development

Basic Insect Development Metamorphosis = change in form over time

• Ametabolous – without metamorphosis

• Hemimetabolous – incomplete metamorphosis

• Holometabolous – complete metamorphosis

Ametabolous • No larvae stage – go from egg to smaller

looking adult (juvenile), then adult

– E.g. Silverfish

Hemimetabolous Egg – Nymph – Adult

E.g. Milkweed Bug

Holometabolous

• Egg – Larvae – Pupa – Adult

– E.g. Butterflies, Blow Flies, and Beetles

Terminology:

• Oviposition – egg laying

• Eclosion – egg hatching

• Molting – shedding outermost skin

• Instar – period between molts

• Prepupa – inactive stage just before pupa

• Pupation – the process of becoming a pupa

• Puparium/Puparia – outer skin forming pupa casing

• Emergence – when the adult leaves the puparium

Oviposition Eclosion 1st molt

2nd molt

Emergence

Temperature, Temperature, Temperature!

• Insects have no internal mechanism for regulating body temperature

• Ways animals regulate body temperature:

– Endothermic – generates own body heat (humans)

– Ectothermic – exchanges heat with surroundings

– Poikilothermic – body heat varies with surroundings **Insects**

Physiological Time

• The rate of growth and development of insects is proportional to ambient temperature

– There is a min. and max. for each stage of development

Typical Blowfly Development – constant temp of 70˚C

Oviposition Eclosion 1st molt

2nd molt

Emergence

23h

27h (50)

22h (72)

130h (202)

143h (345)

Typical Blowfly Development – constant temp of 70˚C

Oviposition Eclosion 1st molt

2nd molt

Emergence

23h

27h (50)

22h (72)

130h (202)

143h (345)

What does this mean?????

• From egg to adult takes about 2 weeks at 70˚C

Forensic Entomology

Decomposition and Insect Succession

Insect Succession on Pig Carcasses

• 1960s – Jerry Payne identified more than 500 species that colonized decomposing pig carcasses

• We will focus on the most COMMON and PREDICTABLE!

Don’t Forget Those That Colonize the Living!

• Insects that may be present prior to death, may remain on the corpse through the fresh stages of decomposition

– E.g. The rate of algor mortis (body temp lower) will influence the presence or absence of body lice

Fresh Stages: 1-2 days

• Insects may appear minutes after death to feed, mate and reproduce

• They are attracted to odors associated with decay and insect activity

Common Insects: Blow flies, Flesh flies, and some predators

Fresh Stages: 1-2 Days

• In the absence of wounds, blow flies colonize the mucus membranes (eyes, nose, mouth)

Bloat Stages: 2-6 Days

• Adult blow flies and flesh flies may still be arriving, but as the body odors change, the flies will seek fresh remains

• Early bloat – early instars will be present

• Late bloat – late instars will be present

Active Decay Stages: 5-11 Days

• Adult blow flies leave

• Maggot mass is present

• Some maggots beginning to wander and pupate

Common insects arriving: secondary flies, predators, beetles

Post Decay Stages: 10-25 Days

• Blow flies complete life-cycle and do not return

• As body starts to dry out – more beetles arrive

• Mature beetle larvae start to appear around 2 week mark

• 80% biomass loss due to maggot feeding

Post Decay Stages: 10-25 Days

• Secondary flies continue to arrive

– E.g. adult Cheese skippers (Piophilidae)

– Some larvae may be present

• How do these flies know when to arrive?

– Butyric acid (what make feet stink and some cheeses smell) is a byproduct of fermenting bodies

Dry, skeletal Remains: 25+ Days

• Some predators

• Early beetles are pupating

• Secondary flies and their larvae

• Dermestid beetles colonize the dry remains

How Can Insect Succession Estimate PMI?

How it can work (scenario): 1. 15 species are collected from corpse 2. All are identified and stage of development

recorded 3. Stage of decomposition, location, and

temperatures noted 4. This assemblage is compared to an ‘occurance

matrix’ 5. PMI is estimated based on where the

assemblage falls *Basically you compare your ‘evidence’ data with a

‘reference’ data table

Groups of 4

• Each person read one section of the paper – Intro

– Methods

– Results

– Discussion

• That person should report to the group what that section was about

• Answer questions together on 1 sheet to turn in

Gruner et al. 2007 article questions 1. What is succession? 2. Besides PMI, what other factors can be concluded based on

entomological evidence? 3. Why are there not very many studies on insect succession? 4. What is the purpose of this study? 5. How were the pig carcasses protected from predators during the study? 6. Why were temperatures near the surface recorded? 7. How many fly species were collected during the 16 trials? 8. During most of the study (except for the coldest days) how long did it

take for the first flies to appear? 9. Compare Figure 2 with Figure 3 (Spring vs. Summer), why do you think

the abundance of each type of species changes from Spring to Summer? 10. Refer to Figure 5 (Winter), why do you think there is such a low

abundance of C. rufifaces compared with other seasons? (Hint: think about rate of decomposition)

11. How would the heavy rain in the Summer months impact the study? 12. What were the results in terms of aerial samples versus larval samples?

Why is this important?